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Learning Java, 4th Edition

Cover of Learning Java, 4th Edition by Daniel Leuck... Published by O'Reilly Media, Inc.
  1. Learning Java
  2. Preface
    1. Who Should Read This Book
    2. New Developments
      1. New in This Edition (Java 6 and 7)
    3. Using This Book
    4. Online Resources
    5. Conventions Used in This Book
    6. Using Code Examples
    7. Safari® Books Online
    8. How to Contact Us
    9. Acknowledgments
  3. 1. A Modern Language
    1. Enter Java
      1. Java’s Origins
      2. Growing Up
    2. A Virtual Machine
    3. Java Compared with Other Languages
    4. Safety of Design
      1. Simplify, Simplify, Simplify...
      2. Type Safety and Method Binding
      3. Incremental Development
      4. Dynamic Memory Management
      5. Error Handling
      6. Threads
      7. Scalability
    5. Safety of Implementation
      1. The Verifier
      2. Class Loaders
      3. Security Managers
    6. Application and User-Level Security
    7. A Java Road Map
      1. The Past: Java 1.0–Java 1.6
      2. The Present: Java 7
      3. The Future
      4. Availability
  4. 2. A First Application
    1. Java Tools and Environment
    2. Configuring Eclipse and Creating a Project
      1. Importing the Learning Java Examples
    3. HelloJava
      1. Classes
      2. The main() Method
      3. Classes and Objects
      4. Variables and Class Types
      5. HelloComponent
      6. Inheritance
      7. The JComponent Class
      8. Relationships and Finger Pointing
      9. Package and Imports
      10. The paintComponent() Method
    4. HelloJava2: The Sequel
      1. Instance Variables
      2. Constructors
      3. Events
      4. The repaint() Method
      5. Interfaces
    5. HelloJava3: The Button Strikes!
      1. Method Overloading
      2. Components
      3. Containers
      4. Layout
      5. Subclassing and Subtypes
      6. More Events and Interfaces
      7. Color Commentary
      8. Static Members
      9. Arrays
      10. Our Color Methods
    6. HelloJava4: Netscape’s Revenge
      1. Threads
      2. The Thread Class
      3. The Runnable Interface
      4. Starting the Thread
      5. Running Code in the Thread
      6. Exceptions
      7. Synchronization
  5. 3. Tools of the Trade
    1. JDK Environment
    2. The Java VM
    3. Running Java Applications
      1. System Properties
    4. The Classpath
      1. javap
    5. The Java Compiler
    6. JAR Files
      1. File Compression
      2. The jar Utility
      3. The pack200 Utility
    7. Policy Files
      1. The Default Security Manager
      2. The policytool Utility
      3. Using a Policy File with the Default Security Manager
  6. 4. The Java Language
    1. Text Encoding
    2. Comments
      1. Javadoc Comments
    3. Types
      1. Primitive Types
      2. Reference Types
      3. A Word About Strings
    4. Statements and Expressions
      1. Statements
      2. Expressions
    5. Exceptions
      1. Exceptions and Error Classes
      2. Exception Handling
      3. Bubbling Up
      4. Stack Traces
      5. Checked and Unchecked Exceptions
      6. Throwing Exceptions
      7. try Creep
      8. The finally Clause
      9. Try with Resources
      10. Performance Issues
    6. Assertions
      1. Enabling and Disabling Assertions
      2. Using Assertions
    7. Arrays
      1. Array Types
      2. Array Creation and Initialization
      3. Using Arrays
      4. Anonymous Arrays
      5. Multidimensional Arrays
      6. Inside Arrays
  7. 5. Objects in Java
    1. Classes
      1. Accessing Fields and Methods
      2. Static Members
    2. Methods
      1. Local Variables
      2. Shadowing
      3. Static Methods
      4. Initializing Local Variables
      5. Argument Passing and References
      6. Wrappers for Primitive Types
      7. Autoboxing and Unboxing of Primitives
      8. Variable-Length Argument Lists
      9. Method Overloading
    3. Object Creation
      1. Constructors
      2. Working with Overloaded Constructors
      3. Static and Nonstatic Initializer Blocks
    4. Object Destruction
      1. Garbage Collection
      2. Finalization
      3. Weak and Soft References
