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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

Drawing Techniques

Now that we’ve learned about the basic tools, let’s put a few of them together. In this section, we’ll look at some techniques for doing fast and flicker-free drawing and painting. If you’re interested in animation, this is for you. Drawing operations take time, and time spent drawing leads to delays and imperfect results. Our goals are to minimize the amount of drawing work we do and, as much as possible, to do that work away from the eyes of the user. To do this, we use two techniques: clipping and double buffering. Fortunately, Swing now handles double buffering by default. You won’t have to implement this logic on your own, but it’s useful to understand it.

Our first example, DragImage, illustrates some of the issues in updating a display. Like many animations, it has two parts: a constant background and a changing object in the foreground. In this case, the background is a checkerboard pattern, and the object is a small, scaled that image we can drag around on top of it, as shown in Figure 20-4:

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

    public class DragImage extends JComponent
        implements MouseMotionListener
      static int imageWidth=60, imageHeight=60;
      int grid = 10;
      int imageX, imageY;
      Image image;

      public DragImage(Image i) {
        image = i;

      public void mouseDragged(MouseEvent e) {
        imageX = e.getX();
        imageY = e.getY();
      public void mouseMoved(MouseEvent e) {}

      public void paint(Graphics g) {
        Graphics2D g2 = (Graphics2D)g;

        int w = getSize().width / grid;
        int h = getSize().height / grid;
        boolean black = false;
        for (int y = 0; y <= grid; y++)
          for (int x = 0; x <= grid; x++) {
            g2.setPaint(black ? : Color.white);
            black = !black;
            g2.fillRect(x * w, y * h, w, h);
        g2.drawImage(image, imageX, imageY, this);

      public static void main(String[] args) {
        String imageFile = "L1-Light.jpg";
        if (args.length > 0)
          imageFile = args[0];

        // Turn off double buffering

        Image image = Toolkit.getDefaultToolkit().getImage(
        image = image.getScaledInstance(
        JFrame frame = new JFrame("DragImage");
        frame.add( new DragImage(image) );
        frame.setSize(300, 300);
        frame.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
The DragImage application

Figure 20-4. The DragImage application

Run the application, optionally specifying an image file as a command-line argument. Then try dragging the image around on the pattern.

DragImage is a custom component that overrides the JComponent paint() method to do its drawing. In the main() method, we load the image and prescale it to improve performance. We then create the DragImage component and place it in the content pane. As the mouse is dragged, DragImage keeps track of its most recent position in two instance variables, imageX and imageY. On each call to mouseDragged(), the coordinates are updated, and repaint() is called to ask that the display be updated. When paint() is called, it looks at some parameters, draws the checkerboard pattern to fill the applet’s area and finally paints the small version of the image at the latest coordinates.

Now for a few arcane details about differences between JComponent and a plain AWT Component. First, the default JComponent update() method simply calls our paint() method. Prior to Java 1.4, the AWT Component class’s default update() method first cleared the screen area using a clearRect() call before calling paint. Remember that the difference between paint() and update() is that paint() draws the entire area and update() assumes the screen region is intact from the last draw. In AWT, update() was overly conservative; in Swing, it’s more optimistic. This is noteworthy if you are working with an older AWT-based application. In that case, you can simply override update() to call paint().

A more important difference between AWT and Swing is that Swing components by default perform double buffering of the output of their paint() method.

Double Buffering

Double buffering means that instead of drawing directly on the screen, Swing first performs drawing operations in an offscreen buffer and then copies the completed work to the display in a single painting operation, as shown in Figure 20-5. It takes the same amount of time to do the drawing work, but once it’s done, double buffering instantaneously updates our display so that the user does not perceive any flickering or progressively rendered output.

You’ll see how to implement this technique yourself when we use an offscreen buffer later in this chapter. However, Swing does this kind of double buffering for you whenever you use a Swing component in a Swing container. AWT components do not have automatic double buffering capability.

It is interesting to take our example and turn off double buffering to see the effect. Each Swing JComponent has a method called setDoubleBuffered() that can be set to false in order to disable the technique. Or you can disable it for all components using a call to the Swing RepaintManager, as we’ve indicated in comments in the example. Try uncommenting that line of DragImage and observe the difference in appearance.

