The important concepts of Java that will be used in learning Selenium would be
See, Java is a programming language and Selenium is just a library, So if you are really good in Java you will have a very short learning curve when learning Selenium.
A lot of students say that they find selenium difficult but in reality it drills down to the Java that they are not good with.
Any interview you give these days For Selenium Automation majority of the questions that will be asked will be on Design Patterns, Java and Data Structure!
But if you still want me to give you some important topics/ concepts that you should be good at then:
- Interfaces and Abstract Class
- Collections [ ArrayList and Maps]
- Handling Files using Java [This will be used for your reporting, test data, and logs]