    5. Enumerations
      1. Enum Values
      2. Customizing Enumerations
  8. 6. Relationships Among Classes
    1. Subclassing and Inheritance
      1. Shadowed Variables
      2. Overriding Methods
      3. Special References: this and super
      4. Casting
      5. Using Superclass Constructors
      6. Full Disclosure: Constructors and Initialization
      7. Abstract Methods and Classes
    2. Interfaces
      1. Interfaces as Callbacks
      2. Interface Variables
      3. Subinterfaces
    3. Packages and Compilation Units
      1. Compilation Units
      2. Package Names
      3. Class Visibility
      4. Importing Classes
    4. Visibility of Variables and Methods
      1. Basic Access Modifiers
      2. Subclasses and Visibility
      3. Interfaces and Visibility
    5. Arrays and the Class Hierarchy
      1. ArrayStoreException
    6. Inner Classes
      1. Inner Classes as Adapters
      2. Inner Classes Within Methods
  9. 7. Working with Objects and Classes
    1. The Object Class
      1. Equality and Equivalence
      2. Hashcodes
      3. Cloning Objects
    2. The Class Class
    3. Reflection
      1. Modifiers and Security
      2. Accessing Fields
      3. Accessing Methods
      4. Accessing Constructors
      5. What About Arrays?
      6. Accessing Generic Type Information
      7. Accessing Annotation Data
      8. Dynamic Interface Adapters
      9. What Is Reflection Good For?
    4. Annotations
      1. Using Annotations
      2. Standard Annotations
      3. The apt Tool
  10. 8. Generics
    1. Containers: Building a Better Mousetrap
      1. Can Containers Be Fixed?
    2. Enter Generics
      1. Talking About Types
    3. “There Is No Spoon”
      1. Erasure
      2. Raw Types
    4. Parameterized Type Relationships
      1. Why Isn’t a List<Date> a List<Object>?
    5. Casts
    6. Writing Generic Classes
      1. The Type Variable
      2. Subclassing Generics
      3. Exceptions and Generics
      4. Parameter Type Limitations
    7. Bounds
      1. Erasure and Bounds (Working with Legacy Code)
    8. Wildcards
      1. A Supertype of All Instantiations
      2. Bounded Wildcards
      3. Thinking Outside the Container
      4. Lower Bounds
      5. Reading, Writing, and Arithmetic
      6. <?>, <Object>, and the Raw Type
      7. Wildcard Type Relationships
    9. Generic Methods
      1. Generic Methods Introduced
      2. Type Inference from Arguments
      3. Type Inference from Assignment Context
      4. Explicit Type Invocation
      5. Wildcard Capture
      6. Wildcard Types Versus Generic Methods
    10. Arrays of Parameterized Types
      1. Using Array Types
      2. What Good Are Arrays of Generic Types?
      3. Wildcards in Array Types
    11. Case Study: The Enum Class
    12. Case Study: The sort() Method
    13. Conclusion
  11. 9. Threads
    1. Introducing Threads
      1. The Thread Class and the Runnable Interface
      2. Controlling Threads
      3. Death of a Thread
    2. Threading an Applet
      1. Issues Lurking
    3. Synchronization
      1. Serializing Access to Methods
      2. Accessing class and instance Variables from Multiple Threads
      3. The wait() and notify() Methods
      4. Passing Messages
      5. ThreadLocal Objects
    4. Scheduling and Priority
      1. Thread State
      2. Time-Slicing
      3. Priorities
      4. Yielding
    5. Thread Groups
      1. Working with ThreadGroups
      2. Uncaught Exceptions
    6. Thread Performance
      1. The Cost of Synchronization
      2. Thread Resource Consumption
    7. Concurrency Utilities
      1. Executors
      2. Locks
      3. Synchronization Constructs
      4. Atomic Operations
    8. Conclusion
  12. 10. Working with Text
    1. Text-Related APIs
    2. Strings
      1. Constructing Strings
      2. Strings from Things
      3. Comparing Strings
      4. Searching
      5. Editing
      6. String Method Summary
      7. StringBuilder and StringBuffer
    3. Internationalization
      1. The java.util.Locale Class
      2. Resource Bundles
    4. Parsing and Formatting Text
      1. Parsing Primitive Numbers
      2. Tokenizing Text
    5. Printf-Style Formatting
      1. Formatter
      2. The Format String
