Java exercises

Mates, I do have one problem. While I regard myself as a full-stack engineer (C,C++,Java,JavaScript,PHP,HTML,CSS, etc.) I get bombed at every single interview exclusively with Java questions. I know Java and I can solve problems in it, but in my last two interviews my Java code was not elegant enough and my solution not fast or smooth enough.

Can you guys advice me a Java exercise reference that includes nice short problems for Object Oriented Java including advanced data structure stuff. It would be important that it also comes with a solution section such that you can actually compare your solution with a suggested one ...

Thanks!