The second edition of Big Java, Late Objects provides an approachable introduction to fundamental programming techniques
571 114 30MB
English Pages [1432] Year 2016
Table of contents :
Cover
Title Page
Copyright
Preface
Contents
Chapter 1 Introduction
1.1 Computer Programs
1.2 The Anatomy of a Computer
1.3 The Java Programming Language
1.4 Becoming Familiar with Your Programming Environment
1.5 Analyzing Your First Program
1.6 Errors
1.7 Problem Solving: Algorithm Design
Chapter 2 Fundamental Data Types
2.1 Variables
2.2 Arithmetic
2.3 Input and Output
2.4 Problem Solving: First Do It By Hand
2.5 Strings
Chapter 3 Decisions
3.1 The if Statement
3.2 Comparing Numbers and Strings
3.3 Multiple Alternatives
3.4 Nested Branches
3.5 Problem Solving: Flowcharts
3.6 Problem Solving: Test Cases
3.7 Boolean Variables and Operators
3.8 Application: Input Validation
Chapter 4 Loops
4.1 The while Loop
4.2 Problem Solving: Hand-Tracing
4.3 The for Loop
4.4 The do Loop
4.5 Application: Processing Sentinel Values
4.6 Problem Solving: Storyboards
4.7 Common Loop Algorithms
4.8 Nested Loops
4.9 Problem Solving: Solve a Simpler Problem First
4.10 Application: Random Numbers and Simulations
Chapter 5 Methods
5.1 Methods as Black Boxes
5.2 Implementing Methods
5.3 Parameter Passing
5.4 Return Values
5.5 Methods Without Return Values
5.6 Problem Solving: Reusable Methods
5.7 Problem Solving: Stepwise Refinement
5.8 Variable Scope
5.9 Recursive Methods (Optional)
Chapter 6 Arrays and Array Lists
6.1 Arrays
6.2 The Enhanced for Loop
6.3 Common Array Algorithms
6.4 Using Arrays with Methods
6.5 Problem Solving: Adapting Algorithms
6.6 Problem Solving: Discovering Algorithms by Manipulating Physical Objects
6.7 Two-Dimensional Arrays
6.8 Array Lists
Chapter 7 Input/Output and Exception Handling
7.1 Reading and Writing Text Files
7.2 Text Input and Output
7.3 Command Line Arguments
7.4 Exception Handling
7.5 Application: Handling Input Errors
Chapter 8 Objects and Classes
8.1 Object-Oriented Programming
8.2 Implementing a Simple Class
8.3 Specifying the Public Interface of a Class
8.4 Designing the Data Representation
8.5 Implementing Instance Methods
8.6 Constructors
8.7 Testing a Class
8.8 Problem Solving: Tracing Objects
8.9 Object References
8.10 Static Variables and Methods
8.11 Problem Solving: Patterns for Object Data
8.12 Packages
Chapter 9 Inheritance and Interfaces
9.1 Inheritance Hierarchies
9.2 Implementing Subclasses
9.3 Overriding Methods
9.4 Polymorphism
9.5 Object: The Cosmic Superclass
9.6 Interface Types
Chapter 10 Graphical User Interfaces
10.1 Frame Windows
10.2 Events and Event Handling
10.3 Processing Text Input
10.4 Creating Drawings
Chapter 11 Advanced User Interfaces
11.1 Layout Management
11.2 Choices
11.3 Menus
11.4 Exploring the Swing Documentation
11.5 Using Timer Events for Animations
11.6 Mouse Events
Chapter 12 Object-Oriented Design
12.1 Classes and Their Responsibilities
12.2 Relationships Between Classes
12.3 Application: Printing an Invoice
Chapter 13 Recursion
13.1 Triangle Numbers
13.2 Recursive Helper Methods
13.3 The Efficiency of Recursion
13.4 Permutations
13.5 Mutual Recursion
13.6 Backtracking
Chapter 14 Sorting and Searching
14.1 Selection Sort
14.2 Profiling the Selection Sort Algorithm
14.3 Analyzing the Performance of the Selection Sort Algorithm
14.4 Merge Sort
14.5 Analyzing the Merge Sort Algorithm
14.6 Searching
14.7 Problem Solving: Estimating the Running Time of an Algorithm
14.8 Sorting and Searching in the Java Library
Chapter 15 The Java Collections Framework
15.1 An Overview of the Collections Framework
15.2 Linked Lists
15.3 Sets
15.4 Maps
15.5 Stacks, Queues, and Priority Queues
15.6 Stack and Queue Applications
Chapter 16 Basic Data Structures
16.1 Implementing Linked Lists
16.2 Implementing Array Lists
16.3 Implementing Stacks and Queues
16.4 Implementing a Hash Table
Chapter 17 Tree Structures
17.1 Basic Tree Concepts
17.2 Binary Trees
17.3 Binary Search Trees
17.4 Tree Traversal
17.5 Red-Black Trees
17.6 Heaps
17.7 The Heapsort Algorithm
Chapter 18 Generic Classes
18.1 Generic Classes and Type Parameters
18.2 Implementing Generic Types
18.3 Generic Methods
18.4 Constraining Type Parameters
18.5 Type Erasure
Chapter 19 Stream Processing
19.1 The Stream Concept
19.2 Producing Streams
19.3 Collecting Results
19.4 Transforming Streams
19.5 Lambda Expressions
19.6 The Optional Type
19.7 Other Terminal Operations
19.8 Primitive-Type Streams
19.9 Grouping Results
19.10 Common Algorithms Revisited
Chapter 20 Advanced Input/Output
20.1 Readers, Writers, and Input/Output Streams
20.2 Binary Input and Output
20.3 Random Access
20.4 Object Input and Output Streams
20.5 File and Directory Operations
Chapter 21 Multithreading
21.1 Running Threads
21.2 Terminating Threads
21.3 Race Conditions
21.4 Synchronizing Object Access
21.5 Avoiding Deadlocks
21.6 Application: Algorithm Animation
Chapter 22 Internet Networking
22.1 The Internet Protocol
22.2 Application Level Protocols
22.3 A Client Program
22.4 A Server Program
22.5 URL Connections
Chapter 23 Relational Databases
23.1 Organizing Database Information
23.2 Queries
23.3 Installing a Database
23.4 Database Programming in Java
23.5 Application: Entering an Invoice
Chapter 24 XML
24.1 XML Tags and Documents
24.2 Parsing XML Documents
24.3 Creating XML Documents
24.4 Validating XML Documents
Chapter 25 Web Applications
25.1 The Architecture of a Web Application
25.2 The Architecture of a JSF Application
25.3 JavaBeans Components
25.4 Navigation Between Pages
25.5 JSF Components
25.6 A Three-Tier Application
Appendix A The Basic Latin and Latin-1 Subsets of Unicode
Appendix B Java Operator Summary
Appendix C Java Reserved Word Summary
Appendix D The Java Library
Appendix E Java Language Coding Guidelines
Introduction
Source Files
Classes
Methods
Variables and Constants
Control Flow
Lexical Issues
Appendix F Tool Summary
The Java Compiler
The Java Virtual Machine Launcher
The JAR Tool
The javadoc Tool
Documentation Comments
Appendix G Number Systems
Binary Numbers
Overflow and Roundoff Errors
Two’s Complement Integers
IEEE Floating-Point Numbers
Hexadecimal Numbers
Bit and Shift Operations
Appendix H UML Summary
CRC Cards
UML Diagrams
Appendix I Java Syntax Summary
Types
Variables
Expressions
Classes
Interfaces
Enumeration Types
Methods
Constructors
Statements
Exceptions
Packages
Generic Types and Methods
Comments
Appendix J HTML Summary
Glossary
Index
Credits
EULA
Variable and Constant Declarations
Type
Name
Initial value
String s = "Hello"; int n = s.length(); // 5 char ch = s.charAt(1); // 'e' String t = s.substring(1, 4); // "ell" String u = s.toUpperCase(); // "HELLO" if (u.equals("HELLO")) ... // Use equals, not == for (int i = 0; i < s.length(); i++) { char ch = s.charAt(i); Process ch }
int cansPerPack = 6; final double CAN_VOLUME = 0.335;
Method Declaration
Modifiers
Return type
Parameter type and name
public static double cubeVolume(double sideLength) { double volume = sideLength * sideLength * sideLength; return volume; Exits method and }
Conditional Statement
Raising to a power x y Square root x Decimal log log10(x) Absolute value |x|
Condition if (floor >= 13) { Executed when condition is true actualFloor = floor - 1; } else if (floor >= 0) Second condition (optional) { actualFloor = floor; } else Executed when { System.out.println("Floor negative"); all conditions are false (optional) }
Sine, cosine, tangent of x (x in radians)
Class Declaration
returns result.
Mathematical Operations Math.pow(x, y) Math.sqrt(x) Math.log10(x) Math.abs(x) Math.sin(x) Math.cos(x)
public class BankAccount { private double balance; private int transactions;
Math.tan(x)
Selected Operators and Their Precedence (See Appendix B for the complete list.) [] ++ -- ! * / % + < >= == != && || =
C-3
String Operations
public BankAccount(double initialBalance) { balance = initialBalance; transactions = 1; }
Array element access Increment, decrement, Boolean not Multiplication, division, remainder Addition, subtraction Comparisons Equal, not equal Boolean and Boolean or Assignment
public void deposit(double amount) { balance = balance + amount; transactions++; } . . .
Condition
Initialization Condition Update for (int i = 0; i < 10; i++) { System.out.println(i); }
Constructor
Method
}
Loop Statements while (balance < TARGET) { year++; balance = balance * (1 + rate / 100); }
Instance variables
do {
Executed while condition is true
Loop body executed at least once System.out.print("Enter a positive integer: "); input = in.nextInt();
} while (input 23);
226 Chapter 5 Methods int minutes; do { System.out.print("Enter a value between 0 and 59: "); minutes = in.nextInt(); } while (minutes < 0 || minutes > 59);
This program segment reads two variables, making sure that each of them is within a certain range. It is easy to extract the common behavior into a method: /**
Prompts a user to enter a value up to a given maximum until the user provides a valid input. @param high the largest allowable input @return the value provided by the user (between 0 and high, inclusive)
*/ public static int readIntUpTo(int high) { int input; Scanner in = new Scanner(System.in); do { System.out.print("Enter a value between 0 and " + high + ": "); input = in.nextInt(); } while (input < 0 || input > high); return input; }
Then use this method twice: int hours = readIntUpTo(23); int minutes = readIntUpTo(59);
Design your methods to be reusable. Supply parameter variables for the values that can vary when the method is reused.
We have now removed the replication of the loop—it only occurs once, inside the method. Note that the method can be reused in other programs that need to read integer values. However, we should consider the possibility that the smallest value need not always be zero. Here is a better alternative: /**
Prompts a user to enter a value within a given range until the user provides a valid input. @param low the smallest allowable input @param high the largest allowable input @return the value provided by the user (between low and high, inclusive)
*/ public static int readIntBetween(int low, int high) { int input; Scanner in = new Scanner(System.in); do { System.out.print("Enter a value between " + low + " and " + high + ": "); input = in.nextInt(); } while (input < low || input > high); return input; }
5.6 Problem Solving: Reusable Methods 227
© Lawrence Sawyer/iStockphoto.
When carrying out the same task multiple times, use a method.
© Lawrence Sawyer/iStockphoto.
FULL CODE EXAMPLE
Go to wiley.com/ go/bjlo2code to © Alex Slobodkin/iStockphoto. download a complete program that demonstrates the readIntBetween method.
SELF CHECK
In our program, we call int hours = readIntBetween(0, 23);
Another program can call int month = readIntBetween(1, 12);
In general, you will want to provide parameter variables for the values that vary when a method is reused. 21.
Consider the following statements: int totalPennies = (int) Math.round(100 * total) % 100; int taxPennies = (int) Math.round(100 * (total * taxRate)) % 100;
Introduce a method to reduce code duplication. Consider this method that prints a page number on the left or right side of a page:
© Nicholas Homrich/iStockphoto.
22.
if (page % 2 == 0) { System.out.println(page); } else { System.out.println("
23.
" + page); }
Introduce a method with return type boolean to make the condition in the if statement easier to understand. Consider the following method that computes compound interest for an account with an initial balance of $10,000 and an interest rate of 5 percent: public static double balance(int years) { return 10000 * Math.pow(1.05, years); }
24.
How can you make this method more reusable? The comment explains what the following loop does. Use a method instead. // Counts the number of spaces int spaces = 0; for (int i = 0; i < input.length(); i++) { if (input.charAt(i) == ' ') { spaces++; } }
25.
Practice It
In Self Check 24, you were asked to implement a method that counts spaces. How can you generalize it so that it can count any character? Why would you want to do this?
Now you can try these exercises at the end of the chapter: R5.7, P5.9.
228 Chapter 5 Methods
Computing & Society 5.1 Personal Computing display lights. Purchasers had to sol der and assemble it, then program it in machine language through the toggle switches. It was not a big hit. The first big hit was the Apple II. It was a real computer with a keyboard, a monitor, and a floppy disk drive. When it was first released, users had a $3,000 machine that could play Space Invaders, run a primitive bookkeep ing program, or let users program it in BASIC. The original Apple II did not even support lowercase letters, mak ing it worthless for word processing. The breakthrough came in 1979 with a new spreadsheet program, VisiCalc. In a spreadsheet, you enter financial data and their relationships into a grid of rows and columns (see the figure). Then you modify some of the data and watch in real time how the others change. For example, you can see how changing the mix of widgets in a manu facturing plant might affect estimated
Reprint Courtesy of International Business Machines Corporation, © International Business Machines Corporation/IBM Corporation.
In 1971, Marcian E. “Ted” Hoff, an engi ©neer Media atBakery. Intel Corporation, was working on a chip for a manufacturer of elec tronic calculators. He realized that it would be a better idea to develop a general-purpose chip that could be programmed to interface with the keys and display of a calculator, rather than to do yet another custom design. Thus, the microprocessor was born. At the time, its primary application was as a con troller for calculators, washing machines, and the like. It took years for the computer industry to notice that a genuine central processing unit was now available as a single chip. Hobbyists were the first to catch on. In 1974 the first computer kit, the Altair 8800, was available from MITS Electronics for about $350. The kit consisted of the microprocessor, a circuit board, a very small amount of memory, toggle switches, and a row of
Reprinted Courtesy of International Businesson Machines Corporation, © International Business The VisiCalc Spreadsheet Running an Apple II Machines Corporation/IBM Corporation.
costs and profits. Corporate managers snapped up VisiCalc and the computer that was needed to run it. For them, the computer was a spreadsheet machine. More importantly, it was a personal device. The managers were free to do the calculations that they wanted to do, not just the ones that the “high priests” in the data center provided. Personal computers have been with us ever since, and countless users have tinkered with their hardware and soft ware, sometimes establishing highly successful companies or creating free software for millions of users. This “freedom to tinker” is an important part of personal computing. On a personal device, you should be able to install the software that you want to install to make you more productive or creative, even if that’s not the same software that most people use. You should be able to add peripheral equipment of your choice. For the first thirty years of personal computing, this freedom was largely taken for granted. We are now entering an era where smart phones, tablets, and smart TV sets are replacing functions that were traditionally fulfilled by personal com puters. While it is amazing to carry more computing power in your cell phone than in the best personal com puters of the 1990s, it is disturbing that we lose a degree of personal con trol. With some phone or tablet brands, you can install only those applications that the manufacturer publishes on the “app store”. For example, Apple rejected MIT’s iPad app for the edu cational language Scratch because it contained a virtual machine. You’d think it would be in Apple’s interest to encourage the next generation to be enthusiastic about programming, but they have a general policy of denying programmability on “their” devices, in order to thwart competitive environ ments such as Flash or Java. When you select a device for mak ing phone calls or watching movies, it is worth asking who is in control. Are you purchasing a personal device that you can use in any way you choose, or are you being tethered to a flow of data that is controlled by somebody else?
5.7 Problem Solving: Stepwise Refinement 229
One of the most powerful strategies for problem solving is the process of stepwise refinement. To solve a difficult task, break it down into simpler tasks. Then keep breaking down the simpler tasks into even simpler ones, until you are left with tasks that you know how to solve. Now apply this process to a problem of everyday life. You get up in the morning and simply must get coffee. How do you get cof fee? You see whether you can get someone else, such as your mother or mate, to bring you some. If that fails, you must make coffee. How do you make coffee? If there is instant
Use the process of stepwise refinement to decompose complex tasks into simpler ones.
Yes Get coffee
Make coffee
Do you No have a microwave?
Fill cup with water Put cup in microwave Heat 3 min.
A process is broken down © production AdShooter/iStockphoto. into sequences of assembly steps.
Can you No ask someone ?
Ask for coffee
Yes
© AdShooter/iStockphoto.
5.7 Problem Solving: Stepwise Refinement
Fill kettle with water
Bring to a boil
Yes
Do you have instant coffee?
No
Make instant coffee
Brew coffee
Boil water
Add water to coffee maker
Mix water and instant coffee
Add filter to coffee maker Grind coffee beans Add coffee beans to filter
Figure 5
Flowchart of Coffee-Making Solution
Turn coffee maker on
Add coffee beans to grinder Grind 60 sec.
coffee available, you can make instant coffee. How do you make instant coffee? Simply boil water and mix the boiling water with the instant coffee. How do you boil water? If there is a microwave, then you fill a cup with water, place it in the microwave and heat it for three minutes. Otherwise, you fill a kettle with water and heat it on the stove until the water comes to a boil. On the other hand, if you don’t have instant coffee, you must brew coffee. How do you brew coffee? You add water to the coffee maker, put in a filter, grind coffee, put the coffee in the filter, and turn the coffee maker on. How do you grind coffee? You add coffee beans to the coffee grinder and push the button for 60 seconds. Figure 5 shows a flowchart view of the coffee-making solution. Refinements are shown as expanding boxes. In Java, you implement a refinement as a method. For example, a method brewCoffee would call grindCoffee, and brewCoffee would be called from a method makeCoffee. Let us apply the process of stepwise refinement to a pro gramming problem. When printing a check, it is customary to write the check amount both as a number (“$274.15”) and as a text string (“two hundred seventy four dollars and 15 cents”). Doing so reduces the recipient’s temptation to add a few digits in front of the amount. For a human, this isn’t p articularly difficult, but how can a computer do this? There is no built-in method that turns 274 into "two hundred seventy four". We need to program this method. Here is the description of the method we want to write: /**
Turns a number into its English name. © YinYang/iStockphoto. @param number a positive integer < 1,000 @return the name of number (e.g., “two hundred seventy four”)
*/ public static String intName(int number) When you discover that you need a method, write a description of the parameter variables and return values.
How can this method do its job? Consider a simple case first. If the number is between 1 and 9, we need to compute "one" … "nine". In fact, we need the same computation again for the hundreds (two hundred). Any time you need something more than once, it is a good idea to turn that into a method. Rather than writing the entire method, write only the comment: /**
Turns a digit into its English name. @param digit an integer between 1 and 9 @return the name of digit (“one” . . . “nine”)
*/ public static String digitName(int digit) A method may require simpler methods to carry out its work.
Numbers between 10 and 19 are special cases. Let’s have a separate method teenName that converts them into strings "eleven", "twelve", "thirteen", and so on: /**
Turns a number between 10 and 19 into its English name. @param number an integer between 10 and 19 @return the name of the number (“ten” . . . “nineteen”)
*/ public static String teenName(int number)
© YinYang/iStockphoto.
230 Chapter 5 Methods
5.7 Problem Solving: Stepwise Refinement 231
Next, suppose that the number is between 20 and 99. The name of such a number has two parts, such as "seventy four". We need a way of producing the first part, "twenty", "thirty", and so on. Again, we will put that computation into a separate method: /**
Gives the name of the tens part of a number between 20 and 99. @param number an integer between 20 and 99 @return the name of the tens part of the number (“twenty” . . . “ninety”)
*/ public static String tensName(int number)
Now let us write the pseudocode for the intName method. If the number is between 100 and 999, then we show a digit and the word "hundred" (such as "two hundred"). We then remove the hundreds, for example reducing 274 to 74. Next, suppose the remaining part is at least 20 and at most 99. If the number is evenly divisible by 10, we use tensName, and we are done. Otherwise, we print the tens with tensName (such as "seventy") and remove the tens, reducing 74 to 4. In a separate branch, we deal with numbers that are at between 10 and 19. Finally, we print any remaining single digit (such as "four").
intName(number) part = number // The part that still needs to be converted name = "" If part >= 100 name = name of hundreds in part + " hundred" Remove hundreds from part. If part >= 20 Append tensName(part) to name. Remove tens from part. Else if part >= 10 Append teenName(part) to name. part = 0 If (part > 0) Append digitName(part) to name. Translating the pseudocode into Java is straightforward. The result is shown in the source listing at the end of this section. Note how we rely on helper methods to do much of the detail work. Using the process of stepwise refinement, we now need to consider these helper methods. Let’s start with the digitName method. This method is so simple to implement that pseudocode is not really required. Simply use an if statement with nine branches: public static String digitName(int digit) { if (digit == 1) { return "one" }; if (digit == 2) { return "two" }; . . . }
The teenName and tensName methods are similar.
