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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

Focus Navigation

We’ve brought up the topic of focus many times in our discussion so far, and we’ve told you that the handling and user navigation of focus is mostly done automatically. The focus system is very powerful and can be heavily customized through the use of “focus traversal policy” objects that control keyboard navigation. For typical application behavior, you won’t have to deal with this directly, but we’ll explain a few features you should know about.

Swing handles keyboard focus navigation through the KeyboardFocusManager class. This class uses FocusTraversalPolicy “strategy” objects that implement the actual schemes for locating the next component to receive focus. There are two primary FocusTraversalPolicy types supplied with Java. The first, DefaultFocusTraversalPolicy, is part of the AWT package. It emulates the legacy AWT-style focus management that navigated components in the order in which they were added to their container. The next, LayoutFocusTraversalPolicy, is the default for all Swing applications. It examines the layout and attempts to provide the more typical navigation from left to right and top to bottom, based on component position and size.

The focus traversal policy is inherited from containers and oriented around groups of components known as “root cycles.” By default, each individual window and JInternalFrame is its own root cycle. In other words, focus traverses all of its child components repeatedly (jumping from the last component back to the first), and won’t, by default, leave the container through keyboard navigation.

The default Swing policy uses the following keys for keyboard navigation:


Tab or Ctrl-Tab (Ctrl-Tab also works inside text areas)


Shift-Tab or Ctrl-Shift-Tab (Ctrl-Shift-Tab also works inside text areas)

You can define your own focus traversal keys for forward and back navigation, as well as for navigation across root cycles using the setFocusTraversalKeys() method of a container. Here is an example that adds the keystroke Ctrl-N to the list of forward key navigation for components in a Frame:

        KeyboardFocusManager.FORWARD_TRAVERSAL_KEYS );
    AWTKeyStroke ks = AWTKeyStroke.getAWTKeyStroke(
        KeyEvent.VK_N, InputEvent.CTRL_DOWN_MASK );
    Set new = new HashSet( old );
    set.add( ks );

Keys are defined by the AWTKeyStroke class, which encapsulates the key and input modifiers—in this case, the Control key. Constants in the KeyboardFocusManager specify forward, back, and up or down root cycle transfer across windows.

Finally, you can also move focus programmatically using the following methods of KeyboardFocusManager:



One of Swing’s advanced components is JTree. Trees are good for representing hierarchical information, like the contents of a disk drive or a company’s organizational chart. As with all Swing components, the data model is distinct from the visual representation. This means you can do things such as update the data model and trust that the visual component will be updated properly.

JTree is powerful and complex. It’s big enough, in fact, that like the text tools, the classes that support JTree have their own package, javax.swing.tree. However, if you accept the default options for almost everything, JTree is very easy to use. Figure 18-6 shows a JTree running in a Swing application that we’ll describe later.

The JTree class in action

Figure 18-6. The JTree class in action

Nodes and Models

A tree’s data model is made up of interconnected nodes. A node has a name—typically, a parent—and some number of children (possibly 0). In Swing, a node is represented by the TreeNode interface. Nodes that can be modified are represented by MutableTreeNode. A concrete implementation of this interface is DefaultMutableTreeNode. One node, called the root node, usually resides at the top of the hierarchy.

A tree’s data model is represented by the TreeModel interface. Swing provides an implementation of this interface called DefaultTreeModel. You can create a DefaultTreeModel by passing a root TreeNode to its constructor.

You could create a TreeModel with just one node like this:

    TreeNode root = new DefaultMutableTreeNode("Root node");
    TreeModel model = new DefaultTreeModel(root);

Here’s another example with a real hierarchy. The root node contains two nodes, Node 1 and Group. The Group node contains Node 2 and Node 3 as subnodes.

    MutableTreeNode root = new DefaultMutableTreeNode("Root node");
    MutableTreeNode group = new DefaultMutableTreeNode("Group");
    root.insert(group, 0);
    root.insert(new DefaultMutableTreeNode("Node 1"), 1);
    group.insert(new DefaultMutableTreeNode("Node 2"), 0);
    group.insert(new DefaultMutableTreeNode("Node 3"), 1);

The second parameter to the insert() method is the index of the node in the parent. After you organize your nodes, you can create a TreeModel in the same way as before:

    TreeModel model = new DefaultTreeModel(root);

Save a Tree

Once you have a tree model, creating a JTree is simple:

    JTree tree = new JTree(model);

The JTree behaves like a souped-up JList. As Figure 18-6 shows, the JTree automatically shows nodes with no children as a sheet of paper, while nodes that contain other nodes are shown as folders. You can expand and collapse nodes by clicking on the little knobs to the left of the folder icons. You can also expand and collapse nodes by double-clicking on them. You can select nodes; multiple selections are possible using the Shift and Control keys. And, as with a JList, you should put a JTree in a JScrollPane if you want it to scroll.

Tree Events

A tree fires off several flavors of events. You can find out when nodes have been expanded and collapsed, when nodes are about to be expanded or collapsed (because the user has clicked on them), and when selections occur. Three distinct event listener interfaces handle this information.