      3. String Conversions
      4. Primitive and Numeric Conversions
      5. Flags
      6. Miscellaneous
    6. Formatting with the java.text Package
      1. MessageFormat
    7. Regular Expressions
      1. Regex Notation
      2. The java.util.regex API
  13. 11. Core Utilities
    1. Math Utilities
      1. The java.lang.Math Class
      2. Big/Precise Numbers
      3. Floating-Point Components
      4. Random Numbers
    2. Dates and Times
      1. Working with Calendars
      2. Time Zones
      3. Parsing and Formatting with DateFormat
      4. Printf-Style Date and Time Formatting
    3. Timers
    4. Collections
      1. The Collection Interface
      2. Iterator
      3. Collection Types
      4. The Map Interface
      5. Collection Implementations
      6. Hash Codes and Key Values
      7. Synchronized and Unsynchronized Collections
      8. Read-Only and Read-Mostly Collections
      9. WeakHashMap
      10. EnumSet and EnumMap
      11. Sorting Collections
      12. A Thrilling Example
    5. Properties
      1. Loading and Storing
      2. System Properties
    6. The Preferences API
      1. Preferences for Classes
      2. Preferences Storage
      3. Change Notification
    7. The Logging API
      1. Overview
      2. Logging Levels
      3. A Simple Example
      4. Logging Setup Properties
      5. The Logger
      6. Performance
    8. Observers and Observables
  14. 12. Input/Output Facilities
    1. Streams
      1. Basic I/O
      2. Character Streams
      3. Stream Wrappers
      4. Pipes
      5. Streams from Strings and Back
      6. Implementing a Filter Stream
    2. File I/O
      1. The java.io.File Class
      2. File Streams
      3. RandomAccessFile
      4. Resource Paths
    3. The NIO File API
      1. FileSystem and Path
      2. NIO File Operations
      3. Directory Operations
      4. Watching Paths
    4. Serialization
      1. Initialization with readObject()
      2. SerialVersionUID
    5. Data Compression
      1. Archives and Compressed Data
      2. Decompressing Data
      3. Zip Archive As a Filesystem
    6. The NIO Package
      1. Asynchronous I/O
      2. Performance
      3. Mapped and Locked Files
      4. Channels
      5. Buffers
      6. Character Encoders and Decoders
      7. FileChannel
      8. Scalable I/O with NIO
  15. 13. Network Programming
    1. Sockets
      1. Clients and Servers
      2. author="pat” timestamp="20120926T110720-0500” comment="one of those sections I hate to get rid of but is less relevant in terms of the example... should probably find a more modern example...”The DateAtHost Client
      3. The TinyHttpd Server
      4. Socket Options
      5. Proxies and Firewalls
    2. Datagram Sockets
      1. author="pat” timestamp="20120926T141346-0500” comment="I actually rewrote this as a standalone client but then decided to leave it as an applet”The HeartBeat Applet
      2. InetAddress
    3. Simple Serialized Object Protocols
      1. A Simple Object-Based Server
    4. Remote Method Invocation
      1. Real-World Usage
      2. Remote and Nonremote Objects
      3. An RMI Example
      4. RMI and CORBA
    5. Scalable I/O with NIO
      1. Selectable Channels
      2. Using Select
      3. LargerHttpd
      4. Nonblocking Client-Side Operations
  16. 14. Programming for the Web
    1. Uniform Resource Locators (URLs)
    2. The URL Class
      1. Stream Data
      2. Getting the Content as an Object
      3. Managing Connections
      4. Handlers in Practice
      5. Useful Handler Frameworks
    3. Talking to Web Applications
      1. Using the GET Method
      2. Using the POST Method
      3. The HttpURLConnection
      4. SSL and Secure Web Communications
      5. URLs, URNs, and URIs
    4. Web Services
      1. XML-RPC
      2. WSDL
      3. The Tools
      4. The Weather Service Client
  17. 15. Web Applications and Web Services
    1. Web Application Technologies
      1. Page-Oriented Versus “Single Page” Applications
      2. JSPs
      3. XML and XSL
      4. Web Application Frameworks
      5. Google Web Toolkit
      6. HTML5, AJAX, and More...
    2. Java Web Applications
      1. The Servlet Lifecycle
      2. Servlets