232 Chapter 5 Methods
This concludes the process of stepwise refinement. Here is the complete program: sec07/IntegerName.java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55
import java.util.Scanner; /**
This program turns an integer into its English name.
*/ public class IntegerName { public static void main(String[] args) { Scanner in = new Scanner(System.in); System.out.print("Please enter a positive integer < 1000: "); int input = in.nextInt(); System.out.println(intName(input)); } /**
Turns a number into its English name. @param number a positive integer < 1,000 @return the name of the number (e.g. “two hundred seventy four”)
*/ public static String intName(int number) { int part = number; // The part that still needs to be converted String name = ""; // The name of the number if (part >= 100) { name = digitName(part / 100) + " hundred"; part = part % 100; } if (part >= 20) { name = name + part = part % } else if (part >= { name = name + part = 0; }
" " + tensName(part); 10; 10) " " + teenName(part);
if (part > 0) { name = name + " " + digitName(part); } return name; } /**
*/
Turns a digit into its English name. @param digit an integer between 1 and 9 @return the name of digit (“one” . . . “nine”)
5.7 Problem Solving: Stepwise Refinement 233 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107
public static String { if (digit == 1) { if (digit == 2) { if (digit == 3) { if (digit == 4) { if (digit == 5) { if (digit == 6) { if (digit == 7) { if (digit == 8) { if (digit == 9) { return ""; }
digitName(int digit) return return return return return return return return return
"one"; } "two"; } "three"; } "four"; } "five"; } "six"; } "seven"; } "eight"; } "nine"; }
/**
Turns a number between 10 and 19 into its English name. @param number an integer between 10 and 19 @return the name of the given number (“ten” . . . “nineteen”)
*/ public static { if (number if (number if (number if (number if (number if (number if (number if (number if (number if (number return ""; }
String teenName(int number) == == == == == == == == == ==
10) 11) 12) 13) 14) 15) 16) 17) 18) 19)
{ { { { { { { { { {
return return return return return return return return return return
"ten"; } "eleven"; } "twelve"; } "thirteen"; } "fourteen"; } "fifteen"; } "sixteen"; } "seventeen"; } "eighteen"; } "nineteen"; }
/**
Gives the name of the tens part of a number between 20 and 99. @param number an integer between 20 and 99 @return the name of the tens part of the number (“twenty” . . . “ninety”)
*/ public static { if (number if (number if (number if (number if (number if (number if (number if (number return ""; }
String tensName(int number) >= >= >= >= >= >= >= >=
90) 80) 70) 60) 50) 40) 30) 20)
{ { { { { { { {
return return return return return return return return
"ninety"; } "eighty"; } "seventy"; } "sixty"; } "fifty"; } "forty"; } "thirty"; } "twenty"; }
}
Program Run Please enter a positive integer < 1000: 729 seven hundred twenty nine
234 Chapter 5 Methods
Explain how you can improve the intName method so that it can handle arguments up to 9999. SELF CHECK 27. Why does line 40 set part = 0? 28. What happens when you call intName(0)? How can you change the intName method to handle this case correctly? © Nicholas Homrich/iStockphoto. 29. Trace the method call intName(72), as described in Programming Tip 5.4. 30. Use the process of stepwise refinement to break down the task of printing the following table into simpler tasks. 26.
+-----+-----------+ | i | i * i * i | +-----+-----------+ | 1 | 1 | | 2 | 8 | . . . . . . | 20 | 8000 | +-----+-----------+
Practice It
Programming Tip 5.3
© Eric Isselé/iStockphoto.
Programming Tip 5.4
Now you can try these exercises at the end of the chapter: R5.12, P5.3, P5.21.
Keep Methods Short There is a certain cost for writing a method. You need to design, code, and test the method. The method needs to be documented. You need to spend some effort to make the method reusable rather than tied to a specific context. To avoid this cost, it is always tempting just to stuff more and more code in one place rather than going through the trouble of breaking up the code into separate methods. It is quite common to see inexperienced programmers pro duce methods that are several hundred lines long. As a rule of thumb, a method that is so long that its code will not fit on a single screen in your development environment should probably be broken up.
Tracing Methods When you design a complex method, it is a good idea to carry out a manual walkthrough before entrusting your program to the computer. Take an index card, or some other piece of paper, and write down the method call that you want to study. Write the name of the method and the names and values of the parameter variables, like this:
© Eric Isselé/iStockphoto.
intName(number = 416)
Then write the names and initial values of the method variables. Write them in a table, because you will update them as you walk through the code.
intName(number = 416) part name 416 ""
5.7 Problem Solving: Stepwise Refinement 235 We enter the test part >= 100. part / 100 is 4 and part % 100 is 16. digitName(4) is easily seen to be "four". (Had digitName been complicated, you would have started another sheet of paper to figure out that method call. It is quite common to accumulate several sheets in this way.) Now name has changed to name + " " + digitName(part / 100) + " hundred", that is "four hundred", and part has changed to part % 100, or 16.
intName(number = 416) part name 416 "" 16 "four hundred"
Now you enter the branch part >= 10. teenName(16) is sixteen, so the variables now have the values
intName(number = 416) part name 416 "" 16 "four hundred" 0 "four hundred sixteen" Now it becomes clear why you need to set part to 0 in line 40. Otherwise, you would enter the next branch and the result would be "four hundred sixteen six". Tracing the code is an effective way to understand the subtle aspects of a method.
© Eric Isselé/iStockphoto.
Stubs When writing a larger program, it is not always feasible to implement and test all methods at once. You often need to test a method that calls another, but the other method hasn’t yet been implemented. Then you can temporarily replace the missing method with a stub. A stub is a method that returns a simple value that is sufficient for testing another method. Here are examples of stub methods: /**
Turns a digit into its English name. @param digit an integer between 1 and 9 Stubs are incomplete methods that can @return the name of digit (“one” . . . nine”) be used for testing. © lillisphotography/iStockphoto.
*/ public static String digitName(int digit) { return "mumble"; } /**
Gives the name of the tens part of a number between 20 and 99. @param number an integer between 20 and 99 @return the tens name of the number (“twenty” . . . “ninety”)
© lillisphotography/iStockphoto.
Programming Tip 5.5
236 Chapter 5 Methods */ public static String tensName(int number) { return "mumblety"; }
If you combine these stubs with the intName method and test it with an argument of 274, you will get a result of "mumble hundred mumblety mumble", which indicates that the basic logic of the intName method is working correctly.
Calculating a Course Grade
Learn how to use stepwise refinement to solve the problem of converting a set of letter grades into an average grade for a course. Go to wiley.com/go/bjlo2examples and download Worked Example 5.2. © Tom Horyn/iStockphoto.
5.8 Variable Scope
The scope of a variable is the part of the program in which it is visible.
© Paul Kline/iStockphoto.
As your programs get larger and contain more variables, you may encounter problems where you cannot access a variable that is defined in a different part of your program, or where two variable definitions conflict with each other. In order to resolve these problems, you need to be familiar with the concept of variable scope. The scope of a variable is the part of the program in which you can access it. For example, the scope of a method’s parameter variable is the entire method. In the following code segment, the scope of the parameter variable sideLength is the entire cubeVolume method but not the main method. public static void main(String[] args) { System.out.println(cubeVolume(10)); } public static double cubeVolume(double sideLength) { return sideLength * sideLength * sideLength; }
A variable that is defined within a method is called a local variable. When a local variable is declared in a block, its scope ranges from its declaration until the end of the block. For example, in the code segment below, the scope of the square variable is highlighted. public static void main(String[] args) { int sum = 0; for (int i = 1; i = 2.5) { return "B-"; } if (x >= 2.15) { return "C+"; } if (x >= 1.85) { return "C"; } if (x >= 1.5) { return "C-"; } if (x >= 1.15) { return "D+"; } if (x >= 0.85) { return "D"; } if (x >= 0.5) { return "D-"; } return "F"; }
Exercise P5.10 suggests an alternate approach. Step 4
Find the minimum of four numbers. Finally, how do we find the smallest of four numbers? The Math class has a method min that computes the minimum of two values, but there is no standard method for computing the
Big Java, Late Objects, 2e, Cay Horstmann, © 2017 John Wiley & Sons, Inc. All rights reserved.
Calculating a Course Grade WE9 minimum of four values. We will write such a method, using the mathematical fact that min(a, b, c, d) = min(min(a, b), min(c, d)). /**
Returns the smallest of four numbers. @param a a number @param b a number @param c a number @param d a number @return the smallest of a, b, c, and d
*/ public static double min4(double a, double b, double c, double d) { return Math.min(Math.min(a, b), Math.min(c, d)); }
Step 5
Process a line. As previously described, to process a line, we follow these steps:
Read in the four input strings. Convert grades to numbers. Compute the average after dropping the lowest grade. Print the grade corresponding to that average. However, if we read the first input string and find a Q, we need to signal to the caller that we have reached the end of the input set and that no further calls should be made. Our method will return a boolean value, true if it was successful, false if it encountered the sentinel. /**
Processes one line of input. @return true if the sentinel was not encountered
*/ public static boolean processLine() { System.out.print("Enter four grades or Q to quit: "); Scanner in = new Scanner(System.in); //
Read four grades
String g1 = in.next(); if (g1.equals("Q")) { return false; } String g2 = in.next(); String g3 = in.next(); String g4 = in.next(); //
Compute and print their average
double x1 = gradeToNumber(g1); double x2 = gradeToNumber(g2); double x3 = gradeToNumber(g3); double x4 = gradeToNumber(g4); double xlow = min4(x1, x2, x3, x4); double avg = (x1 + x2 + x3 + x4 - xlow) / 3; System.out.println(numberToGrade(avg)); return true; }
Big Java, Late Objects, 2e, Cay Horstmann, © 2017 John Wiley & Sons, Inc. All rights reserved.
WE10 Chapter 5 Methods Step 6
Write a class and the main method. The main method is now utterly trivial. We keep calling processLine while it returns true. public static void main(String[] args) { while (processLine()) { } }
We place all methods into a class Grades. See worked_example_2/Grades.java in your source code for the complete program.
Big Java, Late Objects, 2e, Cay Horstmann, © 2017 John Wiley & Sons, Inc. All rights reserved.
CHAPTER
6
A R R AY S A N D A R R AY L I S T S CHAPTER GOALS To collect elements using arrays and array lists
© traveler1116/iStockphoto. © traveler1116/iStockphoto.
To use the enhanced for loop for traversing arrays and array lists To learn common algorithms for processing arrays and array lists To work with two-dimensional arrays
CHAPTER CONTENTS 6.1 ARRAYS 262 CE 1 Bounds Errors 267
6.6 PROBLEM SOLVING: DISCOVERING ALGORITHMS BY MANIPULATING PHYSICAL OBJECTS 291
CE 2 Uninitialized Arrays 267
VE 1 Removing Duplicates from an Array
PT 1 Use Arrays for Sequences of
6.7 TWO-DIMENSIONAL ARRAYS 294
SYN Arrays 263
Related Items 268 C&S Computer Viruses 268
© Alex Slobodkin/iStockp
SYN Two-Dimensional Array Declaration 295 WE 2 A World Population Table
6.2 THE ENHANCED FOR LOOP 269
© Alex Slobodkin/iStockphoto. ST 4 Two-Dimensional Arrays with Variable
SYN The Enhanced for Loop 270
Row Lengths 300 ST 5 Multidimensional Arrays 301
6.3 COMMON ARRAY ALGORITHMS 270 CE 3 Underestimating the Size of a
6.8 ARRAY LISTS 301 SYN Array Lists 302
Data Set 279 ST 1 Sorting with the Java Library 279
CE 4 Length and Size 311
ST 2 Binary Search 279
ST 6 The Diamond Syntax 311
6.4 USING ARRAYS WITH METHODS 280
VE 2 Game of Life © Alex Slobodkin/iStockphoto.
ST 3 Methods with a Variable Number of
Parameters 284 6.5 PROBLEM SOLVING: ADAPTING ALGORITHMS 284 PT 2 Reading Exception Reports 286 HT 1 Working with Arrays 287 WE 1 Rolling the Dice © Alex Slobodkin/iStockphoto.
261
In many programs, you need to collect large numbers of values. In Java, you use the array and array list constructs for this purpose. Arrays have a more concise syntax, whereas array lists can automatically grow to any desired size. In this chapter, you will learn about arrays, array lists, and common algorithms for processing them.
© traveler1116/iStockphoto.
6.1 Arrays We start this chapter by introducing the array data type. Arrays are the fundamental mechanism in Java for collecting multiple values. In the following sections, you will learn how to declare arrays and how to access array elements.
6.1.1 Declaring and Using Arrays Suppose you write a program that reads a sequence of values and prints out the sequence, marking the largest value, like this: 32 54 67.5 29 35 80 115 largest) { largest = values[i]; } }
Note that the loop starts at 1 because we initialize largest with values[0]. To compute the smallest element, reverse the comparison. These algorithms require that the array contain at least one element.
6.3.4 Element Separators When separating elements, don’t place a separator before the first element.
When you display the elements of an array, you usually want to separate them, often with commas or vertical lines, like this: 32 | 54 | 67.5 | 29 | 35
© trutenka/iStockphoto.
Note that there is one fewer separator than there are numbers. Print the separator before each element in the sequence except the initial one (with index 0) like this:
To print five © trutenka/iStockphoto. elements, you need four separators.
for (int i = 0; i < values.length; i++) { if (i > 0) { System.out.print(" | "); } System.out.print(values[i]); }
If you want comma separators, you can use the expression
Arrays.toString
Arrays.toString(values)
returns a string describing the contents of the array values in the form [32, 54, 67.5, 29, 35]
method. The
272 Chapter 6 Arrays and Array Lists
The elements are surrounded by a pair of brackets and separated by commas. This method can be convenient for debugging: System.out.println("values=" + Arrays.toString(values));
© yekorzh/iStockphoto.
6.3.5 Linear Search You often need to search for the position of a specific element in an array so that you can replace or remove it. Visit all elements until you have found a match or you have come to the end of the array. Here we search for the position of the first element in an array that is equal to 100: © yekorzh/iStockphoto.
To search for a specific element, visit the elements and stop when you encounter the match.
A linear search inspects elements in sequence until a match is found.
int searchedValue = 100; int pos = 0; boolean found = false; while (pos < values.length && !found) { if (values[pos] == searchedValue) { found = true; } else { pos++; } } if (found) { System.out.println("Found at position: " + pos); } else { System.out.println("Not found"); }
This algorithm is called linear search or sequential search because you inspect the elements in sequence. If the array is sorted, you can use the more efficient binary search algorithm—see Special Topic 6.2 on page 279.
6.3.6 Removing an Element Suppose you want to remove the element with index pos from the array values. As explained in Section 6.1.3, you need a companion variable for tracking the number of elements in the array. In this example, we use a companion variable called currentSize. If the elements in the array are not in any particular order, simply overwrite the element to be removed with the last element of the array, then decrement the currentSize variable. (See Figure 4.)
currentSize
32 54 67.5 29 34.5 80 115 44.5 100 65
[0] [1] [2] . . . [pos]
Decrement after moving element [currentSize - 1]
Figure 4
Removing an Element in an Unordered Array
1 2 3 4 5
32 54 67.5 29 80 115 44.5 100 65 65
[0] [1] [2] . . . [pos]
Decrement after moving elements [currentSize - 1]
Figure 5
Removing an Element in an Ordered Array
6.3 Common Array Algorithms 273 values[pos] = values[currentSize - 1]; currentSize--;
The situation is more complex if the order of the elements matters. Then you must move all elements following the element to be removed to a lower index, and then decrement the variable holding the size of the array. (See Figure 5.) for (int i = pos + 1; i < currentSize; i++) { values[i - 1] = values[i]; } currentSize--;
6.3.7 Inserting an Element In this section, you will see how to insert an element into an array. Note that you need a companion variable for tracking the array size, as explained in Section 6.1.3. If the order of the elements does not matter, you can simply insert new elements at the end, incrementing the variable tracking the size. if (currentSize < values.length) { currentSize++; values[currentSize - 1] = newElement; }
It is more work to insert an element at a particular position in the middle of an array. First, move all elements after the insertion location to a higher index. Then insert the new element (see Figure 7). Note the order of the movement: When you remove an element, you first move the next element to a lower index, then the one after that, until you finally get to the end of the array. When you insert an element, you start at the end of the array, move that element to a higher index, then move the one before that, and so on until you finally get to the insertion location.
Before inserting an element, move elements to the end of the array starting with the last one.
if (currentSize < values.length) { currentSize++; for (int i = currentSize - 1; i > pos; i--) { values[i] = values[i - 1]; } values[pos] = newElement; }
currentSize
32 54 67.5 29 34.5 80 115 44.5 100
[0] [1] [2] . . .
Insert new element here Incremented before inserting element [currentSize - 1]
5 4 3 2 1
32 54 67.5 29 34.5 34.5 80 115 44.5 100
[0] [1] [2] . . .
Insert new element here
[pos]
Incremented before moving elements [currentSize - 1]
Figure 6
Figure 7
Inserting an Element in an Unordered Array
Inserting an Element in an Ordered Array
274 Chapter 6 Arrays and Array Lists
6.3.8 Swapping Elements You often need to swap elements of an array. For example, you can sort an array by repeatedly swapping elements that are not in order. Consider the task of swapping the elements at positions i and j of an array values. We’d like to set values[i] to values[j]. But that overwrites the value that is currently stored in values[i], so we want to save that first: Use a temporary variable when swapping two elements.
double temp = values[i]; values[i] = values[j];
To swap two elements, you need a temporary variable.
Now we can set values[j] to the saved value. values[j] = temp;
Figure 8 shows the process.
1
values =
32 54 67.5 29 34.5
[0] [1] [i] [2]
Values to be swapped
[3] [j] [4]
2
values =
32 54 67.5 29 34.5
[i] [j]
temp =
54
temp =
54
temp =
54
3
values =
32 29 67.5 29 34.5
[i] [j]
4
values =
32 29 67.5 54 34.5
Figure 8 Swapping Array Elements
[i] [j]
6.3 Common Array Algorithms 275
6.3.9 Copying Arrays Array variables do not themselves hold array elements. They hold a reference to the actual array. If you copy the reference, you get another reference to the same array (see Figure 9): double[] values = new double[6]; . . . // Fill array double[] prices = values; 1 Use the Arrays. copyOf method to copy the elements of an array into a new array.
1
If you want to make a true copy of an array, call the Arrays.copyOf method (as shown in Figure 9). double[] prices = Arrays.copyOf(values, values.length); 2
The call Arrays.copyOf(values, n) allocates an array of length n, copies the first n elements of values (or the entire values array if n > values.length) into it, and returns the new array.
After the assignment prices = values values =
2
After calling Arrays.copyOf values =
double[] prices =
double[] 32 54 67.5 29 35 47.5
32 54 67.5 29 35 47.5 prices =
double[] 32 54 67.5 29 35 47.5
Figure 9 Copying an Array Reference versus Copying an Array
In order to use the Arrays class, you need to add the following statement to the top of your program: import java.util.Arrays;
Another use for Arrays.copyOf is to grow an array that has run out of space. The following statements have the effect of doubling the length of an array (see Figure 10): double[] newValues = Arrays.copyOf(values, 2 * values.length); 1 values = newValues; 2
The copyOf method was added in Java 6. If you use Java 5, replace double[] newValues = Arrays.copyOf(values, n)
with
276 Chapter 6 Arrays and Array Lists
1
2
Move elements to a larger array values =
double[]
Store the reference to the larger array in values values =
0 0 0 0 0 0
newValues =
double[]
double[] 0 0 0 0 0 0
newValues =
0 0 0 0 0 0
double[] 0 0 0 0 0 0
Figure 10 Growing an Array double[] newValues = new double[n]; for (int i = 0; i < n && i < values.length; i++) { newValues[i] = values[i]; }
6.3.10 Reading Input If you know how many inputs the user will supply, it is simple to place them into an array: double[] inputs = new double[NUMBER_OF_INPUTS]; for (i = 0; i < inputs.length; i++) { inputs[i] = in.nextDouble(); }
However, this technique does not work if you need to read a sequence of arbitrary length. In that case, add the inputs to an array until the end of the input has been reached. int currentSize = 0; while (in.hasNextDouble() && currentSize < inputs.length) { inputs[currentSize] = in.nextDouble(); currentSize++; }
6.3 Common Array Algorithms 277
Now inputs is a partially filled array, and the companion variable currentSize is set to the number of inputs. However, this loop silently throws away inputs that don’t fit into the array. A better approach is to grow the array to hold all inputs. double[] inputs = new double[INITIAL_SIZE]; int currentSize = 0; while (in.hasNextDouble()) { // Grow the array if it has been completely filled if (currentSize >= inputs.length) { inputs = Arrays.copyOf(inputs, 2 * inputs.length); // Grow the inputs array } inputs[currentSize] = in.nextDouble(); currentSize++; }
When you are done, you can discard any excess (unfilled) elements: inputs = Arrays.copyOf(inputs, currentSize);
The following program puts these algorithms to work, solving the task that we set ourselves at the beginning of this chapter: to mark the largest value in an input sequence. sec03/LargestInArray.java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33
import java.util.Scanner; /**
This program reads a sequence of values and prints them, marking the largest value.
