It depends on candidate if he/she is a fresher or experienced. If you are experienced candidate they should ask what are the frameworks you used till now and etc., Maximum of 5% to 10% of java will be used in selenium. But interviewer will ask more questions on java only. So we need to prepare more on core java basic concepts.

Here are some Java topics that you need to know to crack an interview.

1.OOP’s concept – Class, Objects Polymorphism, Inheritance and Encapsulation 

2.Java Programming essentials- Object Instances, method overloading/overriding concepts and packages
 
3.Control Statements – While, do-While, Switch, If statements – This will help us in writing the scripts for multiple scenario statements and decision making scenarios. 

4.Looping statements – This will help us in scenarios like, iterating through a large table to find a record that you want and Running the same test for multiple number of times. 

5.Arrays Concepts – This will help us in having some set of datas of same type in a static way.
 
6.Threads and MultiThreading Concepts – This will help us in making run our scripts in different threads that will help us in achieving better performance.
 
7.Java Collections Framework – ArrayLists and HashMaps – This will help us in maintaining a collection of data’s. Particularly useful for scenarios where you need to compare the data from Web app UI with the DB. [OR] From UI to another UI 

8.File Streams – This will be helpful in externalization of data through CSV, Excel or Java Properties file. 

Selenium:

1.What are the limitations of selenium?

2.Can we automate captcha? How we will handle? (If your answer is NO/Yes)


3.Can bar-code be automated?


4.Can webdriver test mobile applications?

Click [Here][1] you can find most popularly asked questions in selenium.

  [1]: http://www.softwaretestinghelp.com/selenium-interview-questions-answers/