      3. The HelloClient Servlet
      4. The Servlet Response
      5. Servlet Parameters
      6. The ShowParameters Servlet
      7. User Session Management
      8. The ShowSession Servlet
      9. The ShoppingCart Servlet
      10. Cookies
      11. The ServletContext API
      12. Asynchronous Servlets
    3. WAR Files and Deployment
      1. Configuration with web.xml and Annotations
      2. URL Pattern Mappings
      3. Deploying HelloClient
      4. Error and Index Pages
      5. Security and Authentication
      6. Protecting Resources with Roles
      7. Secure Data Transport
      8. Authenticating Users
      9. Procedural Authorization
    4. Servlet Filters
      1. A Simple Filter
      2. A Test Servlet
      3. Declaring and Mapping Filters
      4. Filtering the Servlet Request
      5. Filtering the Servlet Response
    5. Building WAR Files with Ant
      1. A Development-Oriented Directory Layout
      2. Deploying and Redeploying WARs with Ant
    6. Implementing Web Services
      1. Defining the Service
      2. Our Echo Service
      3. Using the Service
      4. Data Types
    7. Conclusion
  18. 16. Swing
    1. Components
      1. Peers and Look-and-Feel
      2. The MVC Framework
      3. Painting
      4. Enabling and Disabling Components
      5. Focus, Please
      6. Other Component Methods
      7. Layout Managers
      8. Insets
      9. Z-Ordering (Stacking Components)
      10. The revalidate() and doLayout() Methods
      11. Managing Components
      12. Listening for Components
      13. Windows, Frames and Splash Screens
      14. Other Methods for Controlling Frames
      15. Content Panes
      16. Desktop Integration
    2. Events
      1. Event Receivers and Listener Interfaces
      2. Event Sources
      3. Event Delivery
      4. Event Types
      5. The java.awt.event.InputEvent Class
      6. Mouse and Key Modifiers on InputEvents
      7. Focus Events
    3. Event Summary
      1. Adapter Classes
      2. Dummy Adapters
    4. The AWT Robot!
    5. Multithreading in Swing
  19. 17. Using Swing Components
    1. Buttons and Labels
      1. HTML Text in Buttons and Labels
    2. Checkboxes and Radio Buttons
    3. Lists and Combo Boxes
    4. The Spinner
    5. Borders
    6. Menus
    7. Pop-Up Menus
      1. Component-Managed Pop Ups
    8. The JScrollPane Class
    9. The JSplitPane Class
    10. The JTabbedPane Class
    11. Scrollbars and Sliders
    12. Dialogs
      1. File Selection Dialog
      2. The Color Chooser
  20. 18. More Swing Components
    1. Text Components
      1. The TextEntryBox Application
      2. Formatted Text
      3. Filtering Input
      4. Validating Data
      5. Say the Magic Word
      6. Sharing a Data Model
      7. HTML and RTF for Free
      8. Managing Text Yourself
    2. Focus Navigation
      1. Trees
      2. Nodes and Models
      3. Save a Tree
      4. Tree Events
      5. A Complete Example
    3. Tables
      1. A First Stab: Freeloading
      2. Round Two: Creating a Table Model
      3. Round Three: A Simple Spreadsheet
      4. Sorting and Filtering
      5. Printing JTables
    4. Desktops
    5. Pluggable Look-and-Feel
    6. Creating Custom Components
      1. Generating Events
      2. A Dial Component
      3. Model and View Separation
  21. 19. Layout Managers
    1. FlowLayout
    2. GridLayout
    3. BorderLayout
    4. BoxLayout
    5. CardLayout
    6. GridBagLayout
      1. The GridBagConstraints Class
      2. Grid Coordinates
      3. The fill Constraint
      4. Spanning Rows and Columns
      5. Weighting
      6. Anchoring
      7. Padding and Insets
      8. Relative Positioning
      9. Composite Layouts
    7. Other Layout Managers
    8. Absolute Positioning
  22. 20. Drawing with the 2D API
    1. The Big Picture
    2. The Rendering Pipeline
    3. A Quick Tour of Java 2D
      1. Filling Shapes
      2. Drawing Shape Outlines
      3. Convenience Methods
      4. Drawing Text
      5. Drawing Images
      6. The Whole Iguana
    4. Filling Shapes
      1. Solid Colors
      2. Color Gradients
      3. Textures
      4. Desktop Colors
    5. Stroking Shape Outlines
    6. Using Fonts
      1. Font Metrics
