The basics: what's an object, what are the characteristics of an object (encapsulation, inheritance, etc.... I can never remember, because who uses that language on a daily basis???), maybe how to resolve classpath issues, variable scoping, stuff like that. Pretty much what a senior java dev would have to know. Sadly, interviews for junior devs are often pretty close to what interviews for a senior java dev are. About the main difference is that mistakes are allowed a bit to account for lack of experience. My advice is to just know your stuff.
Check out some resources for questions a junior java dev might get asked:
glassdoor.com/Interview/junior-java-developer-int…