Applet to Applet Communication
The AppletContext class in the java.applet package contains the two member methods getApplet and getApplets. By using those methods, an applet can find other applets and invoke methods on them. That can only be done if the following security requirements are met:
The applets originate from the same server and from the same directory on the server.
The applets are running on the same page and in the same browser window. Those security restrictions may have been designed that way for good reason;
however, the latter requirement limits the ways you can make interesting multiapplet interfaces featuring applet-to-applet (a2a) communication.