What Can an Applet Do?
An applet can:
• Draw pictures on a web page
• Create a new window and draw in it.
• Play sounds.
• Receive input from the user by the keyboard or the mouse.
• Form a network connection to the server from that it came and can send to and receive arbitrary data from that server.
Anything you can do along with these abilities you can do in an applet. An applet cannot:
• Write data on any of the host's disks.
• Read any data from the host's disks without the user's permission. In a few environments, notably Netscape, an applet cannot read data from the user's disks even with permission.
• Delete files
• Read from or write to arbitrary blocks of memory, even on a non-memory-protected operating system like the MacOS. All memory access is strictly controlled.
• Form a network connection to a host on the Internet other than the one from which it was downloaded.
• Call the native API directly (though Java API calls may eventually lead back to native API calls).
• Introduce a virus or trojan horse within the host system.
• An applet is not supposed to be able to crash the host system. Therefore in practice Java isn't quite stable enough to make this claim yet.