Object Oriented programming topics including SOLID principles. (prepare for inheritance puzzles)

Multithreading and Concurrency(prepare for Executor framework and concurrency API

Collection framework (Prepare for the internal working of each collection data structure like HashMap, Concurrent hashmap, HashSet)

Serialization (How it works)

Design Patterns (prepare at least 4–5 design patterns like creational, behavioral, and structural patterns)

Garbage Collections

Java Generics

Java 8 (Stream and Functional Programming-prepare for Java 8 coding programs on streams)

SQL Queries (prepare to write queries on JOINS and employee table like highest salary and all)

Coding practice (prepare Array and String problems as much as you can)

Memory Management (Know about Java 8 and above version memory management changes) 

https://medium.com/javarevisited/how-i-got-more-than-10-offer-letters-as-a-java-developer-in-3-months-8d153605ecda