I've read some short JUnit tutorials and write some test cases myself. But most tutorial teach me on doing testing against standalone app. So my questions come. How to do unit testing against distributed application like RMI? Do I need to learn Mock Object? Thanks.
Creativity is allowing yourself to make mistakes. Art is knowing which ones to keep
If you want to use JUnit, then yes mock objects help. The trick from a testing standpoint I find is that you want to simulate not just correct behaviour, but what the behaviour is when you encounter problems (e.g. simulated network partitions).