java-fundamentals

Choose ArrayList for indexed access with O(1) random access, HashSet for unique elements with O(1) contains checks, and HashMap for key-value pairs with O(1) get and put. Use TreeSet when you need sorted unique elements at O(log n) cost.

Replace iteration loops with stream pipelines using filter, map, and collect or toList. For example, users.stream().filter(User::isActive).map(User::getName).sorted().toList() replaces a manual loop with conditional collection.

Yes, Java 21 supports pattern matching in switch expressions, allowing type checks and casts in case labels. You can match Integer, String, and other types directly with guarded patterns, eliminating manual instanceof and cast chains.

ConcurrentModificationException occurs when a collection is structurally modified during iteration outside the iterator. Fix it by using Iterator.remove(), collecting items to remove first, or using concurrent collections like ConcurrentHashMap.

Use Optional as a return type when a method may legitimately return no value, signaling absence explicitly to callers. Chain operations with map, flatMap, and orElse instead of nested null checks, but avoid Optional for fields or parameters.