*/ public class LargestInArray { public static void main(String[] args) { final int LENGTH = 100; double[] values = new double[LENGTH]; int currentSize = 0; // Read inputs
System.out.println("Please enter values, Q to quit:"); Scanner in = new Scanner(System.in); while (in.hasNextDouble() && currentSize < values.length) { values[currentSize] = in.nextDouble(); currentSize++; } // Find the largest value double largest = values[0]; for (int i = 1; i < currentSize; i++) { if (values[i] > largest) { largest = values[i]; } }
278 Chapter 6 Arrays and Array Lists 34 35 36 37 38 39 40 41 42 43 44 45 46 47
// Print all values, marking the largest for (int i = 0; i < currentSize; i++) { System.out.print(values[i]); if (values[i] == largest) { System.out.print(" 0) { int other = 6 - from - to; move(disks - 1, from, other); System.out.println("Move disk from peg " + from + " to " + to); move(disks - 1, other, to); } } }
Program Run Move Move Move Move Move Move Move Move Move Move Move Move Move Move Move Move Move Move Move Move Move Move Move Move Move Move Move Move Move Move Move
disk disk disk disk disk disk disk disk disk disk disk disk disk disk disk disk disk disk disk disk disk disk disk disk disk disk disk disk disk disk disk
from from from from from from from from from from from from from from from from from from from from from from from from from from from from from from from
peg peg peg peg peg peg peg peg peg peg peg peg peg peg peg peg peg peg peg peg peg peg peg peg peg peg peg peg peg peg peg
1 1 3 1 2 2 1 1 3 3 2 3 1 1 3 1 2 2 1 2 3 3 2 2 1 1 3 1 2 2 1
to to to to to to to to to to to to to to to to to to to to to to to to to to to to to to to
3 2 2 3 1 3 3 2 2 1 1 2 3 2 2 3 1 3 3 1 2 1 1 3 3 2 2 3 1 3 3
These instructions may suffice for the priests, but unfortunately it is not easy for us to see what is going on. Let’s improve the program so that it actually carries out the instructions and shows the contents of the towers after each move. We use a class Tower that manages the disks in one tower. Each disk is represented as an integer indicating its size from 1 to n, the number of disks in the puzzle. We provide methods to remove the top disk, to add a disk to the top, and to show the contents of the tower as a list of disk sizes, for example, [5, 4, 1].
Big Java, Late Objects, 2e, Cay Horstmann, © 2017 John Wiley and Sons, Inc. All rights reserved.
Towers of Hanoi WE7 worked_example_2/Tower.java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43
import java.util.ArrayList; /**
A tower containing disks in the Towers of Hanoi puzzle.
*/ public class Tower { private ArrayList disks; /**
Constructs a tower holding a given number of disks of decreasing size. @param ndisks the number of disks
*/ public Tower(int ndisks) { disks = new ArrayList(); for (int d = ndisks; d >= 1; d--) { disks.add(d); } } /**
Removes the top disk from this tower. @return the size of the removed disk
*/ public int remove() { return disks.remove(disks.size() - 1); } /**
Adds a disk to this tower. @param size the size of the disk to add
*/ public void add(int size) { if (disks.size() > 0 && disks.get(disks.size() - 1) < size) { throw new IllegalStateException("Disk is too large"); } disks.add(size); } public String toString() { return disks.toString(); } }
A TowersOfHanoi puzzle has three towers: public class TowersOfHanoi { private Tower[] towers; public TowersOfHanoi(int ndisks) { towers = new Tower[3]; towers[0] = new Tower(ndisks); towers[1] = new Tower(0); towers[2] = new Tower(0); } . . .
Big Java, Late Objects, 2e, Cay Horstmann, © 2017 John Wiley and Sons, Inc. All rights reserved.
WE8 Chapter 13 Recursion }
Its move method first carries out the move, then prints the contents of the towers: public void move(int disks, int from, int to) { if (disks > 0) { int other = 3 - from - to; move(disks - 1, from, other); towers[to].add(towers[from].remove()); System.out.println(Arrays.toString(towers)); move(disks - 1, other, to); } }
Here, we changed the index values to 0, 1, 2. Therefore, the index of the other peg is 3 - from - to. Here is the main method: public static void main(String[] args) { final int NDISKS = 5; TowersOfHanoi towers = new TowersOfHanoi(NDISKS); towers.move(NDISKS, 0, 2); }
The program output is [[5, 4, 3, 2], [], [1]] [[5, 4, 3], [2], [1]] [[5, 4, 3], [2, 1], []] [[5, 4], [2, 1], [3]] [[5, 4, 1], [2], [3]] [[5, 4, 1], [], [3, 2]] [[5, 4], [], [3, 2, 1]] [[5], [4], [3, 2, 1]] [[5], [4, 1], [3, 2]] [[5, 2], [4, 1], [3]] [[5, 2, 1], [4], [3]] [[5, 2, 1], [4, 3], []] [[5, 2], [4, 3], [1]] [[5], [4, 3, 2], [1]] [[5], [4, 3, 2, 1], []] [[], [4, 3, 2, 1], [5]] [[1], [4, 3, 2], [5]] [[1], [4, 3], [5, 2]] [[], [4, 3], [5, 2, 1]] [[3], [4], [5, 2, 1]] [[3], [4, 1], [5, 2]] [[3, 2], [4, 1], [5]] [[3, 2, 1], [4], [5]] [[3, 2, 1], [], [5, 4]] [[3, 2], [], [5, 4, 1]] [[3], [2], [5, 4, 1]] [[3], [2, 1], [5, 4]] [[], [2, 1], [5, 4, 3]] [[1], [2], [5, 4, 3]] [[1], [], [5, 4, 3, 2]] [[], [], [5, 4, 3, 2, 1]]
That’s better. Now you can see how the disks move. You can check that all moves are legal— the disk size always decreases.
Big Java, Late Objects, 2e, Cay Horstmann, © 2017 John Wiley and Sons, Inc. All rights reserved.
Towers of Hanoi WE9 You can see that it takes 31 = 25 – 1 moves to solve the puzzle for 5 disks. With 64 disks, it takes 264 – 1 = 18446744073709551615 moves. If the priests can move one disk per second, it takes about 585 billion years to finish the job. Because the earth is about 4.5 billion years old at the time this book is written, we don’t have to worry too much whether the world will really come to an end when they are done. worked_example_2/TowersOfHanoiDemo.java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
import java.util.ArrayList; import java.util.Arrays; /**
This program shows a solution for a Towers of Hanoi puzzle.
*/ public class TowersOfHanoiDemo { public static void main(String[] args) { final int NDISKS = 5; TowersOfHanoi towers = new TowersOfHanoi(NDISKS); towers.move(NDISKS, 0, 2); } }
worked_example_2/TowersOfHanoi.java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33
import java.util.Arrays; /**
A Towers of Hanoi puzzle with three towers.
*/ public class TowersOfHanoi { private Tower[] towers; /**
Constructs a puzzle in which the first tower has a given number of disks. @param ndisks the number of disks
*/ public TowersOfHanoi(int ndisks) { towers = new Tower[3]; towers[0] = new Tower(ndisks); towers[1] = new Tower(0); towers[2] = new Tower(0); } /**
Moves a pile of disks from one peg to another and displays the movement. @param disks the number of disks to move @param from the peg from which to move the disks @param to the peg to which to move the disks
*/ public void move(int disks, int from, int to) { if (disks > 0) { int other = 3 - from - to; move(disks - 1, from, other);
Big Java, Late Objects, 2e, Cay Horstmann, © 2017 John Wiley and Sons, Inc. All rights reserved.
WE10 Chapter 13 Recursion 34 35 36 37 38 39
towers[to].add(towers[from].remove()); System.out.println(Arrays.toString(towers)); move(disks - 1, other, to); } } }
Big Java, Late Objects, 2e, Cay Horstmann, © 2017 John Wiley and Sons, Inc. All rights reserved.
CHAPTER
14
SORTING AND SEARCHING CHAPTER GOALS To study several sorting and searching algorithms © Volkan Ersoy/iStockphoto. To appreciate that algorithms for the same task can differ widely in performance © Volkan Ersoy/iStockphoto.
To understand the big-Oh notation To estimate and compare the performance of algorithms To write code to measure the running time of a program
CHAPTER CONTENTS 14.1 SELECTION SORT 650 14.2 PROFILING THE SELECTION SORT ALGORITHM 653 14.3 ANALYZING THE PERFORMANCE OF THE SELECTION SORT ALGORITHM 656 ST 1 Oh, Omega, and Theta 658 ST 2 Insertion Sort 659
14.4 MERGE SORT 661 14.5 ANALYZING THE MERGE SORT ALGORITHM 664
14.7 PROBLEM SOLVING: ESTIMATING THE RUNNING TIME OF AN ALGORITHM 673 14.8 SORTING AND SEARCHING IN THE JAVA LIBRARY 678 CE 1 The compareTo Method Can Return Any
Integer, Not Just –1, 0, and 1 680 ST 4 The Comparator Interface 680 J8 1 Comparators with Lambda
Expressions 681 WE 1 Enhancing the Insertion Sort Algorithm
© Alex Slobodkin/iSt
ST 3 The Quicksort Algorithm 666
14.6 SEARCHING 668 C&S The First Programmer 672
649
One of the most common tasks in data processing is sorting. For example, an array of employees often needs to be displayed in alphabetical order or sorted by salary. In this chapter, you will learn several sorting methods as well as techniques for comparing their performance. These tech niques are useful not just for sorting algorithms, but also for analyzing other algorithms. Once an array of elements is sorted, one can rapidly locate individual elements. You will study the binary search algorithm that carries out this fast lookup. © Volkan Ersoy/iStockphoto.
olkan Ersoy/iStockphoto.
14.1 Selection Sort In this section, we show you the first of several sorting algorithms. A sorting algorithm rearranges the elements of a collection so that they are stored in sorted order. To keep the examples simple, we will discuss how to sort an array of integers before going on to sorting strings or more complex data. Consider the following array a: [0] [1] [2] [3] [4]
11 9 17 5 12 The selection sort algorithm sorts an array by repeatedly finding the smallest element of the unsorted tail region and moving it to the front.
An obvious first step is to find the smallest element. In this case the smallest element is 5, stored in a[3]. We should move the 5 to the beginning of the array. Of course, there is already an element stored in a[0], namely 11. Therefore we cannot simply move a[3] into a[0] without moving the 11 somewhere else. We don’t yet know where the 11 should end up, but we know for certain that it should not be in a[0]. We simply get it out of the way by swapping it with a[3]: [0] [1] [2] [3] [4]
5
9 17 11 12
In selection sort, pick the smallest element and swap it with the first one. Pick the smallest element of the remaining ones and swap it with the next one, and so on.
650
© Zone Creative/iStockphoto.
Now the first element is in the correct place. The darker color in the figure indicates the portion of the array that is already sorted.
14.1 Selection Sort 651
Next we take the minimum of the remaining entries a[1] . . . a[4]. That minimum value, 9, is already in the correct place. We don’t need to do anything in this case and can simply extend the sorted area by one to the right: [0] [1] [2] [3] [4]
5
9 17 11 12
Repeat the process. The minimum value of the unsorted region is 11, which needs to be swapped with the first value of the unsorted region, 17: [0] [1] [2] [3] [4]
5
9 11 17 12
Now the unsorted region is only two elements long, but we keep to the same successful strategy. The minimum value is 12, and we swap it with the first value, 17: [0] [1] [2] [3] [4]
5
9 11 12 17
That leaves us with an unprocessed region of length 1, but of course a region of length 1 is always sorted. We are done. Let’s program this algorithm, called selection sort. For this program, as well as the other programs in this chapter, we will use a utility method to generate an array with random entries. We place it into a class ArrayUtil so that we don’t have to repeat the code in every example. To show the array, we call the static toString method of the Arrays class in the Java library and print the resulting string (see Section 6.3.4). We also add a method for swapping elements to the ArrayUtil class. (See Section 6.3.8 for details about swapping array elements.) This algorithm will sort any array of integers. If speed were not an issue, or if there were no better sorting method available, we could stop the discussion of sorting right here. As the next section shows, however, this algorithm, while entirely correct, shows disappointing performance when run on a large data set. Special Topic 14.2 discusses insertion sort, another simple sorting algorithm. sec01/SelectionSorter.java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
/**
The sort method of this class sorts an array, using the selection sort algorithm.
*/ public class SelectionSorter { /**
Sorts an array, using selection sort. @param a the array to sort
*/ public static void sort(int[] a) { for (int i = 0; i < a.length - 1; i++) { int minPos = minimumPosition(a, i); ArrayUtil.swap(a, minPos, i); } }
652 Chapter 14 Sorting and Searching 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36
/**
Finds the smallest element in a tail range of the array. @param a the array to sort @param from the first position in a to compare @return the position of the smallest element in the range a[from] . . . a[a.length - 1]
*/ private static int minimumPosition(int[] a, int from) { int minPos = from; for (int i = from + 1; i < a.length; i++) { if (a[i] < a[minPos]) { minPos = i; } } return minPos; } }
sec01/SelectionSortDemo.java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
import java.util.Arrays; /**
This program demonstrates the selection sort algorithm by sorting an array that is filled with random numbers.
*/ public class SelectionSortDemo { public static void main(String[] args) { int[] a = ArrayUtil.randomIntArray(20, 100); System.out.println(Arrays.toString(a)); SelectionSorter.sort(a); System.out.println(Arrays.toString(a)); } }
sec01/ArrayUtil.java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
import java.util.Random; /**
This class contains utility methods for array manipulation.
*/ public class ArrayUtil { private static Random generator = new Random(); /**
Creates an array filled with random values. @param length the length of the array @param n the number of possible random values @return an array filled with length numbers between 0 and n - 1
*/ public static int[] randomIntArray(int length, int n) {
14.2 Profiling the Selection Sort Algorithm 653 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40
int[] a = new int[length]; for (int i = 0; i < a.length; i++) { a[i] = generator.nextInt(n); } return a; } /**
Swaps two entries of an array. @param a the array @param i the first position to swap @param j the second position to swap
*/ public static void swap(int[] a, int i, int j) { int temp = a[i]; a[i] = a[j]; a[j] = temp; } }
Program Run [65, 46, 14, 52, 38, 2, 96, 39, 14, 33, 13, 4, 24, 99, 89, 77, 73, 87, 36, 81] [2, 4, 13, 14, 14, 24, 33, 36, 38, 39, 46, 52, 65, 73, 77, 81, 87, 89, 96, 99]
SELF CHECK
1. Why do we need the temp variable in the swap method? What would happen if you simply assigned a[i] to a[j] and a[j] to a[i]? 2. What steps does the selection sort algorithm go through to sort the sequence
6 5 4 3 2 1? 3. How can you change the selection sort algorithm so that it sorts the elements in descending order (that is, with the largest element at the beginning of the array)? 4. Suppose we modified the selection sort algorithm to start at the end of the array, working toward the beginning. In each step, the current position is swapped with the minimum. What is the result of this modification?
© Nicholas Homrich/iStockphoto.
Practice It
Now you can try these exercises at the end of the chapter: R14.2, R14.12, E14.1, E14.2.
14.2 Profiling the Selection Sort Algorithm To measure the performance of a program, you could simply run it and use a stopwatch to measure how long it takes. However, most of our programs run very quickly, and it is not easy to time them accurately in this way. Furthermore, when a program takes a noticeable time to run, a certain amount of that time may simply be used for loading the program from disk into memory and displaying the result (for which we should not penalize it). In order to measure the running time of an algorithm more accurately, we will create a StopWatch class. This class works like a real stopwatch. You can start it, stop
654 Chapter 14 Sorting and Searching
it, and read out the elapsed time. The class uses the System.currentTimeMillis method, which returns the milliseconds that have elapsed since midnight at the start of January 1, 1970. Of course, you don’t care about the absolute number of seconds since this historical moment, but the difference of two such counts gives us the number of milliseconds in a given time interval. Here is the code for the StopWatch class: sec02/StopWatch.java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
/**
A stopwatch accumulates time when it is running. You can repeatedly start and stop the stopwatch. You can use a stopwatch to measure the running time of a program.
*/ public class StopWatch { private long elapsedTime; private long startTime; private boolean isRunning; /**
Constructs a stopwatch that is in the stopped state and has no time accumulated.
*/ public StopWatch() { reset(); } /**
Starts the stopwatch. Time starts accumulating now.
*/ public void start() { if (isRunning) { return; } isRunning = true; startTime = System.currentTimeMillis(); } /**
Stops the stopwatch. Time stops accumulating and is is added to the elapsed time.
*/ public void stop() { if (!isRunning) { return; } isRunning = false; long endTime = System.currentTimeMillis(); elapsedTime = elapsedTime + endTime - startTime; } /**
Returns the total elapsed time. @return the total elapsed time
*/ public long getElapsedTime() { if (isRunning) {
14.2 Profiling the Selection Sort Algorithm 655 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68
long endTime = System.currentTimeMillis(); return elapsedTime + endTime - startTime; } else { return elapsedTime; } } /**
Stops the watch and resets the elapsed time to 0.
*/ public void reset() { elapsedTime = 0; isRunning = false; } }
Here is how to use the stopwatch to measure the sorting algorithm’s performance: sec02/SelectionSortTimer.java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
import java.util.Scanner; /**
This program measures how long it takes to sort an array of a user-specified size with the selection sort algorithm.
*/ public class SelectionSortTimer { public static void main(String[] args) { Scanner in = new Scanner(System.in); System.out.print("Enter array size: "); int n = in.nextInt(); // Construct random array
int[] a = ArrayUtil.randomIntArray(n, 100); // Use stopwatch to time selection sort StopWatch timer = new StopWatch(); timer.start(); SelectionSorter.sort(a); timer.stop(); System.out.println("Elapsed time: " + timer.getElapsedTime() + " milliseconds"); } }
Program Run Enter array size: 50000 Elapsed time: 13321 milliseconds
656 Chapter 14 Sorting and Searching 20
Time (seconds)
15
10
5
10
20
30
40
50
n
Milliseconds
10,000
786
20,000
2,148
30,000
4,796
40,000
9,192
50,000
13,321
60,000
19,299
60
n (thousands)
Figure 1 Time Taken by Selection Sort
To measure the running time of a method, get the current time immediately before and after the method call.
SELF CHECK
By starting to measure the time just before sorting, and stopping the stopwatch just after, you get the time required for the sorting process, without counting the time for input and output. The table in Figure 1 shows the results of some sample runs. These measurements were obtained with an Intel processor with a clock speed of 2 GHz, running Java 6 on the Linux operating system. On another computer the actual numbers will look different, but the relationship between the numbers will be the same. The graph in Figure 1 shows a plot of the measurements. As you can see, when you double the size of the data set, it takes about four times as long to sort it. 5. Approximately how many seconds would it take to sort a data set of 80,000
values? 6. Look at the graph in Figure 1. What mathematical shape does it resemble? © Nicholas Homrich/iStockphoto.
Practice It
Now you can try these exercises at the end of the chapter: E14.3, E14.9.
14.3 Analyzing the Performance of the Selection Sort Algorithm Let us count the number of operations that the program must carry out to sort an array with the selection sort algorithm. We don’t actually know how many machine operations are generated for each Java instruction, or which of those instructions are more time-consuming than others, but we can make a simplification. We will simply count how often an array element is visited. Each visit requires about the same amount of work by other operations, such as incrementing subscripts and comparing values. Let n be the size of the array. First, we must find the smallest of n numbers. To achieve that, we must visit n array elements. Then we swap the elements, which takes
14.3 Analyzing the Performance of the Selection Sort Algorithm 657
two visits. (You may argue that there is a certain probability that we don’t need to swap the values. That is true, and one can refine the computation to reflect that observation. As we will soon see, doing so would not affect the overall conclusion.) In the next step, we need to visit only n − 1 elements to find the minimum. In the following step, n − 2 elements are visited to find the minimum. The last step visits two elements to find the minimum. Each step requires two visits to swap the elements. Therefore, the total number of visits is n + 2 + (n − 1) + 2 + … + 2 + 2 = ( n + (n − 1) + … + 2) + (n − 1) ⋅ 2 = ( 2 + … + (n − 1) + n ) + (n − 1) ⋅ 2
=
n (n + 1) − 1 + (n − 1) ⋅ 2 2
because 1 + 2 + … + (n − 1) + n =
n (n + 1) 2
After multiplying out and collecting terms of n, we find that the number of visits is 1 n2 2
+ 5n − 3 2
We obtain a quadratic equation in n. That explains why the graph of Figure 1 looks approximately like a parabola. Now simplify the analysis further. When you plug in a large value for n (for example, 1,000 or 2,000), then 1 n 2 is 500,000 or 2,000,000. The lower term, 5 n − 3, doesn’t 2 2 contribute much at all; it is only 2,497 or 4,997, a drop in the bucket compared to the hundreds of thousands or even millions of comparisons specified by the 1 n 2 2 term. We will just ignore these lower-level terms. Next, we will ignore the constant factor 12 . We are not interested in the actual count of visits for a single n. We want to compare the ratios of counts for different values of n. For example, we can say that sorting an array of 2,000 numbers requires four times as many visits as sorting an array of 1,000 numbers:
( (
1 2 1 2
)=4 ⋅ 1000 )
⋅ 2000 2 2
1
Computer scientists use the big-Oh notation to describe the growth rate of a function.