    7. Displaying Images
      1. The Image Class
      2. Image Observers
      3. Scaling and Size
    8. Drawing Techniques
      1. Double Buffering
      2. Limiting Drawing with Clipping
      3. Offscreen Drawing
    9. Printing
  23. 21. Working with Images and Other Media
    1. Loading Images
      1. ImageObserver
      2. MediaTracker
      3. ImageIcon
      4. ImageIO
    2. Producing Image Data
      1. Drawing Animations
      2. BufferedImage Anatomy
      3. Color Models
      4. Creating an Image
      5. Updating a BufferedImage
    3. Filtering Image Data
      1. How ImageProcessor Works
      2. Converting an Image to a BufferedImage
      3. Using the RescaleOp Class
      4. Using the AffineTransformOp Class
    4. Saving Image Data
    5. Simple Audio
    6. Java Media Framework
  24. 22. JavaBeans
    1. What’s a Bean?
      1. What Constitutes a Bean?
    2. The NetBeans IDE
      1. Installing and Running NetBeans
    3. Properties and Customizers
    4. Event Hookups and Adapters
      1. Taming the Juggler
      2. Molecular Motion
    5. Binding Properties
      1. Constraining Properties
    6. Building Beans
      1. The Dial Bean
      2. Design Patterns for Properties
    7. Limitations of Visual Design
    8. Serialization Versus Code Generation
    9. Customizing with BeanInfo
      1. Getting Properties Information
    10. Handcoding with Beans
      1. Bean Instantiation and Type Management
      2. Working with Serialized Beans
      3. Runtime Event Hookups with Reflection
    11. BeanContext and BeanContextServices
    12. The Java Activation Framework
    13. Enterprise JavaBeans and POJO-Based Enterprise Frameworks
  25. 23. Applets
    1. The Politics of Browser-Based Applications
    2. Applet Support and the Java Plug-in
    3. The JApplet Class
      1. Applet Lifecycle
      2. The Applet Security Sandbox
      3. Getting Applet Resources
      4. The <applet> Tag
      5. Attributes
      6. Parameters
      7. ¿Habla Applet?
      8. The Complete <applet> Tag
      9. Loading Class Files
      10. Packages
      11. appletviewer
    4. Java Web Start
    5. Conclusion
  26. 24. XML
    1. The Butler Did It
    2. A Bit of Background
      1. Text Versus Binary
      2. A Universal Parser
      3. The State of XML
      4. The XML APIs
      5. XML and Web Browsers
    3. XML Basics
      1. Attributes
      2. XML Documents
      3. Encoding
      4. Namespaces
      5. Validation
      6. HTML to XHTML
    4. SAX
      1. The SAX API
      2. Building a Model Using SAX
      3. XMLEncoder/Decoder
    5. DOM
      1. The DOM API
      2. Test-Driving DOM
      3. Generating XML with DOM
      4. JDOM
    6. XPath
      1. Nodes
      2. Predicates
      3. Functions
      4. The XPath API
      5. XMLGrep
    7. XInclude
      1. Enabling XInclude
    8. Validating Documents
      1. Using Document Validation
      2. DTDs
      3. XML Schema
      4. The Validation API
    9. JAXB Code Binding and Generation
      1. Annotating Our Model
      2. Generating a Java Model from an XML Schema
      3. Generating an XML Schema from a Java Model
    10. Transforming Documents with XSL/XSLT
      1. XSL Basics
      2. Transforming the Zoo Inventory
      3. XSLTransform
      4. XSL in the Browser
    11. Web Services
    12. The End of the Book
  27. A. The Eclipse IDE
    1. The IDE Wars
    2. Getting Started with Eclipse
      1. Importing the Learning Java Examples
    3. Using Eclipse
      1. Getting at the Source
      2. The Lay of the Land
      3. Running the Examples
      4. Building the Ant-Based Examples
      5. Loner Examples
    4. Eclipse Features
      1. Coding Shortcuts
      2. Autocorrection
      3. Refactoring
      4. Diffing Files
      5. Organizing Imports
      6. Formatting Source Code
    5. Conclusion
  28. B. BeanShell: Java Scripting
    1. Running BeanShell
    2. Java Statements and Expressions
      1. Imports
    3. BeanShell Commands
    4. Scripted Methods and Objects
      1. Scripting Interfaces and Adapters
    5. Changing the Classpath
    6. Learning More . . .
  29. Glossary
  30. Index
  31. About the Authors
  32. Colophon
  33. Copyright
O'Reilly logo

