Application frameworks
Libraries are often grouped according to their functionality. Some libraries, for example, are used as is, off the shelf. The standard Java library String and ArrayList classes are examples of these. Other libraries are designed specifically as building blocks to create other classes. A certain category of library is the application framework, whose goal is to help you build applications by providing a class or set of classes that produces the basic behavior that you need in every application of a particular type. Then, to customize the behavior to your own needs, you inherit from the application class and override the methods of interest. The application framework’s default control mechanism will call your overridden methods at the appropriate time. An application framework is a good example of “separating the things that change from the things that stay the same,” since it attempts to localize all the unique parts of a program in the overridden methods.[76]
Applets are built using an application framework. You inherit from class JApplet and override the appropriate methods. There are a few methods that control the creation and execution of an applet on a Web page:
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Method
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Operation
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init( )
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Automatically called to perform first-time initialization of the applet, including component layout. You’ll always override this method.
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start( )
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Called every time the applet moves into sight on the Web browser to allow the applet to start up its normal operations (especially those that are shut off by stop( )). Also called after init( ).
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stop( )
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Called every time the applet moves out of sight on the Web browser to allow the applet to shut off expensive operations. Also called right before destroy( ).
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destroy( )
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Called when the applet is being unloaded from the page to perform final release of resources when the applet is no longer used
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With this information you are ready to create a simple applet:
//: c14:Applet1.java
// Very simple applet.
import javax.swing.*;
import java.awt.*;
public class Applet1 extends JApplet {
public void init() {
getContentPane().add(new JLabel("Applet!"));
}
} ///:~
Note that applets are not required to have a main( ). That’s all wired into the application framework; you put any startup code in init( ).
In this program, the only activity is putting a text label on the applet, via the JLabel class (the old AWT appropriated the name Label as well as other names of components, so you will often see a leading “J” used with Swing components). The constructor for this class takes a String and uses it to create the label. In the preceding program this label is placed on the form.
The init( ) method is responsible for putting all the components on the form using the add( ) method. You might think that you ought to be able to simply call add( ) by itself, and in fact that’s the way it used to be in the old AWT. However, Swing requires that you add all components to the “content pane” of a form, so you must call getContentPane( ) as part of the add( ) process.
