One thing about Java

What's one thing that EVERY Java programmer has to know?  The kind of thing that drives you crazy if the supposed Java programmer didn't know it?

I had to learn Java for a warmup assignment in a class, even though the rest of the class was in C++.  Had to make a multiplayer game with the www.gamegardens.com framework.  What I learned about Java is that it makes me want to kill myself and I never want to see it again.