OOP Concepts: What is the difference between abstraction and encapsulation? Can you explain polymorphism with examples (compile-time vs. runtime)? Collections: How does HashMap work internally? What is the difference between ArrayList and LinkedList? Explain the difference between Set, List, and Map. Multithreading: What is the difference between synchronized methods and blocks? How does ThreadPoolExecutor work? Explain wait() vs. sleep(). Exception Handling: What is the difference between checked and unchecked exceptions? How do you create a custom exception in Java?