Tree selections are a tricky business. You can select any combination of nodes by using the Control key and clicking on nodes. Tree selections are described by a TreePath, which describes how to get from the root node to the selected nodes.

The following example registers an event listener that prints out the last selected node:

    tree.addTreeSelectionListener(new TreeSelectionListener() {
      public void valueChanged(TreeSelectionEvent e) {
        TreePath tp = e.getNewLeadSelectionPath();

A Complete Example

This section contains an example that showcases the following tree techniques:

  • Construction of a tree model, using DefaultMutableTreeNode

  • Creation and display of a JTree

  • Listening for tree selection events

  • Modifying the tree’s data model while the JTree is showing

Here’s the source code for the example:

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

    public class PartsTree {
      public static void main(String[] args) {
        // create a hierarchy of nodes
        MutableTreeNode root = new DefaultMutableTreeNode("Parts");
        MutableTreeNode beams = new DefaultMutableTreeNode("Beams");
        MutableTreeNode gears = new DefaultMutableTreeNode("Gears");
        root.insert(beams, 0);
        root.insert(gears, 1);
        beams.insert(new DefaultMutableTreeNode("1x4 black"), 0);
        beams.insert(new DefaultMutableTreeNode("1x6 black"), 1);
        beams.insert(new DefaultMutableTreeNode("1x8 black"), 2);
        beams.insert(new DefaultMutableTreeNode("1x12 black"), 3);
        gears.insert(new DefaultMutableTreeNode("8t"), 0);
        gears.insert(new DefaultMutableTreeNode("24t"), 1);
        gears.insert(new DefaultMutableTreeNode("40t"), 2);
        gears.insert(new DefaultMutableTreeNode("worm"), 3);
        gears.insert(new DefaultMutableTreeNode("crown"), 4);

        // create the JTree
        final DefaultTreeModel model = new DefaultTreeModel(root);
        final JTree tree = new JTree(model);

        // create a text field and button to modify the data model
        final JTextField nameField = new JTextField("16t");
        final JButton button = new JButton("Add a part");
        button.addActionListener(new ActionListener() {
          public void actionPerformed(ActionEvent e) {
            TreePath tp = tree.getSelectionPath();
            MutableTreeNode insertNode =
            int insertIndex = 0;
            if (insertNode.getParent() != null) {
              MutableTreeNode parent =
              insertIndex = parent.getIndex(insertNode) + 1;
              insertNode = parent;
            MutableTreeNode node =
                new DefaultMutableTreeNode(nameField.getText());
            model.insertNodeInto(node, insertNode, insertIndex);
        JPanel addPanel = new JPanel(new GridLayout(2, 1));

        // listen for selections
        tree.addTreeSelectionListener(new TreeSelectionListener() {
          public void valueChanged(TreeSelectionEvent e) {
            TreePath tp = e.getNewLeadSelectionPath();
            button.setEnabled(tp != null);

        // create a JFrame to hold the tree
        JFrame frame = new JFrame("PartsTree v1.0");

        frame.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
        frame.setSize(200, 200);
        frame.add(new JScrollPane(tree));
        frame.add(addPanel, BorderLayout.SOUTH);

The example begins by creating a node hierarchy. The root node is called Parts. It contains two subnodes named Beams and Gears, as shown:

    MutableTreeNode root = new DefaultMutableTreeNode("Parts");
    MutableTreeNode beams = new DefaultMutableTreeNode("Beams");
    MutableTreeNode gears = new DefaultMutableTreeNode("Gears");
    root.insert(beams, 0);
    root.insert(gears, 1);

The Beams and Gears nodes contain a handful of items each.

The “Add a part” button inserts a new item into the tree at the level of the current node, and just after it. You can specify the name of the new node by typing it in the text field above the button. To determine where the node should be added, the current selection is first obtained in the anonymous inner class ActionListener:

    TreePath tp = tree.getSelectionPath();
    MutableTreeNode insertNode =

The new node should be added to the parent node of the current node, so it ends up being a sibling of the current node. The only hitch here is that if the current node is the root node, it won’t have a parent. If a parent does exist, we determine the index of the currently selected node, and then add the new node at the next index:

    int insertIndex = 0;
    if (insertNode.getParent() != null) {
      MutableTreeNode parent =
      insertIndex = parent.getIndex(insertNode) + 1;
      insertNode = parent;
    MutableTreeNode node =
        new DefaultMutableTreeNode(nameField.getText());
    model.insertNodeInto(node, insertNode, insertIndex);

You must add the new node to the tree’s data model using insertNodeInto()—not to the MutableTableNode itself. The model notifies the JTree that it needs to update itself.

We have another event handler in this example, one that listens for tree selection events. Basically, we want to enable our “Add a part” button only if a current selection exists:

    tree.addTreeSelectionListener(new TreeSelectionListener() {
      public void valueChanged(TreeSelectionEvent e) {
        TreePath tp = e.getNewLeadSelectionPath();
        button.setEnabled(tp != null);

When you first start this application, the button is disabled. As soon as you select something, it is enabled, and you can add nodes to the tree with abandon. If you want to see the button disabled again, you can unselect everything by holding the Control key and clicking on the current selection.

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