Checklist
Know these well enough to explain.
Collections
ArrayList vs LinkedList, HashMap/TreeMap, hashing, iteration costs.
Language basics
Generics, equals/hashCode, immutability, records, exceptions.
Concurrency
Thread-safety, locks, atomics, executors, and common pitfalls.
Clean code
Readable naming, small methods, invariants, and testable design.
Micro-drill
Pick one API (HashMap, PriorityQueue, ArrayDeque) and explain:
complexity, common misuse, and one real-world use case.