Double buffering

Figure 20-5. Double buffering

The difference is most dramatic when you are using a slow system or performing complex drawing operations. Double buffering eliminates all of the flickering. However, on a slow system, it can decrease performance noticeably. In extreme cases (such as a game), it may be beneficial to provide an option to disable double buffering.

Our example is pretty fast, but we’re still doing some wasted drawing. Most of the background stays the same each time it’s painted. You might try to make paint() smarter, so that it wouldn’t redraw these areas, but remember that paint() has to be able to draw the entire scene because it might be called in situations when the display isn’t intact. The solution is to draw only part of the picture whenever the mouse moves. Next, we’ll talk about clipping.

Limiting Drawing with Clipping

Whenever the mouse is dragged, DragImage responds by updating its coordinates and calling repaint(). But repaint() by default causes the entire component to be redrawn. Most of this drawing is unnecessary. It turns out that there’s another version of repaint() that lets you specify a rectangular area that should be drawn—in essence, a clipping region.

Why does it help to restrict the drawing area? Foremost, drawing operations that fall outside the clipping region are not displayed. If a drawing operation overlaps the clipping region, we see only the part that’s inside. A second effect is that, in a good implementation, the graphics context can recognize drawing operations that fall completely outside the clipping region and ignore them altogether. Eliminating unnecessary operations can save time if we’re doing something complex, such as filling a bunch of polygons. This doesn’t save the time our application spends calling the drawing methods, but the overhead of calling these kinds of drawing methods is usually negligible compared to the time it takes to execute them. (If we were generating an image pixel by pixel, this would not be the case, as the calculations would be the major time sink, not the drawing.)

So we can save some time in our application by redrawing only the affected portion of the display. We can pick the smallest rectangular area that includes both the old image position and the new image position, as shown in Figure 20-6. This is the only portion of the display that really needs to change; everything else stays the same.

Determining the clipping region

Figure 20-6. Determining the clipping region

A smarter algorithm could save even more time by redrawing only those regions that have changed. However, the simple clipping strategy we’ve implemented here can be applied to many kinds of drawing and gives good performance, particularly if the area being changed is small.

One important thing to note is that, in addition to looking at the new position, our updating operation now has to remember the last position at which the image was drawn. Let’s fix our application so it will use a specified clipping region. To keep this short and emphasize the changes, we’ll take some liberties with design and make our next example a subclass of DragImage. Let’s call it ClippedDragImage.

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

    public class ClippedDragImage extends DragImage {
      int oldX, oldY;

      public ClippedDragImage( Image i ) { super(i); }

      public void mouseDragged(MouseEvent e) {
        imageX = e.getX();
        imageY = e.getY();
        Rectangle r = getAffectedArea(
            oldX, oldY, imageX, imageY, imageWidth, imageHeight);
        repaint(r);  // repaint just the affected part of the component
        oldX = imageX;
        oldY = imageY;

      private Rectangle getAffectedArea(
        int oldx, int oldy, int newx, int newy, int width, int height)
        int x = Math.min(oldx, newx);
        int y = Math.min(oldy, newy);
        int w = (Math.max(oldx, newx) + width) - x;
        int h = (Math.max(oldy, newy) + height) - y;
        return new Rectangle(x, y, w, h);

      public static void main(String[] args) {
        String imageFile = "L1-Light.jpg";
        if (args.length > 0)
          imageFile = args[0];

        // Turn off double buffering

        Image image = Toolkit.getDefaultToolkit().getImage(
        image = image.getScaledInstance(
        JFrame frame = new JFrame("ClippedDragImage");
        frame.add( new ClippedDragImage(image) );
        frame.setSize(300, 300);
        frame.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );

You may or may not find that ClippedDragImage is significantly faster. Modern desktop computers are so fast that this kind of operation is child’s play for them. However, the fundamental technique is important and applicable to more sophisticated applications.

What have we changed? First, we’ve overridden mouseDragged() so that instead of setting the current coordinates of the image, it figures out the area that has changed by using a new private method. getAffectedArea() takes the new and old coordinates and the width and height of the image as arguments. It determines the bounding rectangle as shown in Figure 20-6, then calls repaint() to draw only the affected area of the screen. mouseDragged() also saves the current position by setting the oldX and oldY variables.