Arrays

An array is a special type of object that can hold an ordered collection of elements. The type of the elements of the array is called the base type of the array; the number of elements it holds is a fixed attribute called its length. Java supports arrays of all primitive and reference types.

The basic syntax of arrays looks much like that of C or C++. We create an array of a specified length and access the elements with the index operator, []. Unlike other languages, however, arrays in Java are true, first-class objects. An array is an instance of a special Java array class and has a corresponding type in the type system. This means that to use an array, as with any other object, we first declare a variable of the appropriate type and then use the new operator to create an instance of it.

Array objects differ from other objects in Java in three respects:

  • Java implicitly creates a special array class type for us whenever we declare a new type of array. It’s not strictly necessary to know about this process in order to use arrays, but it helps in understanding their structure and their relationship to other objects in Java later.

  • Java lets us use the [] operator to access array elements so that arrays look as we expect. We could implement our own classes that act like arrays, but we would have to settle for having methods such as get() and set() instead of using the special [] notation.

  • Java provides a corresponding special form of the new operator that lets us construct an instance of an array with a specified length with the [] notation or initialize it directly from a structured list of values.

Array Types

An array type variable is denoted by a base type followed by the empty brackets, []. Alternatively, Java accepts a C-style declaration with the brackets placed after the array name.

The following are equivalent:

    int [] arrayOfInts;  // preferred
    int arrayOfInts [];  // C-style

In each case, arrayOfInts is declared as an array of integers. The size of the array is not yet an issue because we are declaring only the array type variable. We have not yet created an actual instance of the array class, with its associated storage. It’s not even possible to specify the length of an array when declaring an array type variable. The size is strictly a function of the array object itself, not the reference to it.

An array of reference types can be created in the same way:

    String [] someStrings;
    Button [] someButtons;

Array Creation and Initialization

The new operator is used to create an instance of an array. After the new operator, we specify the base type of the array and its length with a bracketed integer expression:

    arrayOfInts = new int [42];
    someStrings = new String [ number + 2 ];

We can, of course, combine the steps of declaring and allocating the array:

    double [] someNumbers = new double [20];
    Component [] widgets = new Component [12];

Array indices start with zero. Thus, the first element of someNumbers[] is 0, and the last element is 19. After creation, the array elements are initialized to the default values for their type. For numeric types, this means the elements are initially zero:

    int [] grades = new int [30];
    grades[0] = 99;
    grades[1] = 72;
    // grades[2] == 0

The elements of an array of objects are references to the objects—just like individual variables they point to—but do not actually contain instances of the objects. The default value of each element is therefore null until we assign instances of appropriate objects:

    String names [] = new String [4];
    names [0] = new String();
    names [1] = "Boofa";
    names [2] = someObject.toString();
    // names[3] == null

This is an important distinction that can cause confusion. In many other languages, the act of creating an array is the same as allocating storage for its elements. In Java, a newly allocated array of objects actually contains only reference variables, each with the value null.[11] That’s not to say that there is no memory associated with an empty array; memory is needed to hold those references (the empty “slots” in the array). Figure 4-4 illustrates the names array of the previous example.

A Java array

Figure 4-4. A Java array

names is a variable of type String[] (i.e., a string array). This particular String[] object contains four String type variables. We have assigned String objects to the first three array elements. The fourth has the default value null.

Java supports the C-style curly braces {} construct for creating an array and initializing its elements:

    int [] primes = { 2, 3, 5, 7, 7+4 };    // e.g., primes[2] = 5

An array object of the proper type and length is implicitly created, and the values of the comma-separated list of expressions are assigned to its elements. Note that we did not use the new keyword or the array type here. The type of the array was inferred from the assignment.

We can use the {} syntax with an array of objects. In this case, each expression must evaluate to an object that can be assigned to a variable of the base type of the array or the value null. Here are some examples:

    String [] verbs = { "run", "jump", someWord.toString() };
    Button [] controls = { stopButton, new Button("Forwards"),
        new Button("Backwards") };
    // All types are subtypes of Object
    Object [] objects = { stopButton, "A word", null };

The following are equivalent:

    Button [] threeButtons = new Button [3];
    Button [] threeButtons = { null, null, null };

Using Arrays

The size of an array object is available in the public variable length:

    char [] alphabet = new char [26];
    int alphaLen = alphabet.length;             // alphaLen == 26

    String [] musketeers = { "one", "two", "three" };
    int num = musketeers.length;                // num == 3

length is the only accessible field of an array; it is a variable, not a method. (Don’t worry; the compiler tells you when you accidentally use parentheses as if it were a method, as everyone does now and then.)

