Key Java Collections Implementations: List, Set, Map

For List, the two main implementations are ArrayList and LinkedList. ArrayList uses a resizable array internally, so it's great for random access — calling get with an index is constant time. LinkedList uses a doubly linked structure, so it's better when you frequently insert or remove at the beginning or end, and it also implements Deque so you can treat it like a stack or queue. Use ArrayList for indexed access; use LinkedList for queue-like work. For Set you have three choices. HashSet is backed by a hash table, so it's unordered but very fast — add and contains are constant time on average. LinkedHashSet is like HashSet but preserves insertion order. TreeSet uses a red-black tree, so it keeps elements sorted, with log-n operations. Use HashSet for speed, LinkedHashSet when order matters, and TreeSet when you need sorted iteration. For Map it's the same pattern. HashMap is a hash table — unordered, but get and put are constant time on average. LinkedHashMap preserves insertion or access order, which makes it perfect for building an LRU cache. TreeMap is a red-black tree, so keys are sorted and you get range queries like headMap and tailMap, at log-n cost. The way to choose is to ask three questions: do I need fast lookup, insertion order, or sorted order? Two big mistakes to avoid: don't only ever reach for ArrayList and HashMap, and never call get with an index in a loop on a LinkedList, because that's quadratic time. Start with ArrayList, HashSet, and HashMap, then switch to the ordered or sorted variants only when your use case actually needs them.