The factor 2 cancels out in comparisons of this kind. We will simply say, “The num ber of visits is of order n2.” That way, we can easily see that the number of comparisons increases fourfold when the size of the array doubles: (2n)2 = 4n2. To indicate that the number of visits is of order n2, computer scientists often use big-Oh notation: The number of visits is O(n2). This is a convenient shorthand. (See Special Topic 14.1 for a formal definition.) To turn a polynomial expression such as 1 n2 2
+ 5n − 3 2
into big-Oh notation, simply locate the fastest-growing term, n2, and ignore its constant coefficient, no matter how large or small it may be. We observed before that the actual number of machine operations, and the actual amount of time that the computer spends on them, is approximately proportional to the number of element visits. Maybe there are about 10 machine operations
658 Chapter 14 Sorting and Searching
Selection sort is an O(n2 ) algorithm. Doubling the data set means a fourfold increase in processing time.
SELF CHECK
(increments, comparisons, memory loads, and stores) for every element visit. The number of machine operations is then approximately 10 × 1 n 2. As before, we aren’t 2 interested in the coefficient, so we can say that the number of machine operations, and hence the time spent on the sorting, is of the order n2 or O(n2). The sad fact remains that doubling the size of the array causes a fourfold increase in the time required for sorting it with selection sort. When the size of the array increases by a factor of 100, the sorting time increases by a factor of 10,000. To sort an array of a million entries (for example, to create a telephone directory), takes 10,000 times as long as sorting 10,000 entries. If 10,000 entries can be sorted in about 3/4 of a second (as in our example), then sorting one million entries requires well over two hours. We will see in the next section how one can dramatically improve the performance of the sorting process by choosing a more sophisticated algorithm. 7. If you increase the size of a data set tenfold, how much longer does it take to sort
it with the selection sort algorithm? 2 8. How large does n need to be so that 1 n is bigger than 5 n − 3? 2 2 9. Section 6.3.6 has two algorithms for removing an element from an array of © Nicholas Homrich/iStockphoto. length n. How many array visits does each algorithm require on average? 10. Describe the number of array visits in Self Check 9 using the big-Oh notation. 11. What is the big-Oh running time of checking whether an array is already sorted? 12. Consider this algorithm for sorting an array. Set k to the length of the array. Find the maximum of the first k elements. Remove it, using the second algorithm of Section 6.3.6. Decrement k and place the removed element into the kth position. Stop if k is 1. What is the algorithm’s running time in big-Oh notation? Practice It
Special Topic 14.1
Now you can try these exercises at the end of the chapter: R14.4, R14.6, R14.8.
Oh, Omega, and Theta We have used the big-Oh notation somewhat casually in this chapter to describe the growth behavior of a function. Here is the formal definition of the big-Oh notation: Suppose we have a function T(n). Usually, it represents the processing time of an algorithm for a given input of size n. But it could be any function. Also, suppose that we have another function f(n). It is usually chosen to be a simple function, such as f(n) = nk or f(n) = log(n), but it too can be any function. We write T(n) = O(f(n))
© Eric Isselé/iStockphoto.
if T(n) grows at a rate that is bounded by f(n). More formally, we require that for all n larger than some threshold, the ratio T (n) f (n) ≤ C for some constant value C. If T(n) is a polynomial of degree k in n, then one can show that T(n) = O(nk). Later in this chapter, we will encounter functions that are O(log(n)) or O(n log(n)). Some algorithms take much more time. For example, one way of sorting a sequence is to compute all of its permutations, until you find one that is in increasing order. Such an algorithm takes O(n!) time, which is very bad indeed. Table 1 shows common big-Oh expressions, sorted by increasing growth. Strictly speaking, T(n) = O( f(n)) means that T grows no faster than f. But it is permissible for T to grow much more slowly. Thus, it is technically correct to state that T(n) = n2 + 5n − 3 is O(n3) or even O(n10 ).
14.3 Analyzing the Performance of the Selection Sort Algorithm 659
Table 1 Common Big-Oh Growth Rates Big-Oh Expression
Name
O(1)
Constant
O(log(n))
Logarithmic
O(n)
Linear
O(n log(n))
Log-linear
O(n2)
Quadratic
O(n3)
Cubic
O(2n)
Exponential
O(n!)
Factorial
Computer scientists have invented additional notation to describe the growth behavior of functions more accurately. The expression T(n) = Ω(f(n)) means that T grows at least as fast as f, or, formally, that for all n larger than some threshold, the ratio T (n) f (n) ≥ C for some constant value C. (The Ω symbol is the capital Greek letter omega.) For example, T(n) = n2 + 5n − 3 is Ω(n2) or even Ω(n). The expression T(n) = Θ(f(n)) means that T and f grow at the same rate—that is, both T(n) = O( f(n)) and T(n) = Ω(f(n)) hold. (The Θ symbol is the capital Greek letter theta.) The Θ notation gives the most precise description of growth behavior. For example, T(n) = n2 + 5n − 3 is Θ(n2) but not Θ(n) or Θ(n3). The notations are very important for the precise analysis of algorithms. However, in casual conversation it is common to stick with big-Oh, while still giving an estimate as good as one can make.
Special Topic 14.2
Insertion Sort Insertion sort is another simple sorting algorithm. In this algorithm, we assume that the initial sequence a[0] a[1] ... a[k]
of an array is already sorted. (When the algorithm starts, we set k to 0.) We enlarge the initial sequence by inserting the next array element, a[k + 1], at the proper location. When we reach the end of the array, the sorting process is complete. For example, suppose we start with the array © Eric Isselé/iStockphoto.
11 9 16 5
7
Of course, the initial sequence of length 1 is already sorted. We now add a[1], which has the value 9. The element needs to be inserted before the element 11. The result is 9 11 16 5
7
660 Chapter 14 Sorting and Searching Next, we add a[2], which has the value 16. This element does not have to be moved. 9 11 16 5
7
We repeat the process, inserting a[3] or 5 at the very beginning of the initial sequence. 5
9 11 16 7
Finally, a[4] or 7 is inserted in its correct position, and the sorting is completed. The following class implements the insertion sort algorithm: public class InsertionSorter { /**
Sorts an array, using insertion sort. @param a the array to sort
*/ public static void sort(int[] a) { for (int i = 1; i < a.length; i++) { int next = a[i]; // Move all larger elements up int j = i; while (j > 0 && a[j - 1] > next) { a[j] = a[j - 1]; j--; } // Insert the element a[j] = next; } } }
How efficient is this algorithm? Let n denote the size of the array. We carry out n − 1 iterations. In the kth iteration, we have a sequence of k elements that is already sorted, and we need to insert a new element into the sequence. For each insertion, we need to visit the elements of the initial sequence until we have found the location in which the new element can be inserted. Then we need to move up the remaining elements of the sequence. Thus, k + 1 array elements are visited. Therefore, the total number of visits is 2 + 3 + …+ n =
Go to wiley.com/go/ bjlo2code to down © Alex Slobodkin/iStockphoto. load a program that illustrates sorting with insertion sort.
We conclude that insertion sort is an O(n2) algorithm, on the same Insertion sort is an order of efficiency as selection sort. O(n2 ) algorithm. Insertion sort has a desirable property: Its performance is O(n) if the array is already sorted—see Exercise R14.19. This is a useful property in practical applications, in which data sets are often partially sorted. © Kirby Hamilton/iStockphoto.
FULL CODE EXAMPLE
n (n + 1) −1 2
Insertion sort is the method that many people use to sort playing cards. Pick up one card at a time and insert it so that the cards stay sorted.
© Kirby Hamilton/iStockphoto.
14.4 Merge Sort 661
14.4 Merge Sort In this section, you will learn about the merge sort algorithm, a much more efficient algorithm than selection sort. The basic idea behind merge sort is very simple. Suppose we have an array of 10 integers. Let us engage in a bit of wishful thinking and hope that the first half of the array is already perfectly sorted, and the second half is too, like this: 5
9 10 12 17
1
8 11 20 32
The merge sort algorithm sorts an array by cutting the array in half, recursively sorting each half, and then merging the sorted halves.
5
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8 11 20 32
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In fact, you may have performed this merging before if you and a friend had to sort a pile of papers. You and the friend split the pile in half, each of you sorted your half, and then you merged the results together. That is all well and good, but it doesn’t seem to solve the problem for the computer. It still must sort the first and second halves of the array, because it can’t very well ask a few buddies to pitch in. As it turns out, though, if the computer keeps dividing the array into smaller and smaller subarrays, sorting each half and merging them back together, it carries out dramatically fewer steps than the selection sort requires. Let’s write a MergeSorter class that implements this idea. When the MergeSorter sorts an array, it makes two arrays, each half the size of the original, and sorts them recursively. Then it merges the two sorted arrays together: In merge sort, one sorts © Rich Legg/iStockphoto.
each half, then merges public static void sort(int[] a) the sorted halves. { if (a.length pivot
≥ pivot
[j]
[to]
Now swap the values in positions i and j, increasing both areas once more. Keep going while i < j. Here is the code for the partition method: private static int partition(int[] a, int from, int to) { int pivot = a[from]; int i = from - 1; int j = to + 1; while (i < j) { i++; while (a[i] < pivot) { i++; } j--; while (a[j] > pivot) { j--; } if (i < j) { ArrayUtil.swap(a, i, j); } } return j; }
Go to wiley.com/go/ bjlo2code to down© Alex Slobodkin/iStockphoto. load a program that demonstrates the quicksort algorithm.
© Christopher Futcher/iStockphoto.
FULL CODE EXAMPLE
On average, the quicksort algorithm is an O(n log(n)) algorithm. There is just one unfortunate aspect to the quicksort algorithm. Its worst-case run-time behavior is O(n2). Moreover, if the pivot element is chosen as the first element of the region, that worst-case behavior occurs when the input set is already sorted—a common situation in practice. By selecting the pivot element more cleverly, we can make it extremely unlikely for the worst-case behavior to occur. Such “tuned” quicksort algorithms are commonly used because their performance is generally excellent. For example, the sort method in the Arrays class uses a quicksort algorithm. Another improvement that is commonly made in practice is to switch to insertion sort when the array is short, because the total number of operations using insertion sort is lower for short arrays. The Java library makes that switch if the array length is less than seven.
In quicksort, one partitions the elements into two groups, holding the smaller and larger elements. Then one sorts each group. © Christopher Futcher/iStockphoto.
668 Chapter 14 Sorting and Searching
14.6 Searching Searching for an element in an array is an extremely common task. As with sorting, the right choice of algorithms can make a big difference.
14.6.1 Linear Search
A linear search examines all values in an array until it finds a match or reaches the end.
A linear search locates a value in an array in O(n) steps.
Suppose you need to find your friend’s telephone number. You look up the friend’s name in the telephone book, and naturally you can find it quickly, because the telephone book is sorted alphabetically. Now suppose you have a telephone number and you must know to what party it belongs. You could of course call that number, but suppose nobody picks up on the other end. You could look through the telephone book, a number at a time, until you find the number. That would obviously be a tremendous amount of work, and you would have to be desperate to attempt it. This thought experiment shows the difference between a search through an unsorted data set and a search through a sorted data set. The following two sections will analyze the difference formally. If you want to find a number in a sequence of values that occur in arbitrary order, there is nothing you can do to speed up the search. You must simply look through all elements until you have found a match or until you reach the end. This is called a linear or sequential search. How long does a linear search take? If we assume that the element v is present in the array a, then the average search visits n/2 elements, where n is the length of the array. If it is not present, then all n elements must be inspected to verify the absence. Either way, a linear search is an O(n) algorithm. Here is a class that performs linear searches through an array a of integers. When searching for a value, the search method returns the first index of the match, or -1 if the value does not occur in a. sec06_01/LinearSearcher.java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
/**
A class for executing linear searches in an array.
*/ public class LinearSearcher { /**
Finds a value in an array, using the linear search algorithm. @param a the array to search @param value the value to find @return the index at which the value occurs, or -1 if it does not occur in the array
*/ public static int search(int[] a, int value) { for (int i = 0; i < a.length; i++) { if (a[i] == value) { return i; } } return -1;
14.6 Searching 669 21 22
} }
sec06_01/LinearSearchDemo.java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
import java.util.Arrays; import java.util.Scanner; /**
This program demonstrates the linear search algorithm.
*/ public class LinearSearchDemo { public static void main(String[] args) { int[] a = ArrayUtil.randomIntArray(20, 100); System.out.println(Arrays.toString(a)); Scanner in = new Scanner(System.in);
boolean done = false; while (!done) { System.out.print("Enter number to search for, -1 to quit: "); int n = in.nextInt(); if (n == -1) { done = true; } else { int pos = LinearSearcher.search(a, n); System.out.println("Found in position " + pos); } } } }
Program Run [46, 99, 45, 57, 64, 95, 81, 69, 11, 97, 6, 85, 61, 88, 29, 65, 83, 88, 45, 88] Enter number to search for, -1 to quit: 12 Found in position -1 Enter number to search for, -1 to quit: -1
14.6.2 Binary Search Now let us search for an item in a data sequence that has been previously sorted. Of course, we could still do a linear search, but it turns out we can do much better than that. Consider the following sorted array a. The data set is: [0] [1] [2] [3] [4] [5] [6] [7] [8] [9]
1
4
5
8
9 12 17 20 24 32
We would like to see whether the value 15 is in the data set. Let’s narrow our search by finding whether the value is in the first or second half of the array. The last value
670 Chapter 14 Sorting and Searching
in the first half of the data set, a[4], is 9, which is smaller than the value we are looking for. Hence, we should look in the second half of the array for a match, that is, in the sequence: [0] [1] [2] [3] [4] [5] [6] [7] [8] [9]
1
4
5
8
9 12 17 20 24 32
The middle element of this sequence is 20; hence, the value must be located in the sequence: [0] [1] [2] [3] [4] [5] [6] [7] [8] [9]
1
4
5
8
9 12 17 20 24 32
The last value of the first half of this very short sequence is 12, which is smaller than the value that we are searching, so we must look in the second half: [0] [1] [2] [3] [4] [5] [6] [7] [8] [9]
1 A binary search locates a value in a sorted array by determining whether the value occurs in the first or second half, then repeating the search in one of the halves.
4
5
8
9 12 17 20 24 32
It is trivial to see that we don’t have a match, because 15 ≠ 17. If we wanted to insert 15 into the sequence, we would need to insert it just before a[6]. This search process is called a binary search, because we cut the size of the search in half in each step. That cutting in half works only because we know that the sequence of values is sorted. The following class implements binary searches in a sorted array of integers. The search method returns the position of the match if the search succeeds, or –1 if the value is not found in a. Here, we show a recursive version of the binary search algorithm. sec06_02/BinarySearcher.java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
/**
A class for executing binary searches in an array.
*/ public class BinarySearcher { /**
Finds a value in a range of a sorted array, using the binary search algorithm. @param a the array in which to search @param low the low index of the range @param high the high index of the range @param value the value to find @return the index at which the value occurs, or -1 if it does not occur in the array
*/ public int search(int[] a, int low, int high, int value) { if (low v.length() - w.length());
There is a convenient shortcut for this case. Note that the comparison depends on a function that maps each string to a numeric value, namely its length. The static method Comparator.comparing constructs a comparator from a lambda expression. For example, you can call Arrays.sort(words, Comparator.comparing(w -> w.length()));
A comparator is constructed that calls the supplied function on both objects that are to be compared, and then compares the function results. The Comparator.comparing method takes care of many common cases. For example, to sort countries by area, call Arrays.sort(countries, Comparator.comparing(c -> c.getArea()));
W or ked Ex ample 14.1 © Alex Slobodkin/iStockphoto.
© Tom Horyn/iStockphoto.
Enhancing the Insertion Sort Algorithm
Learn how to implement an improvement of the insertion sort algorithm shown in Special Topic 14.2. The enhanced algorithm is called Shell sort after its inventor, Donald Shell. Go to wiley.com/go/bjlo2examples and download Worked Example 14.1.
682 Chapter 14 Sorting and Searching CHAPTER SUMMARY Describe the selection sort algorithm.
• The selection sort algorithm sorts an array by repeatedly finding the smallest element of the unsorted tail region and moving it to the front.
© Zone Creative/iStockphoto. Measure the running
time of a method.
• To measure the running time of a method, get the current time immediately before and after the method call. Use the big-Oh notation to describe the running time of an algorithm.
• Computer scientists use the big-Oh notation to describe the growth rate of a function. • Selection sort is an O(n2) algorithm. Doubling the data set means a fourfold increase in processing time. • Insertion sort is an O(n2) algorithm. Describe the merge sort algorithm.
© Kirby Hamilton/iStockphoto.
• The merge sort algorithm sorts an array by cutting the array in half, recursively sorting each half, and then merging the sorted halves.
Contrast the running © Rich Legg/iStockphoto.
times of the merge sort and selection sort algorithms.
• Merge sort is an O(n log(n)) algorithm. The n log(n) function grows much more slowly than n2. Describe the running times of the linear search algorithm and the binary search algorithm.
• A linear search examines all values in an array until it finds a match or reaches the end. • A linear search locates a value in an array in O(n) steps. • A binary search locates a value in a sorted array by determining whether the value occurs in the first or second half, then repeating the search in one of the halves. • A binary search locates a value in a sorted array in O(log(n)) steps. Practice developing big-Oh estimates of algorithms. 1 • A loop with n iterations has O(n) running time if each step consists of a fixed number of actions. • A loop with n iterations has O(n2) running time if each step takes O(n) time. Lightbulbs: © Kraska/Shutterstock. • The big-Oh running time for doing several steps in a row is the largest of the big-Oh times for each step. 2
Found the value
3
Review Exercises 683
• A loop with n iterations has O(n2) running time if the ith step takes O(i) time. • An algorithm that cuts the size of work in half in each step runs in O(log(n)) time. Use the Java library methods for sorting and searching data.
• The Arrays class implements a sorting method that you should use for your Java programs. • The Collections class contains a sort method that can sort array lists. • The sort method of the Arrays class sorts objects of classes that implement the Comparable interface.
S TA N D A R D L I B R A R Y I T E M S I N T R O D U C E D I N T H I S C H A P T E R java.lang.System currentTimeMillis java.util.Arrays binarySearch sort
java.util.Collections binarySearch sort java.util.Comparator compare comparing
REVIEW EXERCISES • R14.1 What is the difference between searching and sorting? •• R14.2 Checking against off-by-one errors. When writing the selection sort algorithm of
Section 14.1, a programmer must make the usual choices of < versus n1 + n2) return select(k, n1 + n2, n). else return p. Implement this algorithm and measure how much faster it is for computing the median of a random large sequence, when compared to sorting the sequence and taking the middle element. •• P14.4 Implement the following modification of the quicksort algorithm, due to Bentley
and McIlroy. Instead of using the first element as the pivot, use an approximation of the median.
Answers to Self-Check Questions 689
If n ≤ 7, use the middle element. If n ≤ 40, use the median of the first, middle, and last element. Otherwise compute the “pseudomedian” of the nine elements a[i * (n - 1) / 8], where i ranges from 0 to 8. The pseudomedian of nine values is med(med(v0, v1, v2), med(v3, v4, v5), med(v6, v7, v8)). Compare the running time of this modification with that of the original algorithm on sequences that are nearly sorted or reverse sorted, and on sequences with many identical elements. What do you observe? ••• P14.5 Bentley and McIlroy suggest the following modification to the quicksort algorithm
when dealing with data sets that contain many repeated elements. Instead of partitioning as ≤
≥
(where ≤ denotes the elements that are ≤ the pivot), it is better to partition as
However, that is tedious to achieve directly. They recommend to partition as =
=
and then swap the two = regions into the middle. Implement this modification and check whether it improves performance on data sets with many repeated elements. • P14.6 Implement the radix sort algorithm described in Exercise R14.22 to sort arrays of
numbers between 0 and 999.
• P14.7 Implement the radix sort algorithm described in Exercise R14.22 to sort arrays of
numbers between 0 and 999. However, use a single auxiliary array, not ten.
•• P14.8 Implement the radix sort algorithm described in Exercise R14.22 to sort arbitrary int
values (positive or negative).
••• P14.9 Implement the sort method of the merge sort algorithm without recursion, where
the length of the array is an arbitrary number. Keep merging adjacent regions whose size is a power of 2, and pay special attention to the last area whose size is less.
ANSWERS TO SELF-CHECK QUESTIONS 1. Dropping the temp variable would not work.
Then a[i] and a[j] would end up being the same value. 2. 1 | 5 4 3 2 6 12|4356 123456 3. In each step, find the maximum of the remaining elements and swap it with the current element (or see Self Check 4). 4. The modified algorithm sorts the array in descending order. 5. Four times as long as 40,000 values, or about 37 seconds.
