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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
      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 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

HelloJava3: The Button Strikes!

Now we can move on to some fun stuff. HelloJava3 brings us a new graphical interface component: JButton.[4] In this example, we add a JButton component to our application that changes the color of our text each time the button is pressed. The draggable-message capability is still there, too. Our new code looks like this:

    import java.awt.*;
    import java.awt.event.*;
    import javax.swing.*;

    public class HelloJava3

      public static void main( String[] args ) {
        JFrame frame = new JFrame( "HelloJava3" );
        frame.add( new HelloComponent3("Hello, Java!") );
        frame.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
        frame.setSize( 300, 300 );
        frame.setVisible( true );

    class HelloComponent3 extends JComponent
        implements MouseMotionListener, ActionListener
      String theMessage;
      int messageX = 125, messageY = 95;  // Coordinates of the message

      JButton theButton;

      int colorIndex;  // Current index into someColors
      static Color[] someColors = {,,,, Color.magenta };

      public HelloComponent3( String message ) {
        theMessage = message;
        theButton = new JButton("Change Color");
        setLayout( new FlowLayout() );
        add( theButton );
        theButton.addActionListener( this );
        addMouseMotionListener( this );

      public void paintComponent( Graphics g ) {
        g.drawString( theMessage, messageX, messageY );

      public void mouseDragged( MouseEvent e ) {
        messageX = e.getX();
        messageY = e.getY();

      public void mouseMoved( MouseEvent e ) {}

      public void actionPerformed( ActionEvent e ) {
        // Did somebody push our button?
        if (e.getSource() == theButton)

      synchronized private void changeColor() {
        // Change the index to the next color, awkwardly.
        if (++colorIndex == someColors.length)
          colorIndex = 0;
        setForeground( currentColor() ); // Use the new color.

      synchronized private Color currentColor() {
        return someColors[colorIndex];

Compile HelloJava3 in the same way as the other applications. Run the example, and you should see the display shown in Figure 2-9. Drag the text. Each time you press the button, the color should change. Call your friends! Test yourself for color blindness!

The HelloJava3 application

Figure 2-9. The HelloJava3 application

What have we added this time? Well, for starters, we have a new variable:

    JButton theButton;

The theButton variable is of type JButton and is going to hold an instance of the javax.swing.JButton class. The JButton class, as you might expect, represents a graphical button, like other buttons in your windowing system.

Three additional lines in the constructor create the button and display it:

    theButton = new JButton("Change Color");
    setLayout( new FlowLayout() );
    add( theButton );

In the first line, the new keyword creates an instance of the JButton class. The next line affects the way our component will be used as a container to hold the button. It tells HelloComponent3 how it should arrange components that are added to it for display—in this case, to use a scheme called a FlowLayout (more on that coming up). Finally, it adds the button to our component, just like we added HelloComponent3 to the content pane of the JFrame in the main() method.

Method Overloading

JButton has more than one constructor. A class can have multiple constructors, each taking different parameters and presumably using them to do different kinds of setup. When a class has multiple constructors, Java chooses the correct one based on the types of arguments used with them. We call the JButton constructor with a String argument, so Java locates the constructor method of the JButton class that takes a single String argument and uses it to set up the object. This is called method overloading. All methods in Java (not just constructors) can be overloaded; this is another aspect of the object-oriented programming principle of polymorphism.

Overloaded constructors generally provide a convenient way to initialize a new object. The JButton constructor we’ve used sets the text of the button as it is created:

    theButton = new JButton("Change Color");

This is shorthand for creating the button and setting its label, like this:

    theButton = new JButton();
    theButton.setText("Change Color");


We have used the terms component and container somewhat loosely to describe graphical elements of Java applications, but these terms are used in the names of actual classes in the java.awt package.

Component is a base class from which all of Java’s GUI components are derived. It contains variables that represent the location, shape, general appearance, and status of the object as well as methods for basic painting and event handling. javax.swing.JComponent extends the base Component class and refines it for the Swing toolkit. The paintComponent() method we have been using in our example is inherited from the JComponent class. HelloComponent is a kind of JComponent and inherits all its public members, just as other GUI components do.

The JButton class is also derived from JComponent and therefore shares this functionality. This means that the developer of the JButton class had methods such as paintComponent() available with which to implement the behavior of the JButton object, just as we did when creating our example. What’s exciting is that we are perfectly free to further subclass components such as JButton and override their behavior to create our own special types of user-interface components. JButton and HelloComponent3 are, in this respect, equivalent types of things.


The Container class is an extended type of Component that maintains a list of child components and helps to group them. The Container causes its children to be displayed and arranges them on the screen according to a particular layout strategy.

Because a Container is also a Component, it can be placed alongside other Component objects in other Containers in a hierarchical fashion, as shown in Figure 2-10. Our HelloComponent3 class is a kind of Container (by virtue of the JComponent class) and can therefore hold and manage other Java components and containers, such as buttons, sliders, text fields, and panels.

