Sams Teach Yourself Java 2 in 24 Hours

By Rogers Cadenhead

• Table of Contents
  • Copyright
  • About the Author
  • About the Technical Editor
  • Acknowledgments
  • We Want to Hear from You!
  • Reader Services
  • Introduction
  • Hour 1. Becoming a Programmer
  • Hour 2. Writing Your First Program
  • Hour 3. Vacationing in Java
  • Hour 4. Understanding How Java Programs Work
  • Part II: Learning the Basics of Programming
  • Hour 5. Storing and Changing Information in a Program
  • Hour 6. Using Strings to Communicate
  • Hour 7. Using Conditional Tests to Make Decisions
  • Hour 8. Repeating an Action with Loops
  • Part III: Working with Information in New Ways
  • Hour 9. Storing Information with Arrays
  • Hour 10. Creating Your First Object
  • Hour 11. Describing What Your Object Is Like
  • Hour 12. Making the Most of Existing Objects
  • Part IV: Programming a Graphical User Interface
  • Hour 13. Building a Simple User Interface
  • Hour 14. Laying Out a User Interface
  • Hour 15. Responding to User Input
    • Getting Your Programs to Listen
    • Setting Up Components to Be Heard
    • Handling User Events
    • Workshop: A Little Lotto Madness
    • Summary
    • Q&A
    • Quiz
    • Activities
  • Hour 16. Building a Complex User Interface
  • Part V: Creating Multimedia Programs
  • Hour 17. Creating Interactive Web Programs
  • Hour 18. Handling Errors in a Program
  • Hour 19. Creating a Threaded Program
  • Hour 20. Reading and Writing Files
  • Part VI: Creating Multimedia Programs
  • Hour 21. Using Fonts and Color
  • Hour 22. Playing Sound Files
  • Hour 23. Working with Graphics
  • Hour 24. Creating Animation
  • Part VII: Appendixes
  • Appendix A. Tackling New Features of Java 2 Version 1.4
  • Appendix B. Using the Java 2 Software Development Kit
  • Appendix C. Programming with the Java 2 Software Development Kit
  • Appendix D. Using Sun ONE Studio
  • Appendix E. Where to Go from Here: Java Resources
  • Appendix F. This Book's Web Site

Getting Your Programs to Listen

Before you can receive user input in a Java program, you must learn how to teach an object to listen. Responding to user events in a Java program requires the use of one or more EventListener interfaces. Interfaces are special classes that enable a class of objects to inherit behavior it would not be able to use otherwise.

Adding an EventListener interface requires two things right away. First, because the listening classes are part of the java.awt.event group of classes, you must make them available with the following statement:

import java.awt.event.*; 

Second, the class must use the implements statement to declare that it will be using one or more listening interfaces. The following statement creates a class that uses ActionListener, an interface used with buttons and other components:

public class Graph extends javax.swing.JApplet implements ActionListener { 

EventListener interfaces enable a component of a graphical user interface to generate user events. Without one of the listeners in place, a component cannot do anything that can be heard by other parts of a program. A program must include a listener interface for each type of component to which it wants to listen. To have the program respond to a mouse-click on a button or the Enter key being pressed in a text field, you must include the ActionListener interface. To respond to the use of a choice list or check boxes, You need the ItemListener interface to respond to the use of a choice list or check boxes.

When you require more than one interface in the same class, separate their names with commas after the implements statement. The following is an example:

public class Graph3D extends javax.swing.JApplet 
    implements ActionListener, MouseListener { 
    // ... 
} 

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