6. A parabola. 7. It takes about 100 times longer. 8. If n is 4, then 1 n 2 is 8 and 5 n − 3 is 7. 2
2
9. The first algorithm requires one visit, to
store the new element. The second algorithm requires T(p) = 2 × (n – p – 1) visits, where p is the location at which the element is removed. We don’t know where that element is, but if elements are removed at random locations, on average, half of the removals will be above the middle and half below, so we can assume an average p of n / 2 and T(n) = 2 × (n – n / 2 – 1) = n – 2.
690 Chapter 14 Sorting and Searching 10. The first algorithm is O(1), the second O(n).
18. On average, you’d make 500,000 comparisons.
11. We need to check that a[0] ≤ a[1], a[1] ≤ a[2],
19. The search method returns the index at which
and so on, visiting 2n – 2 elements. Therefore, the running time is O(n). 12. Let n be the length of the array. In the kth step, we need k visits to find the minimum. To remove it, we need an average of k – 2 visits (see Self Check 9). One additional visit is required to add it to the end. Thus, the kth step requires 2k – 1 visits. Because k goes from n to 2, the total number of visits is 2n – 1 + 2(n – 1) – 1 + ... + 2 · 3 – 1 + 2 · 2 – 1 = 2(n + (n – 1) + ... + 3 + 2 + 1 – 1) – (n – 1) = n(n + 1) – 2 – n + 1 = n2 – 3 (because 1 + 2 + 3 + ... + (n – 1) + n = n(n + 1)/2) Therefore, the total number of visits is O(n2). 13. When the preceding while loop ends, the loop condition must be false, that is, iFirst >= first.length or iSecond >= second. length (De Morgan’s Law). 14. First sort 8 7 6 5. Recursively, first sort 8 7. Recursively, first sort 8. It’s sorted. Sort 7. It’s sorted. Merge them: 7 8. Do the same with 6 5 to get 5 6. Merge them to 5 6 7 8. Do the same with 4 3 2 1: Sort 4 3 by sorting 4 and 3 and merging them to 3 4. Sort 2 1 by sorting 2 and 1 and merging them to 1 2. Merge 3 4 and 1 2 to 1 2 3 4. Finally, merge 5 6 7 8 and 1 2 3 4 to 1 2 3 4 5 6 7 8. 15. If the array size is 1, return its only element as the sum. Otherwise, recursively compute the sum of the first and second subarray and return the sum of these two values. 16. Approximately (100,000 · log(100,000)) / (50,000 · log(50,000)) = 2 · 5 / 4.7 = 2.13 times the time required for 50,000 values. That’s 2.13 · 192 milliseconds or approximately 409 milliseconds. 17.
2n log(2n) 1 + log(n) . = 2 n log(n) log(n) For n > 2, that is a value < 3.
the match occurs, not the data stored at that location. 20. You would search about 20. (The binary log of 1,024 is 10.) 21.
1
2
3
Lightbulbs: © Kraska/iStockphoto.
22. It is an O(n) algorithm.
23. It is an O(n2) algorithm—the number of visits
follows a triangle pattern. 24. Sort the array, then make a linear scan to check for adjacent duplicates. 25. It is an O(n2) algorithm—the outer and inner loops each have n iterations. 26. Because an n × n array has m = n2 elements, and the algorithm in Section 14.7.4, when applied to an array with m elements, is O(m log(m)), we have an O(n2log(n)) algorithm. Recall that log(n2) = 2 log(n), and the factor of 2 is irrelevant in the big-Oh notation. 27. The Rectangle class does not implement the Comparable interface. 28. The BankAccount class would need to implement the Comparable interface. Its compareTo method must compare the bank balances. 29. Then you know where to insert it so that the array stays sorted, and you can keep using binary search. 30. Otherwise, you would not know whether a value is present when the method returns 0.
Enhancing the Insertion Sort Algorithm WE1 W or ked Ex ample 14.1
© Tom Horyn/iStockphoto.
Enhancing the Insertion Sort Algorithm
Problem Statement Implement an improvement of the insertion sort algorithm (in Special Topic 14.2) called Shell sort after its inventor, Donald Shell. Shell sort is an enhancement of insertion sort that takes advantage of the fact that insertion sort is an O(n) algorithm if the array is already sorted. Shell sort brings parts of the array into sorted order, then runs an insertion sort over the entire array, so that the final sort doesn’t do much work.
A key step in Shell sort is to arrange the sequence into rows and columns, and then to sort each column separately. For example, if the array is 65 46 14 52 38 2 96 39 14 33 13 4 24 99 89 77 73 87 36 81
and we arrange it into four columns, we get 65 46 14 52 38 2 96 39 14 33 13 4 24 99 89 77 73 87 36 81
Now we sort each column: 14 2 13 5 24 33 14 39 38 46 36 52 65 87 89 77 73 99 96 81
Put together as a single array, we get 14 2 13 5 24 33 14 39 38 46 36 52 65 87 89 77 73 99 96 81
Note that the array isn’t completely sorted, but many of the small numbers are now in front, and many of the large numbers are in the back. We will repeat the process until the array is sorted. Each time, we use a different number of columns. Shell had originally used powers of two for the column counts. For example, on an array with 20 elements, he proposed using 16, 8, 4, 2, and finally one column. With one column, we have a plain insertion sort, so we know the array will be sorted. What is surprising is that the preceding sorts greatly speed up the process. However, better sequences have been discovered. We will use the sequence of column counts c1 = 1 c2 = 4 c3 = 13 c4 = 40 … ci + 1 = 3ci + 1 That is, for an array with 20 elements, we first do a 13-sort, then a 4-sort, and then a 1-sort. This sequence is almost as good as the best known ones, and it is easy to compute. We will not actually rearrange the array, but compute the locations of the elements of each column.
Big Java, Late Objects, 2e, Cay Horstmann, © 2017 John Wiley and Sons, Inc. All rights reserved.
WE2 Chapter 14 Sorting and Searching For example, if the number of columns c is 4, the four columns are located in the array as follows: 65
38 46
14 2
14
24 33
96 52
73 99
13 39
87 89
4
36 77
81
Note that successive column elements have distance c from another. The kth column is made up of the elements a[k], a[k + c], a[k + 2 * c], and so on. Now let’s adapt the insertion sort algorithm to sort such a column. The original algorithm was for (int i = 1; i < a.length; i++) { int next = a[i]; // Move all larger elements up int j = i; while (j > 0 && a[j - 1] > next) { a[j] = a[j - 1]; j--; } // Insert the element a[j] = next; }
The outer loop visits the elements a[1], a[2], and so on. In the kth column, the corresponding sequence is a[k + c], a[k + 2 * c], and so on. That is, the outer loop becomes for (int i = k + c; i < a.length; i = i + c)
In the inner loop, we originally visited a[j], a[j - 1], and so on. We need to change that to a[j], a[j - c], and so on. The inner loop becomes while (j >= c && a[j - c] > next) { a[j] = a[j - c]; j = j - c; }
Putting everything together, we get the following method: /**
Sorts a column, using insertion sort. @param a the array to sort @param k the index of the first element in the column @param c the gap between elements in the column
*/ public static void insertionSort(int[] a, int k, int c) { for (int i = k + c; i < a.length; i = i + c) { int next = a[i]; // Move all larger elements up int j = i; while (j >= c && a[j - c] > next) { a[j] = a[j - c]; j = j - c; }
Big Java, Late Objects, 2e, Cay Horstmann, © 2017 John Wiley and Sons, Inc. All rights reserved.
Enhancing the Insertion Sort Algorithm WE3 // Insert the a[j] = next;
element
} }
Now we are ready to implement the Shell sort algorithm. First, we need to find out how many elements we need from the sequence of column counts. We generate the sequence values until they exceed the size of the array to be sorted. ArrayList columns = new ArrayList(); int c = 1; while (c < a.length) { columns.add(c); c = 3 * c + 1; }
For each column count, we sort all columns: for (int s = columns.size() - 1; s >= 0; s--) { c = columns.get(s); for (int k = 0; k < c; k++) { insertionSort(a, k, c); } }
How good is the performance? Let’s compare with the Arrays.sort method in the Java library. int[] a = ArrayUtil.randomIntArray(n, 100); int[] a2 = Arrays.copyOf(a, a.length); StopWatch timer = new StopWatch(); timer.start(); ShellSorter.sort(a); timer.stop(); System.out.println("Elapsed time with Shell sort: " + timer.getElapsedTime() + " milliseconds"); timer.reset(); timer.start(); Arrays.sort(a2); timer.stop(); System.out.println("Elapsed time with Arrays.sort: " + timer.getElapsedTime() + " milliseconds"); if (!Arrays.equals(a, a2)) { throw new IllegalStateException("Incorrect sort result"); }
We make sure to sort the same array with both algorithms. Also, we check that the result of the Shell sort is correct by comparing it against the result of Arrays.sort. Finally, we compare with the insertion sort algorithm. The results show that Shell sort is a dramatic improvement over insertion sort: Enter array size: 1000000 Elapsed time with Shell sort: 205 milliseconds
Big Java, Late Objects, 2e, Cay Horstmann, © 2017 John Wiley and Sons, Inc. All rights reserved.
WE4 Chapter 14 Sorting and Searching Elapsed time with Arrays.sort: 101 milliseconds Elapsed time with insertion sort: 148196 milliseconds
However, quicksort (which is used in Arrays.sort) outperforms Shell sort. For this reason, Shell sort is not used in practice, but it is still an interesting algorithm that is surprisingly effective. You may also find it interesting to experiment with Shell’s original column sizes. In the sort method, simply replace c = 3 * c + 1;
with c = 2 * c;
You will find that the algorithm is about three times slower than the improved sequence. That is still much faster than plain insertion sort. You will find a program to demonstrate Shell sort and compare it to insertion sort in the ch14/worked_example_1 folder of the book’s companion code.
Big Java, Late Objects, 2e, Cay Horstmann, © 2017 John Wiley and Sons, Inc. All rights reserved.
CHAPTER
15
T H E J AVA COLLECTIONS FRAMEWORK CHAPTER GOALS
© nicholas belton/iStockphoto.
To learn how to use the collection classes supplied in the Java library To use iterators to traverse collections To choose appropriate collections for solving programming problems To study applications of stacks and queues
CHAPTER CONTENTS 15.1 AN OVERVIEW OF THE COLLECTIONS FRAMEWORK 692
15.5 STACKS, QUEUES, AND PRIORITY QUEUES 712
15.2 LINKED LISTS 695
15.6 STACK AND QUEUE APPLICATIONS 715
C&S Standardization 700
15.3 SETS 701 PT 1 Use Interface References to Manipulate
Data Structures 705
WE 2 Simulating a Queue of Waiting
Customers © Alexof Slobodkin/iStockphoto. VE 1 Building a Table Contents
ST 2 Reverse Polish Notation 723 © Alex Slobodkin/iStockphoto.
15.4 MAPS 706 J8 1 Updating Map Entries 708 HT 1 Choosing a Collection 708 WE 1 Word Frequency © Alex Slobodkin/iStockphoto. ST 1 Hash Functions 710
691
oto.
If you want to write a program that collects objects (such as the stamps to the left), you have a number of choices. Of course, you can use an array list, but computer scientists have invented other mechanisms that may be better suited for the task. In this chapter, we introduce the collection classes and interfaces that the Java library offers. You will learn how to use the Java collection classes, and how to choose the most appropriate collection type for a problem. © nicholas belton/iStockphoto.
15.1 An Overview of the Collections Framework A collection groups together elements and allows them to be retrieved later.
When you need to organize multiple objects in your program, you can place them into a collection. The ArrayList class that was introduced in Chapter 6 is one of many collection classes that the standard Java library supplies. In this chapter, you will learn about the Java collections framework, a hierarchy of interface types and classes for collecting objects. Each interface type is implemented by one or more classes (see Figure 1). At the root of the hierarchy is the Collection interface. That interface has methods for adding and removing elements, and so on. Table 1 on page 694 shows all the methods. Because all collections implement this interface, its methods are available for all collection classes. For example, the size method reports the number of elements in any collection. The List interface describes an important category of collections. In Java, a list is a collection that remembers the order of its elements (see Figure 2). The ArrayList class implements the List interface. An ArrayList is simply a class containing an array that is expanded as needed. If you are not concerned about efficiency, you can use the ArrayList class whenever you need to collect objects. However, several common operations are inefficient with array lists. In particular, if an element is added or removed, the elements at larger positions must be moved. The Java library supplies another class, LinkedList, that also implements the List interface. Unlike an array list, a linked list allows efficient insertion and removal of elements in the middle of the list. We will discuss that class in the next section.
‹‹interface›› Collection
‹‹interface›› List
ArrayList
Stack
‹‹interface›› Queue
LinkedList
PriorityQueue
‹‹interface›› Map
‹‹interface›› Set
HashSet
HashMap
TreeSet
Figure 1 Interfaces and Classes in the Java Collections Framework
692
TreeMap
Figure 2 A List of Books
© Filip Fuxa/iStockphoto.
A set is an unordered collection of unique elements.
A map keeps associations between key and value objects.
© Vladimir Trenin/iStockphoto. © Vladimir Trenin/iStockphoto.
© parema/iStockphoto.
Figure 3 A Set of Books
Figure 4 A Stack of Books
You use a list whenever you want to retain the order that you established. For example, on your bookshelf, you may order books by topic. A list is an appropriate data structure for such a collection because the ordering matters to you. However, in many applications, you don’t really care about the order of the elements in a collection. Consider a mail-order dealer of books. Without customers browsing the shelves, there is no need to order books by topic. Such a collection without an intrinsic order is called a set—see Figure 3. Because a set does not track the order of the elements, it can arrange the elements so that the operations of finding, adding, and removing elements become more efficient. Computer scientists have invented mechanisms for this purpose. The Java library provides classes that are based on two such mechanisms (called hash tables and binary search trees). You will learn in this chapter how to choose between them. Another way of gaining efficiency in a collection is to reduce the number of operations. A stack remembers the order of its elements, but it does not allow you to insert elements in every position. You can add and remove elements only at the top—see Figure 4. In a queue, you add items to one end (the tail) and remove them from the other end (the head). For example, you could keep a queue of books, adding required reading at the tail and taking a book from the head whenever you have time to read another one. A priority queue is an unordered collection that has an efficient operation for removing the element with the highest priority. You might use a priority queue for organizing your reading assignments. Whenever you have some time, remove the book with the highest priority and read it. We will discuss stacks, queues, and priority queues in Section 15.5. Finally, a map manages associations between keys and values. Every key in the map has an associated value (see Figure 5). The map stores the keys, values, and the associations between them. ISBN 978-0-470-10554-2
ISBN 978-0-470-50948-1
90000
Keys 9
90000
780470 105542
9
780470 105559
Figure 5
A Map from Bar Codes to Books
ISBN 978-0-470-38329-2
Values (books) © david franklin/iStockphoto.
90000
90000
90000
9
780470 509481
ISBN 978-0-471-79191-1
ISBN 978-0-470-10555-9
9
780471 791911
9
780470 383292
(books) © david franklin/iStockphoto.
A list is a collection that remembers the order of its elements.
© parema/iStockphoto.
© Filip Fuxa/iStockphoto.
15.1 An Overview of the Collections Framework 693
694 Chapter 15 The Java Collections Framework FULL CODE EXAMPLE
Go to wiley.com/ go/bjlo2code to © Alex Slobodkin/iStockphoto. download a sample program that dem onstrates several collection classes.
For an example, consider a library that puts a bar code on each book. The program used to check books in and out needs to look up the book associated with each bar code. A map associating bar codes with books can solve this problem. We will discuss maps in Section 15.4. Starting with this chapter, we will use the “diamond syntax” for constructing instances of generic classes (see Special Topic 6.6). For example, when constructing an array list of strings, we will use ArrayList coll = new ArrayList();
Note that there is an empty pair of brackets after new ArrayList on the right-hand side. The compiler infers from the left-hand side that an array list of strings is constructed.
Table 1 The Methods of the Collection Interface Collection coll = new ArrayList();
The ArrayList class implements the Collection interface.
coll = new TreeSet();
The TreeSet class (Section 15.3) also implements the Collection interface.
int n = coll.size();
Gets the size of the collection. n is now 0.
coll.add("Harry"); coll.add("Sally");
Adds elements to the collection.
String s = coll.toString();
Returns a string with all elements in the collection. s is now [Harry, Sally].
System.out.println(coll);
Invokes the toString method and prints [Harry,
coll.remove("Harry"); boolean b = coll.remove("Tom");
Removes an element from the collection, returning false if the element is not present. b is false.
b = coll.contains("Sally");
Checks whether this collection contains a given element. b is now true.
for (String s : coll) { System.out.println(s); }
You can use the “for each” loop with any collection. This loop prints the elements on separate lines.
Iterator iter = coll.iterator();
You use an iterator for visiting the elements in the collection (see Section 15.2.3).
SELF CHECK
Sally].
1. A grade book application stores a collection of quizzes. Should it use a list or
a set? 2. A student information system stores a collection of student records for a university. Should it use a list or a set? © Nicholas Homrich/iStockphoto. 3. Why is a queue of books a better choice than a stack for organizing your required reading? 4. As you can see from Figure 1, the Java collections framework does not consider a map a collection. Give a reason for this decision. Practice It
Now you can try these exercises at the end of the chapter: R15.1, R15.2, R15.3.
15.2 Linked Lists 695
15.2 Linked Lists A linked list is a data structure used for collecting a sequence of objects that allows efficient addition and removal of elements in the middle of the sequence. In the following sections, you will learn how a linked list manages its elements and how you can use linked lists in your programs.
A linked list consists of a number of nodes, each of which has a reference to the next node.
To understand the inefficiency of arrays and the need for a more efficient data structure, imagine a program that maintains a sequence of employee names. If an employee leaves the company, the name must be removed. In an array, the hole in the sequence needs to be closed up by moving all objects that come after it. Conversely, suppose an employee is added in the middle of the sequence. Then all names andrea laurita/iStockphoto. following the new hire must be moved ©Each node in a linked list is connected to the toward the end. Moving a large number of neighboring nodes. elements can involve a substantial amount of processing time. A linked list structure avoids this movement. A linked list uses a sequence of nodes. A node is an object that stores an element and references to the neighboring nodes in the sequence (see Figure 6).
Tom
Diana
Harry
Figure 6
A Linked List
When you insert a new node into a linked list, only the neighboring node references need to be updated (see Figure 7).
Tom
Diana
Harry
Romeo
Figure 7 Inserting a Node into a Linked List
© andrea laurita/iStockphoto.
15.2.1 The Structure of Linked Lists
696 Chapter 15 The Java Collections Framework
The same is true when you remove a node (see Figure 8). What’s the catch? Linked lists allow efficient insertion and removal, but element access can be inefficient.
Figure 8
Removing a Node from a Linked List Adding and removing elements at a given location in a linked list is efficient. Visiting the elements of a linked list in sequential order is efficient, but random access is not.
Tom
Diana
Harry
For example, suppose you want to locate the fifth element. You must first traverse the first four. This is a problem if you need to access the elements in arbitrary order. The term “random access” is used in computer science to describe an access pattern in which elements are accessed in arbitrary (not necessarily random) order. In contrast, sequential access visits the elements in sequence. Of course, if you mostly visit all elements in sequence (for example, to display or print the elements), the inefficiency of random access is not a problem. You use linked lists when you are concerned about the efficiency of inserting or removing elements and you rarely need element access in random order.
15.2.2 The LinkedList Class of the Java Collections Framework The Java library provides a LinkedList class in the java.util package. It is a generic class, just like the ArrayList class. That is, you specify the type of the list elements in angle brackets, such as LinkedList or LinkedList. Table 2 shows important methods of the LinkedList class. (Remember that the LinkedList class also inherits the methods of the Collection interface shown in Table 1.) As you can see from Table 2, there are methods for accessing the beginning and the end of the list directly. However, to visit the other elements, you need a list iterator. We discuss iterators next.
Table 2 Working with Linked Lists LinkedList list = new LinkedList();
An empty list.
list.addLast("Harry");
Adds an element to the end of the list. Same as add.
list.addFirst("Sally");
Adds an element to the beginning of the list. list is now [Sally, Harry].
list.getFirst();
Gets the element stored at the beginning of the list; here "Sally".
list.getLast();
Gets the element stored at the end of the list; here "Harry".
String removed = list.removeFirst();
Removes the first element of the list and returns it. removed is "Sally" and list is [Harry]. Use removeLast to remove the last element.
ListIterator iter = list.listIterator()
Provides an iterator for visiting all list elements (see Table 3 on page 698).