Array access in Java is just like array access in other languages; you access an element by putting an integer-valued expression between brackets after the name of the array. The following example creates an array of Button objects called keyPad and then fills the array with Button objects:

    Button [] keyPad = new Button [ 10 ];
    for ( int i=0; i < keyPad.length; i++ )
       keyPad[ i ] = new Button( Integer.toString( i ) );

Remember that we can also use the enhanced for loop to iterate over array values. Here we’ll use it to print all the values we just assigned:

    for (Button b : keyPad)
        System.out.println(b);

Attempting to access an element that is outside the range of the array generates an ArrayIndexOutOfBoundsException. This is a type of RuntimeException, so you can either catch and handle it yourself if you really expect it, or ignore it, as we’ve already discussed:

    String [] states = new String [50];

    try {
        states[0] = "California";
        states[1] = "Oregon";
        ...
        states[50] = "McDonald's Land";  // Error: array out of bounds
    }
    catch ( ArrayIndexOutOfBoundsException err ) {
        System.out.println( "Handled error: " + err.getMessage() );
    }

It’s a common task to copy a range of elements from one array into another. One way to copy arrays is to use the low-level arraycopy() method of the System class:

    System.arraycopy( source, sourceStart, destination, destStart, length );

The following example doubles the size of the names array from an earlier example:

    String [] tmpVar = new String [ 2 * names.length ];
    System.arraycopy( names, 0, tmpVar, 0, names.length );
    names = tmpVar;

A new array, twice the size of names, is allocated and assigned to a temporary variable, tmpVar. The arraycopy() method is then used to copy the elements of names to the new array. Finally, the new array is assigned to names. If there are no remaining references to the old array object after names has been copied, it is garbage-collected on the next pass.

An easier way is to use the java.util.ArrayscopyOf() and copyOfRange() methods:

        byte [] bar = new byte[] { 1, 2, 3, 4, 5 };

        byte [] barCopy = Arrays.copyOf( bar, bar.length ); 
            // { 1, 2, 3, 4, 5 }
        byte [] expanded = Arrays.copyOf( bar, bar.length+2 );
            // { 1, 2, 3, 4, 5, 0, 0 }
        
        byte [] firstThree = Arrays.copyOfRange( bar, 0, 3 );
            // { 1, 2, 3 }
        byte [] lastThree = Arrays.copyOfRange( bar, 2, bar.length );
            // { 3, 4, 5 }
        byte [] lastThreePlusTwo = Arrays.copyOfRange( bar, 2, bar.length+2 );
            // { 3, 4, 5, 0, 0 }

The copyOf() method takes the original array and a target length. If the target length is larger than the original array length, then the new array is padded (with zeros or nulls) to the desired length. The copyOfRange() takes a starting index (inclusive) and an ending index (exclusive) and a desired length, which will also be padded if necessary.

Anonymous Arrays

Often it is convenient to create “throwaway” arrays, arrays that are used in one place and never referenced anywhere else. Such arrays don’t need a name because you never need to refer to them again in that context. For example, you may want to create a collection of objects to pass as an argument to some method. It’s easy enough to create a normal, named array, but if you don’t actually work with the array (if you use the array only as a holder for some collection), you shouldn’t need to do this. Java makes it easy to create “anonymous” (i.e., unnamed) arrays.

Let’s say you need to call a method named setPets(), which takes an array of Animal objects as arguments. Provided Cat and Dog are subclasses of Animal, here’s how to call setPets() using an anonymous array:

    Dog pokey = new Dog ("gray");
    Cat boojum = new Cat ("grey");
    Cat simon = new Cat ("orange");
    setPets ( new Animal [] { pokey, boojum, simon });

The syntax looks similar to the initialization of an array in a variable declaration. We implicitly define the size of the array and fill in its elements using the curly-brace notation. However, because this is not a variable declaration, we have to explicitly use the new operator and the array type to create the array object.

Anonymous arrays were sometimes used as a substitute for variable-length argument lists to methods, which are discussed in Chapter 5. With the introduction of variable-length argument lists in Java, the usefulness of anonymous arrays has diminished.

