I have started writing Java programs at uni and i'm required to add a Java package called B102.jar to the compiler.
Well, the university doesn't support anyting other than Windows so it was a pain trying to figure out how to use Xcode and use their 'B102.jar' package. Anyway to add a .jar package to Xcode for compilation, all you need to do is add your .jar package to:

/System/Library/Java/Extensions/

That's it. Now when you type import B102.*; for example, it will compile correctly.