15.2 Linked Lists 697
15.2.3 List Iterators You use a list iterator to access elements inside a linked list.
An iterator encapsulates a position anywhere inside the linked list. Conceptually, you should think of the iterator as pointing between two elements, just as the cursor in a word processor points between two characters (see Figure 9). In the conceptual view, think of each element as being like a letter in a word processor, and think of the iterator as being like the blinking cursor between letters. You obtain a list iterator with the listIterator method of the LinkedList class: LinkedList employeeNames = . . .; ListIterator iterator = employeeNames.listIterator();
Note that the iterator class is also a generic type. A ListIterator iterates through a list of strings; a ListIterator visits the elements in a LinkedList. Initially, the iterator points before the first element. You can move the iterator position with the next method: iterator.next();
The next method throws a NoSuchElementException if you are already past the end of the list. You should always call the iterator’s hasNext method before calling next—it returns true if there is a next element. if (iterator.hasNext()) { iterator.next(); }
The
next method returns the element that the iterator is passing. When you use a ListIterator, the return type of the next method is String. In general, the return type of the next method matches the list iterator’s type parameter (which reflects the
type of the elements in the list). You traverse all elements in a linked list of strings with the following loop: while (iterator.hasNext()) { String name = iterator.next(); Do something with name. }
As a shorthand, if your loop simply visits all elements of the linked list, you can use the “for each” loop: for (String name : employeeNames) { Do something with name. }
Then you don’t have to worry about iterators at all. Behind the scenes, the for loop uses an iterator to visit all list elements. Initial ListIterator position
D
H
R
T
After calling next
D
H
R
T
After inserting J
D
J
R H
T R
next returns D
Figure 9
A Conceptual View of the List Iterator
T
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Table 3 Methods of the Iterator and ListIterator Interfaces String s = iter.next();
Assume that iter points to the beginning of the list [Sally] before calling next. After the call, s is "Sally" and the iterator points to the end.
iter.previous(); iter.set("Juliet");
The set method updates the last element returned by next or previous. The list is now [Juliet].
iter.hasNext()
Returns false because the iterator is at the end of the collection.
if (iter.hasPrevious()) { s = iter.previous(); }
hasPrevious returns true because the iterator is not at the beginning of the list. previous and hasPrevious are ListIterator methods.
iter.add("Diana");
Adds an element before the iterator position (ListIterator only). The list is now [Diana, Juliet].
iter.next(); iter.remove();
remove removes the last element returned by next or previous. The list is now [Diana].
The nodes of the LinkedList class store two links: one to the next element and one to the previous one. Such a list is called a doubly-linked list. You can use the previous and hasPrevious methods of the ListIterator interface to move the iterator position backward. The add method adds an object before the iterator position. iterator.add("Juliet");
You can visualize insertion to be like typing text in a word processor. Each character is inserted after the cursor, then the cursor moves past the inserted character (see Figure 9). Most people never pay much attention to this—you may want to try it out and watch carefully how your word processor inserts characters. The remove method removes the object that was returned by the last call to next or previous. For example, this loop removes all names that fulfill a certain condition: while (iterator.hasNext()) { String name = iterator.next(); if (condition is fulfilled for name) { iterator.remove(); } }
You have to be careful when calling remove. It can be called only once after calling next or previous, and you cannot call it immediately after a call to add. If you call the method improperly, it throws an IllegalStateException. Table 3 summarizes the methods of the ListIterator interface. The ListIterator interface extends a more general Iterator interface that is suitable for arbitrary collections, not just lists. The table indicates which methods are specific to list iterators. Following is a sample program that inserts strings into a list and then iterates through the list, adding and removing elements. Finally, the entire list is printed. The comments indicate the iterator position.
15.2 Linked Lists 699 sec02/ListDemo.java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39
import java.util.LinkedList; import java.util.ListIterator; /**
This program demonstrates the LinkedList class. */ public class ListDemo { public static void main(String[] args) { LinkedList staff = new LinkedList(); staff.addLast("Diana"); staff.addLast("Harry"); staff.addLast("Romeo"); staff.addLast("Tom"); // | in the comments indicates the iterator position ListIterator iterator = staff.listIterator(); // |DHRT iterator.next(); // D|HRT iterator.next(); // DH|RT // Add more elements after second element iterator.add("Juliet"); // DHJ|RT iterator.add("Nina"); // DHJN|RT iterator.next(); // DHJNR|T // Remove last traversed element iterator.remove(); // DHJN|T // Print all elements System.out.println(staff); System.out.println("Expected: [Diana, Harry, Juliet, Nina, Tom]"); } }
Program Run [Diana, Harry, Juliet, Nina, Tom] Expected: [Diana, Harry, Juliet, Nina, Tom]
SELF CHECK
5. Do linked lists take more storage space than arrays of the same size? 6. Why don’t we need iterators with arrays? 7. Suppose the list letters contains elements "A", "B", "C", and "D". Draw the con-
tents © Nicholas Homrich/iStockphoto.
of the list and the iterator position for the following operations:
ListIterator iter = letters.iterator(); iter.next(); iter.next(); iter.remove(); iter.next(); iter.add("E");
700 Chapter 15 The Java Collections Framework iter.next(); iter.add("F"); 8. Write a loop that removes all strings with length less than four from a linked list of strings called words. 9. Write a loop that prints every second element of a linked list of strings called words.
Practice It
Now you can try these exercises at the end of the chapter: R15.5, R15.8, E15.1.
Computing & Society 15.1 Standardization
© Denis Vorob’yev/iStockphoto.
You encounter the as the protocol for exchanging e-mail benefits of standard messages. The W3C standardizes the ©ization Media Bakery. every day. When you buy a light Hypertext Markup Language (HTML), bulb, you can be assured that it fits the the format for web pages. These stan socket without having to measure the dards have been instru mental in the socket at home and the light bulb in creation of the World Wide Web as an the store. In fact, you may have experi open platform that is not controlled by enced how painful the lack of stan any one company. dards can be if you have ever pur Many programming languages, chased a flashlight with nonstand ard such as C++ and Scheme, have been bulbs. Replacement bulbs for such a standardized by independent stan flashlight can be difficult and expen dards organizations, such as the sive to obtain. American National Standards Institute Programmers (ANSI) and the International Organiza have a similar desire tion for Standardization—called ISO for standardization. for short (not an acronym; see http:// www.iso.org/iso/about/discover-iso_ Consider the impor isos-name.htm). ANSI and ISO are asso tant goal of plat form independence ciations of industry professionals who for Java programs. develop standards for everything from After you compile car tires to credit card shapes to pro a Java program into gramming languages. class files, you©can Many standards are developed by Denis Vorob’yev/iStockphoto. execute the class files on any computer dedicated experts from a multitude that has a Java virtual machine. For this of vendors and users, with the objec to work, the behavior of the virtual tive of creating a set of rules that codi machine has to be strictly defined. If all fies best practices. But sometimes, virtual machines don’t behave exactly standards are very contentious. By the same way, then the slogan of “write 2005, Microsoft started losing govern once, run anywhere” turns into “write ment contracts when its customers once, debug everywhere”. In order for became concerned that many of their multiple implementors to create com documents were stored in proprietary, patible virtual machines, the virtual undocumented formats. Instead of machine needed to be standardized. supporting existing standard formats, That is, someone needed to create a or working with an industry group to definition of the virtual machine and its improve those standards, Microsoft expected behavior. wrote its own standard that simply Who creates standards? Some of the codified what its product was cur most successful standards have been rently doing, even though that format created by volunteer groups such as is widely regarded as being inconsis the Internet Engineering Task Force tent and very complex. (The descrip (IETF) and the World Wide Web Con tion of the format spans over 6,000 sortium (W3C). The IETF standardizes pages.) The company first proposed protocols used in the Internet, such its standard to the European Computer
Manufacturers Association (ECMA), which approved it with minimal discus sion. Then ISO “fast-tracked” it as an existing standard, bypassing the nor mal technical review mechanism. For similar reasons, Sun Micro systems, the inventor of Java, never agreed to have a third-party organiza tion standardize the Java language. Instead, they put in place their own standardization process, involv ing other companies but refusing to relin quish control. Of course, many important pieces of technology aren’t standardized at all. Consider the Windows operating system. Although Windows is often called a de-facto standard, it really is no standard at all. Nobody has ever attempted to define formally what the Windows operating system should do. The behavior changes at the whim of its vendor. That suits Microsoft just fine, because it makes it impossible for a third party to create its own version of Windows. As a computer professional, there will be many times in your career when you need to make a decision whether to support a particular standard. Con sider a simple example. In this chapter, you learn about the collection classes from the standard Java library. How ever, many computer scientists dislike these classes because of their numer ous design issues. Should you use the Java collections in your own code, or should you implement a better set of collections? If you do the former, you have to deal with a design that is less than optimal. If you do the latter, other programmers may have a hard time understanding your code because they aren’t familiar with your classes.
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15.3 Sets As you learned in Section 15.1, a set organizes its values in an order that is optimized for efficiency, which may not be the order in which you add elements. Inserting and removing elements is more efficient with a set than with a list. In the following sections, you will learn how to choose a set implementation and how to work with sets.
15.3.1 Choosing a Set Implementation
Set implementations arrange the elements so that they can locate them quickly.
You can form hash sets holding objects of type String, Integer, Double, Point, Rectangle, or Color.
The Set interface in the standard Java library has the same methods as the Collection interface, shown in Table 1. However, there is an essential difference between arbitrary collections and sets. A set does not admit duplicates. If you add an element to a set that is already present, the insertion is ignored. The HashSet and TreeSet classes implement the Set interface. These two classes provide set implementations based on two different mechanisms, called hash tables and binary search trees. Both implementations arrange the set elements so that finding, adding, and removing elements is efficient, but they use different strategies. The basic idea of a hash table is simple. Set elements are grouped into smaller collections of elements that share the same characteristic. You can imagine a hash set of books as having a group for each color, so that books of the same color are in the same group. To find whether a book is already present, you just need to check it against the books in the same color group. Actually, hash tables don’t use colors, but integer values (called hash codes) that can be computed from the elements. In order to use a hash table, the elements must have a method to compute those integer values. This method is called hashCode. The elements must also belong to a class with a properly defined equals method (see Section 9.5.2). Many classes in the standard library implement these methods, for example String, Integer, Double, Point, Rectangle, Color, and all the collection classes. Therefore, you can form a HashSet, HashSet, or even a HashSet. Suppose you want to form a set of elements belonging to a class that you declared, such as a HashSet. Then you need to provide hashCode and equals methods for the class Book. There is one exception to this rule. If all elements are distinct (for example, if your program never has two Book objects with the same author and title), then you can simply inherit the hashCode and equals methods of the Object class.
© Alfredo Ragazzoni/iStockphoto.
The HashSet and TreeSet classes both implement the Set interface.
On this shelf, books of the same color are grouped together. Similarly, in a hash table, objects with the same hash code are placed in the same group. © Alfredo Ragazzoni/iStockphoto.
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You can form tree sets for any class that implements the Comparable interface, such as String or Integer.
The TreeSet class uses a different strategy for arranging its ele ments. Elements are kept in sorted order. For example, a set of books might be arranged by height, or alphabetically by author and title. The elements are not stored in an array—that would make adding and removing elements too inefficient. Instead, they are stored in nodes, as in a linked list. However, the nodes © Volkan Ersoy/iStockphoto. are not arranged in a linear sequence but in a tree shape. In order to use a TreeSet, it must be possible to compare the elements and determine which one is “larger”. You can use a TreeSet for classes such as String and Integer that implement the Comparable interface, which we discussed in Section 9.6.3. (That section also shows you how you can implement comparison methods for your own classes.) As a rule of thumb, you should choose a TreeSet if you want to visit the set’s elements in sorted order. Otherwise choose a HashSet––as long as the hash function is well chosen, it is a bit more efficient. When you construct a HashSet or TreeSet, store the reference in a Set variable. For example, Set names = new HashSet();
or Set names = new TreeSet();
After you construct the collection object, the implementation no longer matters; only the interface is important.
15.3.2 Working with Sets You add and remove set elements with the add and remove methods: names.add("Romeo"); names.remove("Juliet"); Sets don’t have duplicates. Adding a duplicate of an element that is already present is ignored.
As in mathematics, a set collection in Java rejects duplicates. Adding an element has no effect if the element is already in the set. Similarly, attempting to remove an element that isn’t in the set is ignored. The contains method tests whether an element is contained in the set: if (names.contains("Juliet")) . . .
The contains method uses the equals method of the element type. If your set collects String or Integer objects, you don’t have to worry. Those classes provide an equals method. However, if you implemented the element type yourself, then you need to define the equals method––see Section 9.5.2. Finally, to list all elements in the set, get an iterator. As with list iterators, you use the next and hasNext methods to step through the set. Iterator iter = names.iterator(); while (iter.hasNext()) {
© Volkan Ersoy/iStockphoto.
A tree set keeps its elements in sorted order.
15.3 Sets 703 String name = iter.next();
}
Do something with name.
You can also use the “for each” loop instead of explicitly using an iterator: for (String name : names) { Do something with name. } A set iterator visits the elements in the order in which the set implementation keeps them.
You cannot add an element to a set at an iterator position.
A set iterator visits the elements in the order in which the set implementation keeps them. This is not necessarily the order in which you inserted them. The order of elements in a hash set seems quite random because the hash code spreads the elements into different groups. When you visit elements of a tree set, they always appear in sorted order, even if you inserted them in a different order. There is an important difference between the Iterator that you obtain from a set and the ListIterator that a list yields. The ListIterator has an add method to add an element at the list iterator position. The Iterator interface has no such method. It makes no sense to add an element at a particular position in a set, because the set can order the elements any way it likes. Thus, you always add elements directly to a set, never to an iterator of the set. However, you can remove a set element at an iterator position, just as you do with list iterators. Also, the Iterator interface has no previous method to go backward through the elements. Because the elements are not ordered, it is not meaningful to distinguish between “going forward” and “going backward”.
Table 4 Working with Sets Set names;
Use the interface type for variable declarations.
names = new HashSet();
Use a TreeSet if you need to visit the elements in sorted order.
names.add("Romeo");
Now names.size() is 1.
names.add("Fred");
Now names.size() is 2.
names.add("Romeo");
names.size() is still 2. You can’t add duplicates.
if (names.contains("Fred"))
The contains method checks whether a value is contained in the set. In this case, the method returns true.
System.out.println(names);
Prints the set in the format [Fred, Romeo]. The elements need not be shown in the order in which they were inserted.
for (String name : names) { . . . }
Use this loop to visit all elements of a set.
names.remove("Romeo");
Now names.size() is 1.
names.remove("Juliet");
It is not an error to remove an element that is not present. The method call has no effect.
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The following program shows a practical application of sets. It reads in all words from a dictionary file that contains correctly spelled words and places them in a set. It then reads all words from a document—here, the book Alice in Wonderland—into a second set. Finally, it prints all words from that set that are not in the dictionary set. These are the potential misspellings. (As you can see from the output, we used an American dictionary, and words with British spelling, such as clamour, are flagged as potential errors.) sec03/SpellCheck.java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48
import import import import import
java.util.HashSet; java.util.Scanner; java.util.Set; java.io.File; java.io.FileNotFoundException;
/**
This program checks which words in a file are not present in a dictionary.
*/ public class SpellCheck { public static void main(String[] args) throws FileNotFoundException { // Read the dictionary and the document
Set dictionaryWords = readWords("words"); Set documentWords = readWords("alice30.txt"); // Print all words that are in the document but not the dictionary for (String word : documentWords) { if (!dictionaryWords.contains(word)) { System.out.println(word); } } } /**
Reads all words from a file. @param filename the name of the file @return a set with all lowercased words in the file. Here, a word is a sequence of upper- and lowercase letters.
*/ public static Set readWords(String filename) throws FileNotFoundException { Set words = new HashSet(); Scanner in = new Scanner(new File(filename)); // Use any characters other than a-z or A-Z as delimiters in.useDelimiter("[^a-zA-Z]+"); while (in.hasNext()) { words.add(in.next().toLowerCase()); } return words;
15.3 Sets 705 49 50
} }
Program Run neighbouring croqueted pennyworth dutchess comfits xii dinn clamour ...
Arrays and lists remember the order in which you added elements; sets do not. Why would you want to use a set instead of an array or list? 11. Why are set iterators different from list iterators? 12. What is wrong with the following test to check whether the Set s con© Nicholas Homrich/iStockphoto. tains the elements "Tom", "Diana", and "Harry"?
SELF CHECK
10.
if (s.toString().equals("[Tom, Diana, Harry]")) . . . 13. 14. 15.
Practice It
Programming Tip 15.1
How can you correctly implement the test of Self Check 12? Write a loop that prints all elements that are in both Set s and Set t. Suppose you changed line 40 of the SpellCheck program to use a TreeSet instead of a HashSet. How would the output change?
Now you can try these exercises at the end of the chapter: E15.3, E15.12, E15.13.
Use Interface References to Manipulate Data Structures It is considered good style to store a reference to a HashSet or TreeSet in a variable of type Set: Set words = new HashSet();
This way, you have to change only one line if you decide to use a TreeSet instead. If a method can operate on arbitrary collections, use the Collection interface type for the parameter variable: © Eric Isselé/iStockphoto.
public static void removeLongWords(Collection words)
In theory, we should make the same recommendation for the List interface, namely to save ArrayList and LinkedList references in variables of type List. However, the List interface has get and set methods for random access, even though these methods are very inefficient for linked lists. You can’t write efficient code if you don’t know whether the methods that you are calling are efficient or not. This is plainly a serious design error in the standard library, and it makes the List interface somewhat unattractive.
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15.4 Maps The HashMap and TreeMap classes both implement the Map interface.
A map allows you to associate elements Values Keys from a key set with elements from a value Romeo collection. You use a map when you want to look up objects by using a key. For Adam example, Figure 10 shows a map from the Eve names of people to their favorite colors. Juliet Just as there are two kinds of set implementations, the Java library has two implementations for the Map interface: Figure 10 A Map HashMap and TreeMap. After constructing a HashMap or TreeMap, you can store the reference to the map object in a Map reference: Map favoriteColors = new HashMap();
Use the put method to add an association: favoriteColors.put("Juliet", Color.RED);
You can change the value of an existing association, simply by calling put again: favoriteColors.put("Juliet", Color.BLUE);
The get method returns the value associated with a key. Color julietsFavoriteColor = favoriteColors.get("Juliet");
If you ask for a key that isn’t associated with any values, the get method returns null. To remove an association, call the remove method with the key: favoriteColors.remove("Juliet");
Table 5 Working with Maps Map scores;
Keys are strings, values are Integer wrappers. Use the interface type for variable declarations.
scores = new TreeMap();
Use a HashMap if you don’t need to visit the keys in sorted order.
scores.put("Harry", 90); scores.put("Sally", 95);
Adds keys and values to the map.
scores.put("Sally", 100);
Modifies the value of an existing key.
int n = scores.get("Sally"); Integer n2 = scores.get("Diana");
Gets the value associated with a key, or null if the key is not present. n is 100, n2 is null.
System.out.println(scores);
Prints scores.toString(), a string of the form {Harry=90,
for (String key : scores.keySet()) { Integer value = scores.get(key); . . . }
Iterates through all map keys and values.
scores.remove("Sally");
Removes the key and value.
Sally=100}
15.4 Maps 707
To find all keys and values in a map, iterate through the key set and find the values that correspond to the keys.
Sometimes you want to enumerate all keys in a map. The keySet method yields the set of keys. You can then ask the key set for an iterator and get all keys. From each key, you can find the associated value with the get method. Thus, the following instructions print all key/value pairs in a map m whose keys have type String: Set keySet = m.keySet(); for (String key : keySet) { Color value = m.get(key); System.out.println(key + "->" + value); }
This sample program shows a map in action: sec04/MapDemo.java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
import import import import
java.awt.Color; java.util.HashMap; java.util.Map; java.util.Set;
/**
This program demonstrates a map that maps names to colors.
*/ public class MapDemo { public static void main(String[] args) { Map favoriteColors = new HashMap(); favoriteColors.put("Juliet", Color.BLUE); favoriteColors.put("Romeo", Color.GREEN); favoriteColors.put("Adam", Color.RED); favoriteColors.put("Eve", Color.BLUE); // Print all keys and values in the map Set keySet = favoriteColors.keySet(); for (String key : keySet) { Color value = favoriteColors.get(key); System.out.println(key + " : " + value); } } }
Program Run Juliet : java.awt.Color[r=0,g=0,b=255] Adam : java.awt.Color[r=255,g=0,b=0] Eve : java.awt.Color[r=0,g=0,b=255] Romeo : java.awt.Color[r=0,g=255,b=0]
What is the difference between a set and a map? 17. Why is the collection of the keys of a map a set and not a list? 18. Why is the collection of the values of a map not a set? © Nicholas Homrich/iStockphoto. 19. Suppose you want to track how many times each word occurs in a document. Declare a suitable map variable.
SELF CHECK
16.
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Practice It
Java 8 Note 15.1
What is a Map? Give a possible use for such a structure.
Now you can try these exercises at the end of the chapter: R15.20, E15.4, E15.5.
Updating Map Entries Maps are commonly used for counting how often an item occurs. For example, Worked Example 15.1 uses a Map to track how many times a word occurs in a file. It is a bit tedious to deal with the special case of inserting the first value. Consider the following code from Worked Example 15.1:
© subjug/iStockphoto.
