If they are planning to interview you specifically on JAVA, it guarantees questions about OOP. I will list a number of very common questions asked in JAVA interviews:

1. It could be about garbage collection:
   1. What is garbage collection?
   2. How does JAVA achieve garbage collection?
   3. How would you implement a garbage collector?
2. They may ask you about multithreading:
   1. How do you avoid synchronisation problems among threads?
   2. What is a deadlock?
   3. How do you avoid deadlocks?
   4. How do you ensure that variables are safe when there are two threads involved
3. Then it must be questions about OOP in general:
   1. 4 main principles of OOP
   2. Difference between abstract class and interface (this question pops up a lot)
4. What is the difference between ArrayLists and arrays?
5. When would you prefer to use one over another?
6. If you state on your CV that you know C/C++ they will ask you to state the differences between JAVA and C or C++

So in general the list of questions that could be asked is very broad and mainly it is about concepts like OOP, Multithreading, Garbage Collection, about available data structures Collection(s) Interface( trees, maps, lists ).