The
Simple
applet, like every other applet, features a subclass of theApplet
class. TheSimple
class overrides fourApplet
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 is 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
, anddestroy
methods are discussed and used throughout this tutorial. For more information, you can also refer to theJApplet
API Specification.
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. (Thepaint
method is described on the following page.) Most applets, however, do more.
The
init
method is useful for one-time initialization that doesn't take very long. In general, theinit
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 itsinit
method is called. For example, theApplet
image loading methods simply don't work inside of an applet constructor. Theinit
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 theinit
method more than once after the applet has been loaded. See the previous page for more details.
Every applet that does something after initialization (except in direct response to user actions) must override the
start
method. Thestart
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.
Most applets that override
start
should also override thestop
method. Thestop
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.
Many applets don't need to override the
destroy
method, since theirstop
method (which is called beforedestroy
) 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 thedestroy
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.