Integer count = frequencies.get(word); // Get the old frequency // If there was none, put 1; otherwise, increment the count if (count == null) { count = 1; } else { count = count + 1; } frequencies.put(word, count);
count
Java 8 adds a useful merge method to the Map interface. You specify • A key. • A value to be used if the key is not yet present. • A function to compute the updated value if the key is present. The function is specified as a lambda expression (see Java 8 Note 9.3). For example, frequencies.merge(word, 1, (oldValue, notPresentValue) -> oldValue + notPresentValue);
does the same as the four lines of code above. If word is not present, the value is set to 1. Otherwise, the old value is incremented. The merge method is also useful if the map values are sets or comma-separated strings—see Exercises E15.6 and E15.7.
H ow T o 15.1
Choosing a Collection
Step 1 © Steve Simzer/iStockphoto.
Determine how you access the values.
© Tom Hahn/ iStockphoto.
Suppose you need to store objects in a collection. You have now seen a number of different data structures. This How To reviews how to pick an appropriate collection for your application.
© Tom Hahn/iStockphoto.
You store values in a collection so that you can later retrieve them. How do you want to access individual values? You have several choices: • Values are accessed by an integer position. Use an ArrayList. • Values are accessed by a key that is not a part of the object. Use a map. • Values are accessed only at one of the ends. Use a queue (for first-in, first-out access) or a stack (for last-in, first-out access). • You don’t need to access individual values by position. Refine your choice in Steps 3 and 4. Step 2
Determine the element types or key/value types. For a list or set, determine the type of the elements that you want to store. For example, if you collect a set of books, then the element type is Book.
15.4 Maps 709 Similarly, for a map, determine the types of the keys and the associated values. If you want to look up books by ID, you can use a Map or Map, depending on your ID type. Step 3
Determine whether element or key order matters. When you visit elements from a collection or keys from a map, do you care about the order in which they are visited? You have several choices: • Elements or keys must be sorted. Use a TreeSet or TreeMap. Go to Step 6. • Elements must be in the same order in which they were inserted. Your choice is now narrowed down to a LinkedList or an ArrayList. • It doesn’t matter. As long as you get to visit all elements, you don’t care in which order. If you chose a map in Step 1, use a HashMap and go to Step 5.
Step 4
For a collection, determine which operations must be efficient. You have several choices: • Finding elements must be efficient. Use a HashSet. • It must be efficient to add or remove elements at the beginning, or, provided that you are already inspecting an element there, another position. Use a LinkedList. • You only insert or remove at the end, or you collect so few elements that you aren’t concerned about speed. Use an ArrayList.
Step 5
For hash sets and maps, decide whether you need to implement the methods.
hashCode
and
equals
• If your elements or keys belong to a class that someone else implemented, check whether the class has its own hashCode and equals methods. If so, you are all set. This is the case for most classes in the standard Java library, such as String, Integer, Rectangle, and so on. • If not, decide whether you can compare the elements by identity. This is the case if you never construct two distinct elements with the same contents. In that case, you need not do anything—the hashCode and equals methods of the Object class are appropriate. • Otherwise, you need to implement your own equals and hashCode methods––see Section 9.5.2 and Special Topic 15.1. Step 6
If you use a tree, decide whether to supply a comparator. Look at the class of the set elements or map keys. Does that class implement the Comparable interface? If so, is the sort order given by the compareTo method the one you want? If yes, then you don’t need to do anything further. This is the case for many classes in the standard library, in particular for String and Integer. If not, then your element class must implement the Comparable interface (Section 9.6.3), or you must declare a class that implements the Comparator interface (see Special Topic 14.4).
© Alex Slobodkin/iStockphoto.
© Tom Horyn/iStockphoto.
Word Frequency
© Ermin Gutenberger/ iStockphoto.
W orked Ex ample 15.1
Learn how to create a program that reads a text file and prints a list of all words in the file in alphabetical order, together with a count that indicates how often each word occurred in the file. Go to wiley.com/go/bjlo2examples and download Worked Example 15.1. © Ermin Gutenberger/iStockphoto.
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Hash Functions If you use a hash set or hash map with your own classes, you may need to implement a hash function. A hash function is a function that computes an integer value, the hash code, from an object in such a way that different objects are likely to yield different hash codes. Because hashing is so important, the Object class has a hashCode method. The call
© Eric Isselé/iStockphoto.
int h = x.hashCode();
computes the hash code of any object x. If you want to put objects of a given class into a HashSet © one clear vision/iStockphoto. or use the objects as keys in a HashMap, the class A good hash function produces different should override this method. The method should hash values for each object so that they be implemented so that different objects are likely are scattered about in a hash table. to have different hash codes. For example, the String class declares a hash function for A hash function strings that does a good job of producing different integer values computes an integer for different strings. Table 6 shows some examples of strings and value from an object. their hash codes. It is possible for two or more distinct objects to have the same A good hash function hash code; this is called a collision. For example, the strings "Ugh" minimizes collisions— and "VII" happen to have the same hash code, but these collisions identical hash codes for are very rare for strings (see Exercise P15.5). different objects. The hashCode method of the String class combines the characters of a string into a numerical code. The code isn’t simply the sum of the character values— that would not scramble the character values enough. Strings that are permutations of another (such as "eat" and "tea") would all have the same hash code. Here is the method the standard library uses to compute the hash code for a string: final int HASH_MULTIPLIER = 31; int h = 0; for (int i = 0; i < s.length(); i++) { h = HASH_MULTIPLIER * h + s.charAt(i); }
For example, the hash code of "eat" is 31 * (31 * 'e' + 'a') + 't' = 100184
Table 6 Sample Strings and Their Hash Codes String
Hash Code
"eat"
100184
"tea"
114704
"Juliet"
–2065036585
"Ugh"
84982
"VII"
84982
© one clear vision/iStockphoto.
Special Topic 15.1
15.4 Maps 711 The hash code of "tea" is quite different, namely 31 * (31 * 't' + 'e') + 'a' = 114704
(Use the Unicode table from Appendix A to look up the character values: 'a' is 97, 'e' is 101, and 't' is 116.) For your own classes, you should make up a hash code that Override hashCode combines the hash codes of the instance variables in a similar way. methods in your own For example, let us declare a hashCode method for the Country class classes by combining the hash codes for the from Section 9.6.1. instance variables. There are two instance variables: the country name and the area. First, compute their hash codes. You know how to compute the hash code of a string. To compute the hash code of a floating-point number, first wrap the floating-point number into a Double object, and then compute its hash code. public class Country { . . . public int hashCode() { int h1 = name.hashCode(); int h2 = new Double(area).hashCode(); . . . } }
Then combine the two hash codes: final int HASH_MULTIPLIER = 31; int h = HASH_MULTIPLIER * h1 + h2; return h;
However, it is easier to use the Objects.hash method which takes the hash codes of all arguments and combines them with a multiplier. public int hashCode() { return Objects.hash(name, area); }
FULL CODE EXAMPLE
Go to wiley.com/go/ bjlo2code to down© Alex Slobodkin/iStockphoto.
load a program that demonstrates a hash set with objects of the Country class.
When you supply your own hashCode method for a class, you must also provide a compatible equals method. The equals method is used to differentiate between two objects that happen to have the same hash code. The equals and hashCode methods must be compatible with each A class’s hashCode other. Two objects that are equal must yield the same hash code. method must be You get into trouble if your class declares an equals method compatible with its equals method. but not a hashCode method. Suppose the Country class declares an equals method (checking that the name and area are the same), but no hashCode method. Then the hashCode method is inherited from the Object superclass. That method computes a hash code from the memory location of the object. Then it is very likely that two objects with the same contents will have different hash codes, in which case a hash set will store them as two distinct objects. However, if you declare neither equals nor hashCode, then there is no problem. The equals method of the Object class considers two objects equal only if their memory location is the same. That is, the Object class has compatible equals and hashCode methods. Of course, then the notion of equality is very restricted: Only identical objects are considered equal. That can be a perfectly valid notion of equality, depending on your application.
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15.5 Stacks, Queues, and Priority Queues In the following sections, we cover stacks, queues, and priority queues. These data structures each have a different policy for data removal. Removing an element yields the most recently added element, the least recently added, or the element with the highest priority.
© budgetstockphoto/iStockphoto.
A stack is a collection of elements with “last-in, first-out” retrieval.
A stack lets you insert and remove elements only at one end, traditionally called the top of the stack. New items can be added to the top of the stack. Items are removed from the top of the stack as well. Therefore, they are removed in the order that is opposite from the order in which they have been added, called last-in, first-out or LIFO order. For example, if you add items A, B, and C and then remove them, you obtain C, B, and A. With stacks, the addition and removal operations are called push and pop. Stack s = new Stack(); The last pancake that has been s.push("A"); s.push("B"); s.push("C"); added to this stack will be the while (s.size() > 0) first one that is consumed. { © John Madden/iStockphoto. System.out.print(s.pop() + " "); // Prints C B A }
There are many applications for stacks in computer science. Consider the undo feaof a word processor. It keeps the issued commands in a stack. When you select “Undo”, the last command is undone, then the next-to-last, and so on. Another important example is the run-time stack that a processor or virtual machine keeps to store the values of variables in nested methods. Whenever a new method is called, its parameter variables and local variables are pushed onto a stack. When the method exits, they are popped off again. You will see other applications in Section 15.6. The Java library provides a simple Stack class with methods push, pop, and peek—the latter gets the top element of the stack but does not remove it (see Table 7).
© budgetstockphoto/iStockphoto. ture
The Undo key pops commands off a stack so that the last command is the first to be undone.
Table 7 Working with Stacks Stack s = new Stack();
Constructs an empty stack.
s.push(1); s.push(2); s.push(3);
Adds to the top of the stack; s is now [1, 2, 3]. (Following the toString method of the Stack class, we show the top of the stack at the end.)
int top = s.pop();
Removes the top of the stack; top is set to 3 and s is now [1, 2].
head = s.peek();
Gets the top of the stack without removing it; head is set to 2.
© John Madden/iStockphoto.
15.5.1 Stacks
15.5 Stacks, Queues, and Priority Queues 713
A queue is a collection of elements with “firstin, first-out” retrieval.
A queue lets you add items to one end of the queue (the tail) and remove them from the other end of the queue (the head). Queues yield items in a first-in, first-out or FIFO fashion. Items are removed in the same order in which they were added. A typical application is a print queue. A printer may be accessed by several applications, perhaps running on different computers. If each of the applications Photodisc/Punchstock. tried to access the printer at the same time, To visualize a queue, think of people lining up. the printout would be garbled. Instead, each application places its print data into a file and adds that file to the print queue. When the printer is done printing one file, it retrieves the next one from the queue. Therefore, print jobs are printed using the “first-in, first-out” rule, which is a fair arrangement for users of the shared printer. The Queue interface in the standard Java library has methods add to add an element to the tail of the queue, remove to remove the head of the queue, and peek to get the head element of the queue without removing it (see Table 8). The LinkedList class implements the Queue interface. Whenever you need a queue, simply initialize a Queue variable with a LinkedList object: Queue q = new LinkedList(); q.add("A"); q.add("B"); q.add("C"); while (q.size() > 0) { System.out.print(q.remove() + " "); } // Prints A B C
The standard library provides several queue classes that we do not discuss in this book. Those classes are intended for work sharing when multiple activities (called threads) run in parallel.
Table 8 Working with Queues Queue q = new LinkedList();
The LinkedList class implements the Queue interface.
q.add(1); q.add(2); q.add(3);
Adds to the tail of the queue; q is now [1,
int head = q.remove();
Removes the head of the queue; head is set to 1 and q is [2,
head = q.peek();
Gets the head of the queue without removing it; head is set to 2.
2, 3].
3].
15.5.3 Priority Queues When removing an element from a priority queue, the element with the most urgent priority is retrieved.
A priority queue collects elements, each of which has a priority. A typical example of a priority queue is a collection of work requests, some of which may be more urgent than others. Unlike a regular queue, the priority queue does not maintain a first-in, first-out discipline. Instead, elements are retrieved according to their priority. In other words, new items can be inserted in any order. But whenever an item is removed, it is the item with the most urgent priority.
Photodisc/Punchstock.
15.5.2 Queues
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© paul kline/iStockphoto.
It is customary to give low values to urgent priorities, with priority 1 denoting the most urgent priority. Thus, each removal operation extracts the minimum element from the queue. For example, consider this code in which we add objects of a class Work Order into a priority queue. Each work order has a priority and a description. PriorityQueue q = new PriorityQueue(); q.add(new WorkOrder(3, "Shampoo carpets")); q.add(new WorkOrder(1, "Fix broken sink")); q.add(new WorkOrder(2, "Order cleaning supplies"));
When calling q.remove() for the first time, the work order with priority 1 is removed. The next call to q.remove() removes the work order whose priority When you retrieve an item from is highest among those remaining in the queue—in our example, the work a priority queue, you always order with priority 2. If there happen to be two elements with the same priget the most urgent one. ority, the priority queue will break ties arbitrarily. © paul kline/iStockphoto. Because the priority queue needs to be able to tell which element is the smallest, the added elements should belong to a class that implements the Comparable interface. FULL CODE EXAMPLE (See Section 9.6.3 for a description of that interface type.) Go to wiley.com/go/ bjlo2code to down Table 9 shows the methods of the PriorityQueue class in the standard Java library. © Alex Slobodkin/iStockphoto. load programs that demonstrate stacks, queues, and priority queues.
SELF CHECK
Table 9 Working with Priority Queues PriorityQueue q = new PriorityQueue();
This priority queue holds Integer objects. In practice, you would use objects that describe tasks.
q.add(3); q.add(1); q.add(2);
Adds values to the priority queue.
int first = q.remove(); int second = q.remove();
Each call to remove removes the most urgent item: first is set to 1, second to 2.
int next = q.peek();
Gets the smallest value in the priority queue without removing it.
21.
Why would you want to declare a variable as Queue q = new LinkedList();
instead of simply declaring it as a linked list? © Nicholas Homrich/iStockphoto. 22. Why wouldn’t you want to use an array list for implementing a queue? 23. What does this code print? Queue q = new LinkedList(); q.add("A"); q.add("B"); q.add("C"); while (q.size() > 0) { System.out.print(q.remove() + " "); } 24. 25.
Why wouldn’t you want to use a stack to manage print jobs? In the sample code for a priority queue, we used a WorkOrder class. Could we have used strings instead? PriorityQueue q = new PriorityQueue(); q.add("3 - Shampoo carpets"); q.add("1 - Fix broken sink"); q.add("2 - Order cleaning supplies");
15.6 Stack and Queue Applications 715 Practice It
Now you can try these exercises at the end of the chapter: R15.15, E15.8, E15.9.
15.6 Stack and Queue Applications Stacks and queues are, despite their simplicity, very versatile data structures. In the following sections, you will see some of their most useful applications.
15.6.1 Balancing Parentheses A stack can be used to check whether parentheses in an expression are balanced.
In Common Error 2.5, you saw a simple trick for detecting unbalanced parentheses in an expression such as -(b * b - (4 * a * c ) ) / (2 * a) 1 2 1 0 1 0
Increment a counter when you see a ( and decrement it when you see a ). The counter should never be negative, and it should be zero at the end of the expression. That works for expressions in Java, but in mathematical notation, one can have more than one kind of parentheses, such as –{ [b ⋅ b – (4 ⋅ a ⋅ c ) ] / (2 ⋅ a) } To see whether such an expression is correctly formed, place the parentheses on a stack: FULL CODE EXAMPLE
Go to wiley.com/go/ bjlo2code to down © Alex Slobodkin/iStockphoto. load a program for checking balanced parentheses.
When you see an opening parenthesis, push it on the stack. When you see a closing parenthesis, pop the stack. If the opening and closing parentheses don’t match The parentheses are unbalanced. Exit. If at the end the stack is empty The parentheses are balanced. Else The parentheses are not balanced. Here is a walkthrough of the sample expression:
Stack Empty { {[ {[( {[ { {( { Empty
Unread expression -{ [b * b - (4 * a * c ) ] / (2 * a) } [b * b - (4 * a * c ) ] / (2 * a) } b * b - (4 * a * c ) ] / (2 * a) } 4 * a * c ) ] / (2 * a) } ] / (2 * a) } / (2 * a) } 2 * a) } } No more input
Comments
( matches ) [ matches ] ( matches ) { matches } The parentheses are balanced
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15.6.2 Evaluating Reverse Polish Expressions Use a stack to evaluate expressions in reverse Polish notation.
Consider how you write arithmetic expressions, such as (3 + 4) × 5. The parentheses are needed so that 3 and 4 are added before multiplying the result by 5. However, you can eliminate the parentheses if you write the operators after the numbers, like this: 3 4 + 5 × (see Special Topic 15.2 on page 723). To evaluate this expression, apply + to 3 and 4, yielding 7, and then simplify 7 5 × to 35. It gets trickier for complex expressions. For example, 3 4 5 + × means to compute 4 5 + (that is, 9), and then evaluate 3 9 ×. If we evaluate this expression left-to-right, we need to leave the 3 somewhere while we work on 4 5 +. Where? We put it on a stack. The algorithm for evaluating reverse Polish expressions is simple:
If you read a number Push it on the stack. Else if you read an operand Pop two values off the stack. Combine the values with the operand. Push the result back onto the stack. Else if there is no more input Pop and display the result. Here is a walkthrough of evaluating the expression 3 4 5 + ×:
Stack Empty 3 34 345 39 27 Empty
Unread expression 345+x 45+x 5+x +x x No more input
Comments Numbers are pushed on the stack
Pop 4 and 5, push 4 5 + Pop 3 and 9, push 3 9 x Pop and display the result, 27
The following program simulates a reverse Polish calculator: sec06_02/Calculator.java 1 2 3 4 5 6 7 8 9 10 11 12 13 14
import java.util.Scanner; import java.util.Stack; /**
This calculator uses the reverse Polish notation.
*/ public class Calculator { public static void main(String[] args) { Scanner in = new Scanner(System.in); Stack results = new Stack(); System.out.println("Enter one number or operator per line, Q to quit. "); boolean done = false;
15.6 Stack and Queue Applications 717 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52
while (!done) { String input = in.nextLine(); // If the command is an operator, pop the arguments and push the result if (input.equals("+")) { results.push(results.pop() + results.pop()); } else if (input.equals("-")) { Integer arg2 = results.pop(); results.push(results.pop() - arg2); } else if (input.equals("*") || input.equals("x")) { results.push(results.pop() * results.pop()); } else if (input.equals("/")) { Integer arg2 = results.pop(); results.push(results.pop() / arg2); } else if (input.equals("Q") || input.equals("q")) { done = true; } else { // Not an operator--push the input value results.push(Integer.parseInt(input)); } System.out.println(results); } } }
15.6.3 Evaluating Algebraic Expressions In the preceding section, you saw how to evaluate expressions in reverse Polish notation, using a single stack. If you haven’t found that notation attractive, you will be glad to know that one can evaluate an expression in the standard algebraic notation using two stacks—one for numbers and one for operators.
© Jorge Delgado/iStockphoto.
Using two stacks, you can evaluate expressions in standard algebraic notation.
Use two stacks to evaluate algebraic expressions. © Jorge Delgado/iStockphoto.
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First, consider a simple example, the expression 3 + 4. We push the numbers on the number stack and the operators on the operator stack. Then we pop both numbers and the operator, combine the numbers with the operator, and push the result. Number stack Empty
Operator stack Empty
1
3
2
3
+
3
4 3
+
4
Unprocessed input 3+4
Comments
+4 4 No more input
Evaluate the top.
The result is 7.
7
This operation is fundamental to the algorithm. We call it “evaluating the top”. In algebraic notation, each operator has a precedence. The + and - operators have the lowest precedence, * and / have a higher (and equal) precedence. Consider the expression 3 × 4 + 5. Here are the first processing steps: Number stack Empty
Operator stack Empty
1
3
2
3
×
3
4 3
×
Unprocessed input 3×4+5
Comments
×4+5 4+5 +5
Evaluate × before +.
Because × has a higher precedence than +, we are ready to evaluate the top: Number stack
Operator stack
Comments
4
12
+
5
5
5 12
+
No more input
6
17
Evaluate the top.
That is the result.
With the expression, 3 + 4 × 5, we add × to the operator stack because we must first read the next number; then we can evaluate × and then the +: Number stack Empty 1
3
2
3
Operator stack Empty
Unprocessed input 3+4×5 +4×5
+
4+5
Comments
15.6 Stack and Queue Applications 719
3
4
×5
4 3
+
4 3
× +
Don’t evaluate + yet.
5
In other words, we keep operators on the stack until they are ready to be evaluated. Here is the remainder of the computation:
5
6
7
Number stack
Operator stack
5 4 3
× +
20 3
+
Comments No more input
Evaluate the top.
Evaluate top again.
That is the result.
23
To see how parentheses are handled, consider the expression 3 × (4 + 5). A ( is pushed on the operator stack. The + is pushed as well. When we encounter the ), we know that we are ready to evaluate the top until the matching ( reappears: Number stack Empty
Operator stack Empty
Unprocessed input 3 × (4 + 5)
1
3
2
3
×
(4 + 5)
( ×
4 + 5)
3 4 3
( ×
+ 5)
+ ( ×
5)
4 3 6
5 4 3
+ ( ×
)
7
9 3
( ×
No more input
8
9 3
×
3
4
5
9
27
Comments
× (4 + 5)
Don’t evaluate × yet.
