Methods for Milestones - Java Tutorial 5.0 英文版

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Trail: Deployment
Lesson: Applets

Methods for Milestones

The Simple applet, like every other applet, features a subclass of the Applet class. The Simple class overrides four Applet methods so that it can respond to major events:

init
To initialize the applet each time it's loaded (or reloaded).
start
To start the applet's execution, such as when the applet's loaded or when the user revisits a page that contains the applet.
stop
To stop the applet's execution, such as when the user leaves the applet's page or quits the browser.
destroy
To perform a final cleanup in preparation for unloading.

Following is the interface for these methods:
public class Simple extends JApplet {
    . . .
    public void init() { . . . }
    public void start() { . . . }
    public void stop() { . . . }
    public void destroy() { . . . }
    . . .
}
The init, start, stop, and destroy methods are discussed and used throughout this tutorial. For more information, you can also refer to the JApplet API Specification.

Overriding These Methods

Not every applet needs to override every one of these methods. Some very simple applets override none of them. For example, the HelloWorld applet at the beginning of this section doesn't override any of these methods, since it doesn't do anything except draw itself.

The "Hello World" applet just displays a string once, using its paint method. (The paint method is described on the following page.) Most applets, however, do more.

The init Method

The init method is useful for one-time initialization that doesn't take very long. In general, the init method should contain the code that you would normally put into a constructor. The reason applets shouldn't usually have constructors is that an applet isn't guaranteed to have a full environment until its init method is called. For example, the Applet image loading methods simply don't work inside of a applet constructor. The init method, on the other hand, is a great place to call the image loading methods, since the methods return quickly.

Browser note: Some browsers sometimes call the init method more than once after the applet has been loaded. See the previous page for more details.

The start Method

Every applet that does something after initialization (except in direct response to user actions) must override the start method. The start method either performs the applet's work or (more likely) starts up one or more threads to perform the work. You'll learn more about threads later in this trail, in the Threads in Applets section. You'll learn more about handling the events that represent user actions on the next page.

The stop Method

Most applets that override start should also override the stop method. The stop method should suspend the applet's execution, so that it doesn't take up system resources when the user isn't viewing the applet's page. For example, an applet that displays animation should stop trying to draw the animation when the user isn't looking at it.

The destroy Method

Many applets don't need to override the destroy method, since their stop method (which is called before destroy) does everything necessary to shut down the applet's execution. However, destroy is available for applets that need to release additional resources.

Note: You should keep implementations of the destroy method as short as possible, because there is no guarantee that this method will be completely executed. The Java Virtual Machine might exit before a long destroy method has completed.

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