Why can't my applet read or write to files?
Applets execute under the control of a web browser. Netscape and Internet Explorer impose a security restriction, that prohibits access to the local filesystem by applets. While this may cause frustration for developers, this is an important security feature for the end-user. Without it, applets would be free to modify the contents of a user's hard-drive, or to read its contents and send this information back over a network.
Digitally signed applets can request permission to access the local filesystem, but the easiest way around the problem is to read and write to remote files located on a network drive. For example, in conjunction with a CGI script or servlet, you could send HTTP requests to store and retrieve data.