Applet Life-Cycle Methods

Now that you have created an applet, let's consider the five applet methods that are called by the applet container from the time the applet is loaded into the browser to the time that the applet is terminated by the browser. These methods correspond to various aspects of an applet's life cycle. Figure 20.9 lists these methods, which are inherited into your applet classes from class JApplet. The table specifies when each method gets called and explains its purpose. Other than method paint, these methods have empty bodies by default. If you would like to declare any of these methods in your applets and have the applet container call them, you must use the method headers shown in Fig. 20.9. If you modify the method headers (e.g., by changing the method names or by providing additional parameters), the applet container will not call your methods. Instead, it will call the superclass methods inherited from JApplet.

Figure 20.9. JApplet life cycle methods that are called by an applet container during an applet's execution.

Method

When the method is called and its purpose

public void init()

 

Called once by the applet container when an applet is loaded for execution. This method initializes an applet. Typical actions performed here are initializing fields, creating GUI components, loading sounds to play, loading images to display (see Chapter 20, Multimedia: Applets and Applications) and creating threads (see Chapter 23, Multithreading).

public void start()

 

Called by the applet container after method init completes execution. In addition, if the user browses to another Web site and later returns to the applet's HTML page, method start is called again. The method performs any tasks that must be completed when the applet is loaded for the first time and that must be performed every time the applet's HTML page is revisited. Actions performed here might include starting an animation (see Chapter 21) or starting other threads of execution (see Chapter 23).

public void paint( Graphics g )

 

Called by the applet container after methods init and start. Method paint is also called when the applet needs to be repainted. For example, if the user covers the applet with another open window on the screen and later uncovers the applet, the paint method is called. Typical actions performed here involve drawing with the Graphics object g that is passed to the paint method by the applet container.

public void stop()

 

This method is called by the applet container when the user leaves the applet's Web page by browsing to another Web page. Since it is possible that the user might return to the Web page containing the applet, method stop performs tasks that might be required to suspend the applet's execution, so that the applet does not use computer processing time when it is not displayed on the screen. Typical actions performed here would stop the execution of animations and threads.

public void destroy()

 

This method is called by the applet container when the applet is being removed from memory. This occurs when the user exits the browsing session by closing all the browser windows and may also occur at the browser's discretion when the user has browsed to other Web pages. The method performs any tasks that are required to clean up resources allocated to the applet.

Common Programming Error 20.2

Declaring methods init, start, paint, stop or destroy with method headers that differ from those shown in Figure 20.9 results in methods that will not be called by the applet container. The code specified in your versions of the methods will not execute.


Introduction to Computers, the Internet and the World Wide Web

Introduction to Java Applications

Introduction to Classes and Objects

Control Statements: Part I

Control Statements: Part 2

Methods: A Deeper Look

Arrays

Classes and Objects: A Deeper Look

Object-Oriented Programming: Inheritance

Object-Oriented Programming: Polymorphism

GUI Components: Part 1

Graphics and Java 2D™

Exception Handling

Files and Streams

Recursion

Searching and Sorting

Data Structures

Generics

Collections

Introduction to Java Applets

Multimedia: Applets and Applications

GUI Components: Part 2

Multithreading

Networking

Accessing Databases with JDBC

Servlets

JavaServer Pages (JSP)

Formatted Output

Strings, Characters and Regular Expressions

Appendix A. Operator Precedence Chart

Appendix B. ASCII Character Set

Appendix C. Keywords and Reserved Words

Appendix D. Primitive Types

Appendix E. (On CD) Number Systems

Appendix F. (On CD) Unicode®

Appendix G. Using the Java API Documentation

Appendix H. (On CD) Creating Documentation with javadoc

Appendix I. (On CD) Bit Manipulation

Appendix J. (On CD) ATM Case Study Code

Appendix K. (On CD) Labeled break and continue Statements

Appendix L. (On CD) UML 2: Additional Diagram Types

Appendix M. (On CD) Design Patterns

Appendix N. Using the Debugger

Inside Back Cover



Java(c) How to Program
Java How to Program (6th Edition) (How to Program (Deitel))
ISBN: 0131483986
EAN: 2147483647
Year: 2003
Pages: 615

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