Need some help about running .jar files on PC?

I've installed JAVA Runtime Environment already. But whenever i try to open a jar file(By double clicking or run with/installation folder/bin/java.exe), It always shows error messages like- 'Failed to load main class attribute from ......... .jar' ; 'Invalid or corrupt jar file'..etc. What should i do? [I need to run Opera Mini 5 primarily and what are the jad files for?] Thanks for your help.