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17. The java.applet Package

An applet is a small, embeddable Java program. The java.applet package is a small one. It contains the Applet class, which is the superclass of all applets, and three related interfaces. Figure 17.1 shows the class hierarchy of this package. See Chapter 6, Applets, for more information about this package.

17.1 java.applet.Applet (JDK 1.0)

This class implements an applet. To create your own applet, you should create a subclass of this class and override some or all of the following methods. Note that you never need to call these methods--they are called when appropriate by a Web browser or other applet viewer.

init() should perform any initialization for the applet; it is called when the applet first starts. destroy() should free up any resources that the applet is holding; it is called when the applet is about to be permanently stopped. start() is called to make the applet start doing whatever it is that it does. Often, it starts a thread to perform an animation or similar task. stop() should temporarily stop the applet from executing. It is called when the applet temporarily becomes hidden or non-visible.

getAppletInfo() should return text suitable for display in an About dialog posted by the Web browser or applet viewer. getParameterInfo() should return an arbitrary-length array of three-element arrays of strings where each element describes one of the parameters that this applet understands. The three elements of each parameter description are strings that specify, respectively, the parameter's name, type, and description.

In addition to these methods, an applet also typically overrides several of the methods of java.awt.Component, notably the paint() method to draw the applet on the screen. There are also several Applet methods that you do not override but may call from applet code: showStatus() displays text in the Web browser or applet viewer's status line. getImage() and getAudioClip() read image (GIF and JPEG formats) and audio files (AU format) over the network and return corresponding Java objects. getParameter() looks up the value of a parameter specified with a <PARAM> tag within an <APPLET>...</APPLET> pair. getCodeBase() returns the base URL from which the applet's code was loaded, and getDocumentBase() returns the base URL from which the HTML document containing the applet was loaded. getAppletContext() returns an AppletContext object, which also has useful methods.

public class Applet extends Panel {
    // Default Constructor: public Applet()
    // Public Instance Methods
            public void destroy();
            public AppletContext getAppletContext();
            public String getAppletInfo();
            public AudioClip getAudioClip(URL url);
            public AudioClip getAudioClip(URL url, String name);
            public URL getCodeBase();
            public URL getDocumentBase();
            public Image getImage(URL url);
            public Image getImage(URL url, String name);
            public Locale getLocale();  // Overrides Component
            public String getParameter(String name);
            public String[][] getParameterInfo();
            public void init();
            public boolean isActive();
            public void play(URL url);
            public void play(URL url, String name);
            public void resize(int width, int height);  // Overrides Component
            public void resize(Dimension d);  // Overrides Component
            public final void setStub(AppletStub stub);
            public void showStatus(String msg);
            public void start();
            public void stop();


Object->Component(ImageObserver, MenuContainer, Serializable)->Container->Panel->Applet

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