Communicating with Other Programs in Java

By invoking the public methods of other applets on the same page
By using an API defined in the java.applet package, which allows it to communicate in limited way with a browser or applet viewer that contains it.
By using the API defined in the java.net package to communicate over network with the other programs. Other programs must be running on a host which the applet originated from.

Sending Messages to Other Applets On Same Page

An applet can find other applet either by looking up by its name (using the AppletContext getApplet() method) or by finding all the applets present on the page (by using AppletContext getApplets() method). Both methods, if are successful, give the caller one or more applet objects. Once if the caller finds an Applet object, it can invoke methods on the object.

Finding up an Applet by Name: getApplet(receiverName)

This section gives the code that Sender uses to look up an applet by its name. Here is the Sender program

Applet receiver = null;
String receiverName = nameField.getText(); //get name to search for
receiver = getAppletContext().getApplet(receiverName);

The Sender goes to make sure that Receiver was found and that it is an instance of correct class (Receiver). If everything goes well, then Sender sends a message to the Receiver. Here is the Receivers program.

if (receiver != null)
{
if (!(receiver instanceof Receiver))
{
status.appendText("Found applet named "
+ receiverName + ", "
+ "but it's not a Receiver object.\n");
} else {
status.appendText("Found applet named "
+ receiverName + ".\n"
+ " Sending message to it.\n");
((Receiver)receiver).processRequestFrom(myName);
}
} . . .

Finding All the Applets on a Page: getApplets()

A getApplets() method returns the list (an Enumeration ) of all the applets on present in the page. For the security purpose, many browsers and the applet viewers implement the getApplets() method so that it will return only those applets which are originated from the same host as the applet calling getApplets(). Here is an applet that simply lists all the applets it finds on the page:

public void printApplets()
{
//Enumeration will contain all applets on this page (including
//this one) that we can send messages to.
Enumeration e = getAppletContext().getApplets();
. . .
while (e.hasMoreElements())
{
Applet applet = (Applet)e.nextElement();
String info = ((Applet)applet).getAppletInfo();
if (info != null)
{
textArea.appendText("- " + info + "\n");
} else
{
textArea.appendText("- " + applet.getClass().getName() + "\n");
}
}
. . .
}

Communicating with a Browser

Many of the Applets and AppletContext methods involves some sort of the communication with a browser or applet viewer. Consider for example, Applet getDocumentBase() and getCodeBase() methods get the information browser or applet viewer where the applet and the HTML page came from. The method Applet showStatus() tells the browser or the viewer to display a status message. The Applets getParameterInfo() method can give a browser the list of parameters an applet understands. And, of course the browser or the applet viewer calls up Applet init, start, stop, and destroy methods to inform a applet of the changes in its state. Two other interesting methods are AppletContext showDocument() methods

public void showDocument(java.net.URL url)
public void showDocument(java.net.URL url, String targetWindow)

...//In the Applet subclass...
urlWindow = new URLWindow(getAppletContext());
. . .

class URLWindow extends Frame
{
. . .
public URLWindow(AppletContext appletContext)
{
. . .
this.appletContext = appletContext;
. . .
}

public boolean action(Event event, Object o)
{
. . .
URL url = null;
try
{
url = new URL(urlString);
} catch (MalformedURLException e)
{
...//Inform the user, somehow, and return..
. }

if (url != null)
{
if (/* user doesn't want to specify the window */)
{
appletContext.showDocument(url);
} else {
appletContext.showDocument(url, /* user's choice */);
}
}
. . .

Working with the Server-Side Application

Applets, like any other Java programs, can make use of the API defined in the java.net package to communicate across the network. Here the only difference is that, for security reasons, the only host with which an applet can communicate is the host from which it was delivered.

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