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
    • How Object-Oriented Programming Works
    • Objects in Action
    • What Objects Are
    • Understanding Inheritance
    • Building an Inheritance Hierarchy
    • Converting Objects and Simple Variables
    • Workshop: Creating an Object
    • Summary
    • Q&A
    • Quiz
    • Activities
  • 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
  • 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

Building an Inheritance Hierarchy

Inheritance, which enables a variety of related classes to be developed without a lot of redundant work, can be passed down from one class to another class to another class. This system of classes is called a class hierarchy, and all of the standard classes you can use in your Java programs are part of a hierarchy.

Understanding a hierarchy is easier if you understand what subclasses and superclasses are. A class that inherits from another class is called a subclass, and the class that is inherited from is called a superclass.

In the preceding WarGames example, the Modem class is the superclass of the ErrorCorrectionModem class, and ErrorCorrectionModem is the subclass of Modem. A class can have more than one class that inherits from it in the hierarchy—another subclass of Modem could be ISDNModem, since ISDN modems have behavior and attributes that make them different from error-correcting modems. If there were a subclass of ErrorCorrectionModem such as InternalErrorCorrectionModem, it would inherit from all classes above it in the hierarchy—both ErrorCorrectionModem and Modem. These inheritance relationships are shown in Figure 10.2.

Figure 10.2 An example of a class hierarchy.

The programs you write as you are learning about Java won't use complicated class hierarchies. However, the classes that are part of the standard Java language make full use of inheritance. Understanding it is essential to working with the classes that are part of the Java language. You'll learn more about inheritance during Hour 12, "Making the Most of Existing Objects."

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