Applet Life-Cycle Methods

2017-11-03 09:05:04

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.

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