Evaluate the top.
Pop (.
Evaluate top again.
That is the result.
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Here is the algorithm:
If you read a number Push it on the number stack. Else if you read a ( Push it on the operator stack. Else if you read an operator op While the top of the stack has a higher precedence than op Evaluate the top. Push op on the operator stack. Else if you read a ) While the top of the stack is not a ( Evaluate the top. Pop the (. Else if there is no more input While the operator stack is not empty Evaluate the top. At the end, the remaining value on the number stack is the value of the expression. The algorithm makes use of this helper method that evaluates the topmost operator with the topmost numbers: FULL CODE EXAMPLE
Go to wiley.com/ go/bjlo2code to get © Alex Slobodkin/iStockphoto. the complete code for the expression calculator.
Evaluate the top: Pop two numbers off the number stack. Pop an operator off the operator stack. Combine the numbers with that operator. Push the result on the number stack.
Use a stack to remember choices you haven’t yet made so that you can backtrack to them.
Suppose you are inside a maze. You need to find the exit. What should you do when you come to an intersection? You can continue exploring one of the paths, but you will want to remember the other ones. If your chosen path didn’t work, you can go back to one of the other choices and try again. Of course, as you go along one path, you may reach Skip ODonnell/iStockphoto. further intersections, and you need to remember your A© stack can be used to track choice again. Simply use a stack to remember the paths positions in a maze. that still need to be tried. The process of returning to a choice point and trying another choice is called backtracking. By using a stack, you return to your more recent choices before you explore the earlier ones. Figure 11 shows an example. We start at a point in the maze, at position (3, 4). There are four possible paths. We push them all on a stack 1 . We pop off the topmost one, traveling north from (3, 4). Following this path leads to position (1, 4). We now push two choices on the stack, going west or east 2 . Both of them lead to dead ends 3 4 . Now we pop off the path from (3, 4) going east. That too is a dead end 5 . Next is the path from (3, 4) going south. At (5, 4), it comes to an intersection. Both choices are pushed on the stack 6 . They both lead to dead ends 7 8 . Finally, the path from (3, 4) going west leads to an exit 9 .
© Skip ODonnell/ iStockphoto.
15.6.4 Backtracking
15.6 Stack and Queue Applications 721
1
2
3
4
5
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
34↑ 34→ 34↓ 34←
14→ 14← 34→ 34↓ 34←
14← 34→ 34↓ 34←
34→ 34↓ 34←
6
7
8
9
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
54↓ 54← 34←
54← 34←
34←
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
34↓ 34←
Figure 11 Backtracking Through a Maze
Using a stack, we have found a path out of the maze. Here is the pseudocode for our maze-finding algorithm:
Push all paths from the point on which you are standing on a stack. While the stack is not empty Pop a path from the stack. Follow the path until you reach an exit, intersection, or dead end. If you found an exit Congratulations! Else if you found an intersection Push all paths meeting at the intersection, except the current one, onto the stack. This algorithm will find an exit from the maze, provided that the maze has no cycles. If it is possible that you can make a circle and return to a previously visited intersection along a different sequence of paths, then you need to work harder––see Exercise E15.21.
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How you implement this algorithm depends on the description of the maze. In the example code, we use a two-dimensional array of characters, with spaces for corridors and asterisks for walls, like this: ******** * * **** *** * **** *** * *** **** *** ******** FULL CODE EXAMPLE
Go to wiley.com/ go/bjlo2code to © Alex Slobodkin/iStockphoto. download a complete program demonstrat ing backtracking.
SELF CHECK
In the example code, a Path object is constructed with a starting position and a direction (North, East, South, or West). The Maze class has a method that extends a path until it reaches an intersection or exit, or until it is blocked by a wall, and a method that computes all paths from an intersection point. Note that you can use a queue instead of a stack in this algorithm. Then you explore the earlier alternatives before the later ones. This can work just as well for finding an answer, but it isn’t very intuitive in the context of exploring a maze—you would have to imagine being teleported back to the initial intersections rather than just walking back to the last one. 26. 27.
What is the value of the reverse Polish notation expression 2 3 4 + 5 × ×? Why does the branch for the subtraction operator in the Calculator program not simply execute
results.push(results.pop() - results.pop()); © Nicholas Homrich/iStockphoto.
29. 30.
Practice It
In the evaluation of the expression 3 – 4 + 5 using the algorithm of Section 15.6.3, which operator gets evaluated first? In the algorithm of Section 15.6.3, are the operators on the operator stack always in increasing precedence? Consider the simple maze at right. Assuming that we start at A B C D the marked point and push paths in the order West, South, East, North, in which order are the lettered points visited, E F G using the algorithm of Section 15.6.4?
Now you can try these exercises at the end of the chapter: R15.25, E15.18, E15.20, E15.21, E15.22.
W orked Ex ample 15.2 © Alex Slobodkin/iStockphoto.
H L
I M
J K N
Simulating a Queue of Waiting Customers
Learn how to use a queue to simulate an actual queue of waiting customers. Go to wiley.com/go/bjlo2examples and download Worked Example 15.2.
© Tom Horyn/iStockphoto.
V ideo Ex ample 15.1 © Alex Slobodkin/iStockphoto.
Building a Table of Contents
Photodisc/Punchstock.
In this Video Example, you will see how to build a table of contents for a book. Go to wiley. com/go/bjlo2videos to view Video Example 15.1.
Photodisc/Punchstock.
28.
15.6 Stack and Queue Applications 723
Special Topic 15.2
Reverse Polish Notation In the 1920s, the Polish mathematician Jan Łukasiewicz realized that it is possible to dispense with parentheses in arithmetic expressions, provided that you write the operators before their arguments, for example, + 3 4 instead of 3 + 4. Thirty years later, Australian computer scientist Charles Hamblin noted that an even better scheme would be to have the operators follow the operands. This was termed reverse Polish notation or RPN.
© Eric Isselé/iStockphoto.
Standard Notation
Reverse Polish Notation
3 + 4
3 4 +
3 + 4 × 5
3 4 5 × +
3 × (4 + 5)
3 4 5 + ×
(3 + 4) × (5 + 6)
3 4 + 5 6 + ×
3 + 4 + 5
3 4 + 5 +
Courtesy of Nigel Tout.
Reverse Polish notation might look strange to you, but that is just an accident of history. Had earlier mathematicians realized its advantages, today’s schoolchildren might be using it and not worrying about precedence rules and parentheses. In 1972, Hewlett-Packard introduced the HP 35 calculator that used reverse Polish notation. The calculator had no keys labeled with parentheses or an equals symbol. There is just a key labeled ENTER to push a number onto a stack. For that reason, Hewlett-Packard’s marketing department used to refer to their product as “the calculators that have no equal”. Over time, calculator vendors have adapted to the standard algebraic notation rather than forcing its users to learn a new notation. However, those users who have made the effort to learn reverse Polish notation tend to be fanatic proponents, and to this day, some HewlettPackard calculator models still support it.
Courtesy of Nigel Tout.
The Calculator with No Equal
724 Chapter 15 The Java Collections Framework CHAPTER SUMMARY Understand the architecture of the Java collections framework.
• A collection groups together elements and allows them to be retrieved later. • A list is a collection that remembers the order of its elements. • A set is an unordered collection of unique elements. • A map keeps associations between key and value objects. © Tom Hahn/iStockphoto.
Understand and use linked lists.
• A linked list consists of a number of nodes, each of which has a reference to the next node. • Adding and removing elements at a given position in a linked list is efficient.
© andrea laurita/iStockphoto.
• Visiting the elements of a linked list in sequential order is efficient, but random access is not. • You use a list iterator to access elements inside a linked list.
Choose a set implementation and use it to manage sets of values.
• The HashSet and TreeSet classes both implement the Set interface. • Set implementations arrange the elements so that they can locate them quickly. • You can form hash sets holding objects of type String, Integer, Double, Point, Rectangle, or Color. • You can form tree sets for any class that implements the Comparable interface, such as String or Integer.
© parema/iStockphoto.
• Sets don’t have duplicates. Adding a duplicate of an element that is already present is ignored.
© Alfredo Ragazzoni/iStockphoto.
• A set iterator visits the elements in the order in which the set implementation keeps them.
© Volkan Ersoy/iStockphoto.
• You cannot add an element to a set at an iterator position.
Use maps to model associations between keys and values. Keys
ISBN 978-0-470-10554-2
90000
780470 105542
9
780470 509481 ISBN 978-0-470-38329-2
ISBN 978-0-471-79191-1
90000
90000
90000
780470 105559
• The HashMap and TreeMap classes both implement the Map interface.
ISBN 978-0-470-50948-1
90000
9 ISBN 978-0-470-10555-9
9
9
780471 791911
9
780470 383292
Values
© david franklin/iStockphoto.
• To find all keys and values in a map, iterate through the key set and find the values that correspond to the keys. • A hash function computes an integer value from an object. • A good hash function minimizes collisions—identical hash codes for different objects. • Override hashCode methods in your own classes by combining the hash codes for the instance variables. • A class’s hashCode method must be compatible with its equals method.
© one clear vision/iStockphoto.
Review Exercises 725 Use the Java classes for stacks, queues, and priority queues.
• A stack is a collection of elements with “last-in, first-out” retrieval. • A queue is a collection of elements with “first-in, first-out” retrieval. • When removing an element from a priority queue, the element with the most urgent priority is retrieved. © John Madden/iStockphoto.
Solve programming problems using stacks and queues.
Photodisc/Punchstock.
• A stack can be used to check whether parentheses in an expression are balanced. • Use a stack to evaluate expressions in reverse Polish notation. • Using two stacks, you can evaluate expressions in standard algebraic notation. © Jorge Delgado/iStockphoto.
• Use a stack to remember choices you haven’t yet made so that you can backtrack to them.
S TA N D A R D L I B R A R Y I T E M S I N T R O D U C E D I N T H I S C H A P T E R java.util.Collection add contains iterator remove size java.util.HashMap java.util.HashSet java.util.Iterator hasNext next remove java.util.LinkedList addFirst addLast getFirst getLast removeFirst removeLast
java.util.List listIterator java.util.ListIterator add hasPrevious previous set java.util.Map get keySet put remove java.util.Objects hash
java.util.PriorityQueue remove java.util.Queue peek java.util.Set java.util.Stack peek pop push java.util.TreeMap java.util.TreeSet
REVIEW EXERCISES •• R15.1 An invoice contains a collection of purchased items. Should that collection be imple-
mented as a list or set? Explain your answer.
•• R15.2 Consider a program that manages an appointment calendar. Should it place the
appointments into a list, stack, queue, or priority queue? Explain your answer.
••• R15.3 One way of implementing a calendar is as a map from date objects to event objects.
However, that only works if there is a single event for a given date. How can you use another collection type to allow for multiple events on a given date?
726 Chapter 15 The Java Collections Framework •• R15.4 Look up the descriptions of the methods addAll, removeAll, retainAll, and containsAll
in the Collection interface. Describe how these methods can be used to implement common operations on sets (union, intersection, difference, subset).
• R15.5 Explain what the following code prints. Draw a picture of the linked list after
each step.
LinkedList staff = new LinkedList(); staff.addFirst("Harry"); staff.addFirst("Diana"); staff.addFirst("Tom"); System.out.println(staff.removeFirst()); System.out.println(staff.removeFirst()); System.out.println(staff.removeFirst());
• R15.6 Explain what the following code prints. Draw a picture of the linked list after each
step.
LinkedList staff = new LinkedList(); staff.addFirst("Harry"); staff.addFirst("Diana"); staff.addFirst("Tom"); System.out.println(staff.removeLast()); System.out.println(staff.removeFirst()); System.out.println(staff.removeLast());
• R15.7 Explain what the following code prints. Draw a picture of the linked list after each
step.
LinkedList staff = new LinkedList(); staff.addFirst("Harry"); staff.addLast("Diana"); staff.addFirst("Tom"); System.out.println(staff.removeLast()); System.out.println(staff.removeFirst()); System.out.println(staff.removeLast());
• R15.8 Explain what the following code prints. Draw a picture of the linked list and the
iterator position after each step.
LinkedList staff = new LinkedList(); ListIterator iterator = staff.listIterator(); iterator.add("Tom"); iterator.add("Diana"); iterator.add("Harry"); iterator = staff.listIterator(); if (iterator.next().equals("Tom")) { iterator.remove(); } while (iterator.hasNext()) { System.out.println(iterator.next()); }
• R15.9 Explain what the following code prints. Draw a picture of the linked list and the
iterator position after each step.
LinkedList staff = new LinkedList(); ListIterator iterator = staff.listIterator(); iterator.add("Tom"); iterator.add("Diana"); iterator.add("Harry"); iterator = staff.listIterator(); iterator.next(); iterator.next();
Review Exercises 727 iterator.add("Romeo"); iterator.next(); iterator.add("Juliet"); iterator = staff.listIterator(); iterator.next(); iterator.remove(); while (iterator.hasNext()) { System.out.println(iterator.next()); }
•• R15.10 You are given a linked list of strings. How do you remove all elements with length
less than or equal to three?
•• R15.11 Repeat Exercise R15.10, using the removeIf method. (Read the description in the API
of the Collection interface.) Use a lambda expression (see Java 8 Note 9.3).
© subjug/iStockphoto.
•• R15.12 What advantages do linked lists have over arrays? What disadvantages do they have? •• R15.13 Suppose you need to organize a collection of telephone numbers for a company
division. There are currently about 6,000 employees, and you know that the phone switch can handle at most 10,000 phone numbers. You expect several hundred look ups against the collection every day. Would you use an array list or a linked list to store the information?
•• R15.14 Suppose you need to keep a collection of appointments. Would you use a linked list
or an array list of Appointment objects?
• R15.15 Suppose you write a program that models a card deck. Cards are taken from the
top of the deck and given out to players. As cards are returned to the deck, they are placed on the bottom of the deck. Would you store the cards in a stack or a queue?
• R15.16 Suppose the strings "A" . . . "Z" are pushed onto a stack. Then they are popped off the
stack and pushed onto a second stack. Finally, they are all popped off the second stack and printed. In which order are the strings printed?
• R15.17 What is the difference between a set and a map? •• R15.18 The union of two sets A and B is the set of all elements that are contained in A, B, or
both. The intersection is the set of all elements that are contained in A and B. How can you compute the union and intersection of two sets, using the add and contains methods, together with an iterator?
•• R15.19 How can you compute the union and intersection of two sets, using some of the
methods that the java.util.Set interface provides, but without using an iterator? (Look up the interface in the API documentation.)
• R15.20 Can a map have two keys with the same value? Two values with the same key? •• R15.21 A map can be implemented as a set of (key, value) pairs. Explain. • R15.22 How can you print all key/value pairs of a map, using the keySet method? The © subjug/iStockphoto.
entrySet method? The forEach method with a lambda expression? (See Java 8 Note 9.3 on lambda expressions.)
••• R15.23 Verify the hash code of the string "Juliet" in Table 6. ••• R15.24 Verify that the strings "VII" and "Ugh" have the same hash code.
728 Chapter 15 The Java Collections Framework • R15.25 Consider the algorithm for traversing a maze from Section 15.6.4 Assume that we
start at position A and push in the order West, South, East, and North. In which order will the lettered locations of the sample maze be visited? O
P
L
Q
R
M N J
G
H
I
A
B
C
K F D E
• R15.26 Repeat Exercise R15.25, using a queue instead of a stack.
PRACTICE EXERCISES •• E15.1 Write a method public static void downsize(LinkedList employeeNames, int n)
that removes every nth employee from a linked list. •• E15.2 Write a method public static void reverse(LinkedList strings)
•• E15.3 Implement the sieve of Eratosthenes: a method for computing
prime numbers, known to the ancient Greeks. This method computes all prime numbers up to n. Choose an n. First insert all numbers from 2 to n into a set. Then erase all multiples of 2 (except 2); that is, 4, 6, 8, 10, 12, . . . . Erase all multiples of 3; that is, 6, 9, 12, 15, . . . . Go up to n. Then print the set.
•• E15.4 Write a program that keeps a map in which both keys and
© martin mcelligott/iStockphoto.
that reverses the entries in a linked list.
values are strings—the names of students and their course grades. Prompt the user of the program to add or remove © martin mcelligott/iStockphoto. students, to modify grades, or to print all grades. The printout should be sorted by name and formatted like this: Carl: B+ Joe: C Sarah: A
••• E15.5 Write a program that reads a Java source file and produces an index of all identifiers
in the file. For each identifier, print all lines in which it occurs. For simplicity, we will consider each string consisting only of letters, numbers, and underscores an identifier. Declare a Scanner in for reading from the source file and call in.useDelimiter("[^A-Za-z0-9_]+")
Then each call to next returns an identifier.
Practice Exercises 729 •• E15.6 Read all words from a file and add them to a map whose keys are the first letters of © subjug/iStockphoto.
the words and whose values are sets of words that start with that same letter. Then print out the word sets in alphabetical order. Provide two versions of your solution, one that uses the merge method (see Java 8 Note 15.1) and one that updates the map as in Worked Example 15.1.
•• E15.7 Read all words from a file and add them to a map whose keys are word lengths and © subjug/iStockphoto.
whose values are comma-separated strings of words of the same length. Then print out those strings, in increasing order by the length of their entries. Provide two versions of your solution, one that uses the merge method (see Java 8 Note 15.1) and one that updates the map as in Worked Example 15.1.
•• E15.8 Use a stack to reverse the words of a sentence. Keep reading words until you have a
word that ends in a period, adding them onto a stack. When you have a word with a period, pop the words off and print them. Stop when there are no more words in the input. For example, you should turn the input Mary had a little lamb. Its fleece was white as snow.
into Lamb little a had mary. Snow as white was fleece its.
Pay attention to capitalization and the placement of the period. • E15.9 Your task is to break a number into its individual digits, for example, to turn 1729
into 1, 7, 2, and 9. It is easy to get the last digit of a number n as n % 10. But that gets the numbers in reverse order. Solve this problem with a stack. Your program should ask the user for an integer, then print its digits separated by spaces.
•• E15.10 A homeowner rents out parking spaces in a driveway during special events. The
driveway is a “last-in, first-out” stack. Of course, when a car owner retrieves a vehicle that wasn’t the last one in, the cars blocking it must temporarily move to the street so that the requested vehicle can leave. Write a program that models this behavior, using one stack for the driveway and one stack for the street. Use integers as license plate numbers. Positive numbers add a car, negative numbers remove a car, zero stops the simulation. Print out the stack after each operation is complete.
• E15.11 Implement a to do list. Tasks have a priority between 1 and 9, and a description.
When the user enters the command add priority description, the program adds a new task. When the user enters next, the program removes and prints the most urgent task. The quit command quits the program. Use a priority queue in your solution.
• E15.12 Write a program that reads text from a file and breaks it up into individual words.
Insert the words into a tree set. At the end of the input file, print all words, followed by the size of the resulting set. This program determines how many unique words a text file has.
• E15.13 Insert all words from a large file (such as the novel “War and Peace”, which is avail
able on the Internet) into a hash set and a tree set. Time the results. Which data structure is more efficient?
• E15.14 Supply compatible hashCode and equals methods to the BankAccount class of Chapter 8.
Test the hashCode method by printing out hash codes and by adding BankAccount objects to a hash set.
730 Chapter 15 The Java Collections Framework •• E15.15 A labeled point has x- and y-coordinates and a string label. Provide a class
LabeledPoint with a constructor LabeledPoint(int x, int y, String label) and hashCode and equals methods. Two labeled points are considered the same when they have the same location and label.
•• E15.16 Reimplement the LabeledPoint class of Exercise E15.15 by storing the location in a
java.awt.Point object. Your hashCode and equals methods should call the hashCode and equals methods of the Point class.
•• E15.17 Modify the LabeledPoint class of Exercise E15.15 so that it implements the Comparable
interface. Sort points first by their x-coordinates. If two points have the same x-coordinate, sort them by their y-coordinates. If two points have the same x- and y-coordinates, sort them by their label. Write a tester program that checks all cases by inserting points into a TreeSet.
• E15.18 Add a % (remainder) operator to the expression calculator of Section 15.6.3. •• E15.19 Add a ^ (power) operator to the expression calculator of Section 15.6.3. For example,
2 ^ 3 evaluates to 8. As in mathematics, your power operator should be evaluated from the right. That is, 2 ^ 3 ^ 2 is 2 ^ (3 ^ 2), not (2 ^ 3) ^ 2. (That’s more useful because you could get the latter as 2 ^ (3 × 2).)
• E15.20 Write a program that checks whether a sequence of HTML tags is properly nested.
For each opening tag, such as
, there must be a closing tag
. A tag such asmay have other tags inside, for example
User name:
Password:
The current time is #{timeBean.time}
Set time zone:
The current time in #{timeZoneBean.city} is #{timeZoneBean.time}
Sorry, no information is available for #{timeZoneBean.city}
Enter city:
Sorry, no information is available for #{timeZoneBean.cityToAdd}.