Focus on core java - Really Important.
Deep implementation on Data structures like questions can be
How does search implemented in Array list ? and how is it difference with regards to HashMap ?
Why interfaces are created when you have abstract concepts ? ?
Few basic questions on polymorphism, perhaps a quick review on OOP concepts would do.
Questions on all kinds of indexing algorithms & Cache related questions
Reasons of introducing new features from java version to version ( 8, 11, 17, 21 )