AppletContext and showDocument( ) - The Applet Class

AppletContext and showDocument( )

One application of Java is to use active images and animation to provide a graphical means of navigating the Web that is more interesting than simple text-based links. To allow your applet to transfer control to another URL, you must use the showDocument( ) method defined by the AppletContext interface. AppletContext is an interface that lets you

get information from the applet’s execution environment. The methods defined by AppletContext are shown in Table 23-2. The context of the currently executing applet is obtained by a call to the getAppletContext( ) method defined by Applet.

Method : Description

Applet getApplet(String appletName) : Returns the applet specified by appletName if it is within the current applet context. Otherwise, null is returned.

Enumeration<Applet> getApplets( ) : Returns an enumeration that contains all of the applets within the current applet context.

AudioClip getAudioClip(URL url) : Returns an AudioClip object that encapsulates the audio clip found at the location specified by url.

Image getImage(URL url) : Returns an Image object that encapsulates the image found at the location specified by url.

InputStream getStream(String key) : Returns the stream linked to key. Keys are linked to streams by using the setStream( ) method. A null reference is returned if no stream is linked to key.

Iterator<String> getStreamKeys( ) : Returns an iterator for the keys associated with the invoking object. The keys are linked to streams. See getStream( ) and setStream( ).

void setStream(String key, InputStream strm) throws IOException : Links the stream specified by strm to the key passed in key. The key is deleted from the invoking object if strm is null.

void showDocument(URL url) : Brings the document at the URL specified by url into view. This method may not be supported by applet viewers.

void showDocument(URL url, String where) : Brings the document at the URL specified by url into view. This method may not be supported by applet viewers. The placement of the document is specified by where as described in the text.

void showStatus(String str) : Displays str in the status window.

Table 23-2   The Methods Defined by the AppletContext Interface (continued)

Within an applet, once you have obtained the applet’s context, you can bring another document into view by calling showDocument( ). This method has no return value and throws no exception if it fails, so use it carefully. There are two showDocument( ) methods. The method showDocument(URL) displays the document at the specified URL. The method showDocument(URL, String) displays the specified document at the specified location within the browser window. Valid arguments for where are "_self" (show in current frame), "_parent" (show in parent frame), "_top" (show in topmost frame), and "_blank" (show in new browser window). You can also specify a name, which causes the document to be shown in a new browser window by that name.

The following applet demonstrates AppletContext and showDocument( ). Upon execution, it obtains the current applet context and uses that context to transfer control to a file called Test.html. This file must be in the same directory as the applet. Test.html can contain any valid hypertext that you like.

/*       Using an applet context, getCodeBase(), and showDocument() to display an HTML file.

*/

import java.awt.*; import java.applet.*; import java.net.*;

/*

<applet code="ACDemo" width=300 height=50> </applet>

*/

public class ACDemo extends Applet { public void start() {

AppletContext ac = getAppletContext();

URL url = getCodeBase(); // get url of this applet

try {

ac.showDocument(new URL(url+"Test.html"));

} catch(MalformedURLException e) { showStatus("URL not found");

}

}