Also,

You should understand common OOP design concepts such as polymorphism, abstraction + interfaces, and simple things like multiple ways to iterate through a collection.

1. Don't just say polymorphism is done through inheritance and overriding etc etc. This is obvious and they don't care if you can define types of polymorphism.. make sure you can actually EXPLAIN what it is not how it works in java.
Polymorphism means an object can do different things in different situations. My favorite example is a bluray player: it doesn't care if you put a dvd or a bluray in... it'll do something different depending what disc is inserted. This is an example of method overloading but without giving a specific example in java. If they want you to actually write the code out then go ahead and show them your overloaded methods accepting a DVD object or a Bluray object.

2. Be able to explain what an interface is. Not "the class implements it" but state that it is an agreement to implement all methods in said interface somewhere down the line. Also be able to explain when to use interfaces and when to use abstraction explicitly.

3. They love questions like "iterate through this arraylist"
You could do a simple for loop as you've done for years... and you can show them the for(List s : thisList){} it just shows them you can do things multiple ways and understand the language and aren't just going off memorization.