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
  • 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
    • Threads
    • Working with Threads
    • Starting with init()
    • Catching Errors as You Set Up URLs
    • Handling Screen Updates in the paint() Method
    • Starting the Thread
    • Handling Mouse Clicks
    • Workshop: Revolving Links
    • Summary
    • Q&A
    • Quiz
    • Questions
    • Activities
  • 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

Handling Screen Updates in the paint() Method

The paint() method of any applet is handled when the screen needs to be updated. This situation can be caused by the Web browser or operating system outside of the applet if they obscure part of an applet window or change its dimensions in some way. The paint() method can also be called manually within an applet when the screen needs to be updated.

If you put a repaint(); statement in an applet, it forces the paint() method to be handled. This statement is a way you can tell the program that you have done something that requires a screen update. For example, if you are writing an animation program and you move an image from one place to another, you need to use repaint(); so the image is shown in its new location.

The Revolve applet has a short paint() method:

public void paint(Graphics screen) { 
    Graphics2D screen2D = (Graphics2D) screen; 
    screen2D.setColor(butterscotch); 
    screen2D.fillRect(0, 0, getSize().width, getSize().height); 
    screen2D.setColor(Color.black); 
    screen2D.drawString(pageTitle[current], 5, 60); 
    screen2D.drawString("" + pageLink[current], 5, 80); 
} 

The first statement in this method creates a screen2D object that represents the area of the applet window where things such as strings can be drawn. All drawing will be done by calling the methods of this object.

The setColor() method of Graphics2D selects the color that will be used for subsequent drawing. The color is set to butterscotch before a rectangle is drawn that fills the entire applet window. Next, the color is set to black and lines of text are displayed on the screen at the (x,y) positions of (5,60) and (5,80). The first line that is displayed is an element of the pageTitle array. The second line displayed is the address of the URL object, which is stored in the pageLink array. The current variable is used to determine which elements of these arrays to display.

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