Java is a very popular programming language and Java has been the program of choice when making games and applications for mobile phones. The great thing about Java applications and games is that it takes up very little space on your mobile phone and since some mobile phones don't have external storage, it is imperative that programs and applications have smaller sizes.

The file extension JAR and jad are associated with Java applications. However, just because a file has the file extension JAR and jad, it doesn't mean that it is automatically a file for mobile phones. Some applications such as Azureus, makes use of Java so plugins that are downloaded sometimes have Jar file extensions.

Anyways, if your mobile phone is java capable, you can install jar and jad files by either using your mobile phone's included CD, or transferring the Java application to your phone's memory card (if it supports one, that is), and then launching the files right from your phone (using your phone's file explorer). If you've successfully installed the JAD or JAR file but the application is not working, that application you have downloaded may not be compatible with your phone model.