Greetings;
Your question is sketchy. Although you can search Java Damage.
Here is a bit that may help otherwise you could go to Sun Micro Systems to get some info.
Java-applet normally need a lot of system resources, to be interpreted and executed. Bad-willed applets or just bad-programmes applets can consume resources in rude or malicious ways, so that a lot of or all the CPU or memory resources of your computer are consumed, slowing your computer down to a locking state, where your machine hangs..
In earlier JAVA-versions, there was a bug in the Verifier ( a part of Java's runtime system, which checks if applets adhere to the Java safety rules ), which allowed applets, to delete files or do other damage. Fixed in JDK 1.0.2 and in Netscape Navigator 2.02
Malicious Java applets can generate and execute raw machine code, meaning those applets can perform any action that the user, running this applet via his WWW-browser, can legally perform ( e.g.: read, delete or corrupt files ).
** Crylynx **