Multidimensional Arrays

Java supports multidimensional arrays in the form of arrays of array type objects. You create a multidimensional array with C-like syntax, using multiple bracket pairs, one for each dimension. You also use this syntax to access elements at various positions within the array. Here’s an example of a multidimensional array that represents a chess board:

    ChessPiece [][] chessBoard;
    chessBoard = new ChessPiece [8][8];
    chessBoard[0][0] = new ChessPiece.Rook;
    chessBoard[1][0] = new ChessPiece.Pawn;
    ...

Here, chessBoard is declared as a variable of type ChessPiece[][] (i.e., an array of ChessPiece arrays). This declaration implicitly creates the type ChessPiece[] as well. The example illustrates the special form of the new operator used to create a multidimensional array. It creates an array of ChessPiece[] objects and then, in turn, makes each element into an array of ChessPiece objects. We then index chessBoard to specify values for particular ChessPiece elements. (We’ll neglect the color of the pieces here.)

Of course, you can create arrays with more than two dimensions. Here’s a slightly impractical example:

    Color [][][] rgbCube = new Color [256][256][256];
    rgbCube[0][0][0] = Color.black;
    rgbCube[255][255][0] = Color.yellow;
    ...

We can specify a partial index of a multidimensional array to get a subarray of array type objects with fewer dimensions. In our example, the variable chessBoard is of type ChessPiece[][]. The expression chessBoard[0] is valid and refers to the first element of chessBoard, which, in Java, is of type ChessPiece[]. For example, we can populate our chess board one row at a time:

    ChessPiece [] homeRow =  {
        new ChessPiece("Rook"), new ChessPiece("Knight"),
        new ChessPiece("Bishop"), new ChessPiece("King"),
        new ChessPiece("Queen"), new ChessPiece("Bishop"),
        new ChessPiece("Knight"), new ChessPiece("Rook")
    };

    chessBoard[0] = homeRow;

We don’t necessarily have to specify the dimension sizes of a multidimensional array with a single new operation. The syntax of the new operator lets us leave the sizes of some dimensions unspecified. The size of at least the first dimension (the most significant dimension of the array) has to be specified, but the sizes of any number of trailing, less significant array dimensions may be left undefined. We can assign appropriate array-type values later.

We can create a checkerboard of Boolean values (which is not quite sufficient for a real game of checkers either) using this technique:

    boolean [][] checkerBoard;
    checkerBoard = new boolean [8][];

Here, checkerBoard is declared and created, but its elements, the eight boolean[] objects of the next level, are left empty. Thus, for example, checkerBoard[0] is null until we explicitly create an array and assign it, as follows:

    checkerBoard[0] = new boolean [8];
    checkerBoard[1] = new boolean [8];
    ...
    checkerBoard[7] = new boolean [8];

The code of the previous two examples is equivalent to:

    boolean [][] checkerBoard = new boolean [8][8];

One reason we might want to leave dimensions of an array unspecified is so that we can store arrays given to us by another method.

Note that because the length of the array is not part of its type, the arrays in the checkerboard do not necessarily have to be of the same length; that is, multidimensional arrays don’t have to be rectangular. Here’s a defective (but perfectly legal in Java) checkerboard:

    checkerBoard[2] = new boolean [3];
    checkerBoard[3] = new boolean [10];

And here’s how you could create and initialize a triangular array:

    int [][] triangle = new int [5][];
    for (int i = 0; i < triangle.length; i++) {
        triangle[i] = new int [i + 1];
        for (int j = 0; j < i + 1; j++)
            triangle[i][j] = i + j;
    }

Inside Arrays

We said earlier that arrays are instances of special array classes in the Java language. If arrays have classes, where do they fit into the class hierarchy and how are they related? These are good questions, but we need to talk more about the object-oriented aspects of Java before answering them. That’s the subject of the next chapter. For now, take it on faith that arrays fit into the class hierarchy.



[11] The analog in C or C++ is an array of pointers to objects. However, pointers in C or C++ are themselves two- or four-byte values. Allocating an array of pointers is, in actuality, allocating the storage for some number of those pointer objects. An array of references is conceptually similar, although references are not themselves objects. We can’t manipulate references or parts of references other than by assignment, and their storage requirements (or lack thereof) are not part of the high-level Java language specification.

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