Sometimes it happens that after downloading a Jar file for an App, there appears to be several catch with it, like activation code required. Is there any tool that I can use to simulate the Jar file on desktop prior to downloading/transferring to mobile?
What you are looking for is a J2ME emulator, there are many of them and it appears that many have some limitations such as limited 3D support, slowness and other bugs.
I also found Java Emulator which looks to be more game orientated.
There are just too many emulators to count, and you're going to have to review them to see which suits your needs best.