Core java interviews mostly focus on Java fundamentals such as Classes and Interfaces, Exception handling, Garbage collection, Threads, Wrapper classes and Collections. Questions can be based on the practical examples on these topics. I would focus more on Threads which is a favourite topic amongst most interviewers.
If you have done your SCJP or studying for it, you would understand the nature of questions to be asked.
You can study Swings so as to get a brief understanding of how it works. Study MVC and three tier patterns.