Collections Framework trong Java là gì?

Collections Framework (JDK 1.2) là kiến trúc thống nhất để lưu và thao tác nhóm object.

Cấu trúc interface (đơn giản hoá):

Collection<E>
├── List<E>     — có thứ tự, cho phép trùng     → ArrayList, LinkedList
├── Set<E>      — không trùng                    → HashSet, LinkedHashSet, TreeSet
└── Queue<E>    — FIFO/priority                  → ArrayDeque, PriorityQueue

Map<K,V>        — KHÔNG extend Collection        → HashMap, LinkedHashMap, TreeMap

3 tầng: Interface (contract) → Implementation (cụ thể) → Utility (Collections.sort(), Collections.unmodifiableList()...).

Lợi ích: type-safe với generics (Java 5+), API nhất quán, polymorphism (đổi implementation dễ).

Java 9+ factory ngắn gọn:

List<Integer> nums = List.of(1, 2, 3);          // immutable
Map<String, Integer> ages = Map.of("Ada", 30);  // immutable

⚠ List.of(...) immutable — add() throw UnsupportedOperationException. Cần mutable thì new ArrayList<>(List.of(...)).

Concurrent variants: ConcurrentHashMap, CopyOnWriteArrayList, BlockingQueue (multi-thread).

The Collections Framework (JDK 1.2) is a unified architecture for storing and manipulating groups of objects.

Interface hierarchy (simplified):

Collection<E>
├── List<E>     — ordered, allows duplicates    → ArrayList, LinkedList
├── Set<E>      — no duplicates                  → HashSet, LinkedHashSet, TreeSet
└── Queue<E>    — FIFO/priority                  → ArrayDeque, PriorityQueue

Map<K,V>        — does NOT extend Collection     → HashMap, LinkedHashMap, TreeMap

Three layers: Interfaces (contracts) → Implementations → Utilities (Collections.sort(), Collections.unmodifiableList()...).

Benefits: type-safe via generics (Java 5+), consistent API, polymorphism (swap implementations easily).

Java 9+ factory methods:

List<Integer> nums = List.of(1, 2, 3);          // immutable
Map<String, Integer> ages = Map.of("Ada", 30);  // immutable

⚠ List.of(...) is immutable — add() throws UnsupportedOperationException. For mutable, use new ArrayList<>(List.of(...)).

Concurrent variants: ConcurrentHashMap, CopyOnWriteArrayList, BlockingQueue (multi-threading).