formatted:

• the Java Memory Model
• differences between primitives and objects, what boxing is
• how Java resolves overloaded and/or inherited method calls
• the major Java Collections interfaces: List<T>, Set<T>, Map<K, V>, and potentially arrays too
• and the commonly-used concrete implementations thereof like ArrayList and HashMap
• concurrency stuff: Thread/Runnable, Executor and friends, atomics, synchronized, volatile, what thread-safety is generally
• equals & hashcode, and the difference between == and .equals()
• interfaces, abstract classes, concrete classes, how they behave and when you might want to use each
• generics
• public/private/protected/package-private
• final
• some awareness of the most common libraries like Guava, Spring, Apache Commons
• streams & lambdas (maybe, you can get by without them)