Layout of Java containers (in bold) and components (in italics)

Figure 2-10. Layout of Java containers (in bold) and components (in italics)

In Figure 2-10, the italicized items are Components, and the bold items are Containers. The keypad is implemented as a container object that manages a number of keys. The keypad itself is contained in the GizmoTool container object.

Since JComponent descends from Container, it can be both a component and a container. In fact, we’ve already used it in this capacity in the HelloComponent3 example. It does its own drawing and handles events, just like a component, but it also contains a button, just like a container.


Having created a JButton object, we need to place it in the container, but where? An object called a LayoutManager determines the location within the HelloComponent3 container at which to display the JButton. A LayoutManager object embodies a particular scheme for arranging components on the screen and adjusting their sizes. There are several standard layout managers to choose from, and we can, of course, create new ones. In our case, we specify one of the standard managers, a FlowLayout . The net result is that the button is centered at the top of the HelloComponent3 container. Our JFrame has another kind of layout, called BorderLayout. You’ll learn more about layout managers in Chapter 19.

To add the button to the layout, we invoke the add() method that HelloComponent3 inherits from Container, passing the JButton object as a parameter:

    add( theButton );

add() is a method inherited by our class from the Container class. It appends our JButton to the list of components that the HelloComponent3 container manages. Thereafter, HelloComponent3 is responsible for the JButton: it causes the button to be displayed and it determines where in its window the button should be placed.

Subclassing and Subtypes

If you look up the add() method of the Container class, you’ll see that it takes a Component object as an argument. In our example, we’ve given it a JButton object. What’s going on?

As we’ve said, JButton is a subclass of the Component class. Because a subclass is a kind of its superclass and has, at minimum, the same public methods and variables, Java allows us to use an instance of a subclass anywhere we could use an instance of its superclass. JButton is a kind of Component, so any method that expects a Component as an argument will accept a JButton. The converse, however, is not true. A method signature expecting a particular class will not accept its superclass as a parameter.

More Events and Interfaces

Now that we have a JButton, we need some way to communicate with it—that is, to get the events it generates. We could just listen for mouse clicks within the button and act accordingly, but that would require customization, via subclassing of the JButton, and we would be giving up the advantages of using a pre-fab component. Instead, we have the HelloComponent3 object listen for higher-level events, corresponding to button presses. A JButton generates a special kind of event called an ActionEvent when someone clicks on it with the mouse. To receive these events, we have added another method to the HelloComponent3 class:

    public void actionPerformed( ActionEvent e ) {
      if ( e.getSource() == theButton )

If you followed the previous example, you shouldn’t be surprised to see that HelloComponent3 now declares that it implements the ActionListener interface in addition to MouseMotionListener. ActionListener requires us to implement an actionPerformed() method that is called whenever an ActionEvent occurs. You also shouldn’t be surprised to see that we added a line to the HelloComponent3 constructor, registering itself (this) as a listener for the button’s action events:

    theButton.addActionListener( this );

Note that this time, we’re registering our component as a listener with a different object—the button—whereas previously we were asking for our own events.

The actionPerformed() method takes care of any action events that arise. First, it checks to make sure that the event’s source (the component generating the event) is what we think it should be: theButton. This may seem superfluous; after all, there is only one button. What else could possibly generate an action event? In this application, nothing, but it’s a good idea to check because another application may have many buttons, and you may need to figure out which one has been clicked. Or you may add a second button to this application later, and you don’t want it to break something when you do. To check this, we call the getSource() method of the ActionEvent object, e. We then use the == operator to make sure the event source matches theButton.


In Java, == is a test for identity, not equality; it is true if the event source and theButton are the same object. The distinction between equality and identity is important. We would consider two String objects to be equal if they have the same characters in the same sequence. However, they might not be the same object. In Chapter 7, we’ll look at the equals() method, which tests for equality.

Once we establish that event e comes from the right button, we call our changeColor() method, and we’re finished.

You may wonder why we don’t have to change mouseDragged() now that we have a JButton in our application. The rationale is that the coordinates of the event are all that matter for this method. We are not particularly concerned if the event falls within an area of the screen occupied by another component. This means you can drag the text right through the JButton: try it and see! In this case, the arrangement of containers means that the button is on top of our component, so the text is dragged beneath it.

Color Commentary

To support HelloJava3’s colorful side, we have added a couple of new variables and two helpful methods. We create and initialize an array of Color objects representing the colors through which we cycle when the button is pressed. We also declare an integer variable that serves as an index into this array, specifying the position of the current color:

    int colorIndex;
    static Color[] someColors = {,,,, Color.magenta };

A number of things are going on here. First, let’s look at the Color objects we are putting into the array. Instances of the java.awt.Color class represent colors; they are used by all classes in the java.awt package that deal with basic color graphics. Notice that we are referencing variables such as and These look like examples of an object’s instance variables, but Color is not an object, it’s a class. What is the meaning of this? We’ll discuss that next.

