Computer Science
Java Interview Prep — Part 4
Multithreading & concurrency, Collections internals, OOP & design patterns, exception handling, system design/LLD, and pattern-printing programs (Q99–Q122) — producer-consumer, executors, HashMap internals, Singleton/Factory/Builder/Observer, and a vending-machine LLD walkthrough.
Why it's on this site
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This is the part of Java interview prep people skip because it's harder — concurrency, Collections internals, and low-level design. I kept the vending-machine LLD walkthrough in because that kind of question trips people up not from lack of knowledge but from not knowing how to structure the answer out loud under pressure. If you're comfortable with basic Java and want to go up a level, this is where to start.