Try turning off double buffering on this example and compare it to the unbuffered previous example to see how much less work is being done. You probably won’t see the difference; computers are just too fast nowadays. If you were using the 2D API to do some fancy rendering, it might help a lot.

Offscreen Drawing

In addition to serving as buffers for double buffering, offscreen images are useful for saving complex, hard-to-produce, background information. We’ll look at a fun, simple example: the doodle pad. DoodlePad is a simple drawing tool that lets us scribble by dragging the mouse, as shown in Figure 20-7. It draws into an offscreen image; its paint() method simply copies the image to the display area.

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

    public class DoodlePad
      public static void main(String[] args)
        JFrame frame = new JFrame("DoodlePad");
        frame.setLayout(new BorderLayout());
        final DrawPad drawPad = new DrawPad();
        frame.add(drawPad, BorderLayout.CENTER);
        JPanel panel = new JPanel();
        JButton clearButton = new JButton("Clear");
        clearButton.addActionListener(new ActionListener() {
          public void actionPerformed(ActionEvent e) {
        frame.add(panel, BorderLayout.SOUTH);
        frame.setSize(280, 300);
        frame.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );

    } // end of class DoodlePad

    class DrawPad extends JComponent
      Image image;
      Graphics2D graphics2D;
      int currentX, currentY, oldX, oldY;

      public DrawPad() {
        addMouseListener(new MouseAdapter() {
          public void mousePressed(MouseEvent e) {
            oldX = e.getX();
            oldY = e.getY();
        addMouseMotionListener(new MouseMotionAdapter() {
          public void mouseDragged(MouseEvent e) {
            currentX = e.getX();
            currentY = e.getY();
            if (graphics2D != null)
              graphics2D.drawLine(oldX, oldY, currentX, currentY);
            oldX = currentX;
            oldY = currentY;

      public void paintComponent(Graphics g) {
        if (image == null) {
          image = createImage(getSize().width, getSize().height);
          graphics2D = (Graphics2D)image.getGraphics();
        g.drawImage(image, 0, 0, null);

      public void clear() {
        graphics2D.fillRect(0, 0, getSize().width, getSize().height);
The DoodlePad application

Figure 20-7. The DoodlePad application

Give it a try. Draw a nice moose or a sunset. We just drew a lovely cartoon of Bill Gates. If you make a mistake, hit the Clear button and start over.

The parts should be familiar by now. We made a type of JComponent called DrawPad. The new DrawPad component uses inner classes to supply handlers for the MouseListener and MouseMotionListener interfaces. We used the JComponent createImage() method to create an empty offscreen image buffer to hold our scribble. Mouse-dragging events trigger us to draw lines into the offscreen image and call repaint() to update the display. DrawPad’s paint() method does a drawImage() to copy the offscreen drawing area to the display. In this way, DrawPad saves our sketch information.

What is unusual about DrawPad is that it does some drawing outside of paint(). In this example, we want to let the user scribble with the mouse, so we should respond to every mouse movement. Therefore, we do our work, drawing to the offscreen buffer in mouseDragged() itself. As a rule, we should be careful about doing heavy work in event-handling methods because we don’t want to interfere with other tasks that the windowing system’s painting thread is performing. In this case, our line drawing option should not be a burden, and our primary concern is getting as close a coupling as possible between the mouse movement events and the sketch on the screen. A more elaborate example might push coordinates into a queue for some other drawing thread to consume, thus freeing up the event handler thread.

In addition to drawing a line as the user drags the mouse, the mouseDragged() handler maintains a pair of previous coordinates to be used as a starting point for the next line segment. The mousePressed() handler resets the previous coordinates to the current mouse position whenever the user moves the mouse. Finally, DrawPad provides a clear() method that clears the offscreen buffer and calls repaint() to update the display. The DoodlePad application ties the clear() method to an appropriately labeled button through another anonymous inner class.

What if we wanted to do something with the image after the user has finished scribbling on it? As we’ll see in the next chapter, we could get the pixel data for the image and work with that. It wouldn’t be hard to create a save facility that stores the pixel data and reproduces it later. Think about how you might go about creating a networked “bathroom wall,” where people could scribble on your web pages.

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