Static Members

A class can contain variables and methods that are shared among all instances of the class. These shared members are called static variables and static methods. The most common use of static variables in a class is to hold predefined constants or unchanging objects that all the instances can use.

This approach has two advantages. One advantage is that static values are shared by all instances of the class; the same value can be seen by all instances. More importantly, static members can be accessed even if no instances of the class exist. In this example, we use the static variable without having to create an instance of the Color class.

An instance of the Color class represents a visible color. For convenience, the Color class contains some static, predefined objects with friendly names such as GREEN, RED, and (the happy color) MAGENTA. The variable GREEN, for example, is a static member in the Color class. The data type of the variable GREEN is Color. Internally, in Java-land, it is initialized like this:

    public final static Color GREEN = new Color(0, 255, 0);

The GREEN variable and the other static members of Color cannot be modified (after they’ve been initialized) so that they are effectively constants and can be optimized as such by the Java VM. The alternative to using these predefined colors is to create a color manually by specifying its red, green, and blue (RGB) components using a Color class constructor.


Next, we turn our attention to the array. We have declared a variable called someColors, which is an array of Color objects. In Java, arrays are first-class objects. This means that an array itself is a type of object—one that knows how to hold an indexed list of some other type of object. An array is indexed by integers; when you index an array, the resulting value is an object reference—that is, a reference to the object that is located in the array’s specified slot. Our code uses the colorIndex variable to index someColors. It’s also possible to have an array of simple primitive types, such as floats, rather than objects.

When we declare an array, we can initialize it using the curly brace construct. Specifying a comma-separated list of elements inside curly braces is a convenience that instructs the compiler to create an instance of the array with those elements and assign it to our variable. Alternatively, we could have just declared our someColors variable and, later, allocated an array object for it and assigned individual elements to that array’s slots. See Chapter 5 for a complete discussion of arrays.

Our Color Methods

Now we have an array of Color objects and a variable with which to index the array. Two private methods do the actual work for us. The private modifier on these methods specifies that they can be called only by other methods in the same instance of the class. They cannot be accessed outside the object that contains them. We declare members to be private to hide the detailed inner workings of a class from the outside world. This is called encapsulation and is another tenet of object-oriented design as well as good programming practice. Private methods are created as helper functions for use solely in the class implementation.

The first method, currentColor(), is simply a convenience routine that returns the Color object representing the current text color. It returns the Color object in the someColors array at the index specified by our colorIndex variable:

    synchronized private Color currentColor() {
      return someColors[colorIndex];

We could just as readily have used the expression someColors[colorIndex] everywhere we use currentColor(); however, creating methods to wrap common tasks is another way of shielding ourselves from the details of our class. In an alternative implementation, we might have shuffled off details of all color-related code into a separate class. We could have created a class that takes an array of colors in its constructor and then provides two methods: one to ask for the current color and one to cycle to the next color (just some food for thought).

The second method, changeColor(), is responsible for incrementing the colorIndex variable to point to the next Color in the array. changeColor() is called from our actionPerformed() method whenever the button is pressed:

    synchronized private void changeColor() {
        // Change the index to the next color, awkwardly.
        if ( ++colorIndex == someColors.length )
          colorIndex = 0;
        setForeground( currentColor() ); // Use the new color.

Here we increment colorIndex and compare it to the length of the someColors array. All array objects have a variable called length that specifies the number of elements in the array. If we have reached the end of the array, we wrap around to the beginning by resetting the index to 0. We’ve flagged this with a comment to indicate that we’re doing something fishy here. But we’ll come back to that in a moment. After changing the currently selected color, we do two things. First, we call the component’s setForeground() method, which changes the color used to draw text in our component. Then we call repaint() to cause the component to be redrawn with the new color for the draggable message.

What is the synchronized keyword that appears in front of our currentColor() and changeColor() methods? Synchronization has to do with threads, which we’ll examine in the next section. For now, all you need to know is that the synchronized keyword indicates that these two methods can never be running at the same time. They must always run in a mutually exclusive way.

The reason for this is related to the fishy way we increment our index. Notice that in changeColor(), we increment colorIndex before testing its value. Strictly speaking, this means that for some brief period of time while Java is running through our code, colorIndex can have a value that is past the end of our array. If our currentColor() method happened to run at that same moment, we would see a runtime “array out of bounds” error. Now, it would be easy for us to fix the problem in this case with some simple arithmetic before changing the value, but this simple example is representative of more general synchronization issues that we need to address. We’ll use it to illustrate the use of the synchronized keyword. In the next section, you’ll see that Java makes dealing with these problems relatively easy through language-level synchronization support.

[4] Why isn’t it just called a Button? Button is the name that was used in Java’s original GUI toolkit, AWT. AWT had some significant shortcomings, so it was extended and essentially replaced by Swing in Java 1.2. Since AWT already took the reasonable names, such as Button and MenuBar, and mixing them in code could be confusing, Swing user interface component names start with J, such as JButton and JMenuBar.

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