Java Collections Framework 是什么？

List<String> list = new ArrayList<>();   // backed by an array — fast random access
list.add("a");
list.get(0);            // index access — O(1)
list.contains("a");     // O(n) search
// LinkedList — fast insert/remove at ends, O(n) random access

Set<String> set = new HashSet<>();    // O(1) add/contains, NO order
set.add("a");
set.add("a");           // ignored — duplicates not allowed
set.contains("a");      // O(1) — great for membership tests
// TreeSet — sorted order; LinkedHashSet — insertion order

Map<String, Integer> map = new HashMap<>();   // O(1) get/put, no order
map.put("age", 30);
map.get("age");                 // 30
map.getOrDefault("x", 0);        // safe access with default
map.computeIfAbsent("list", k -> new ArrayList<>()); // handy for grouping
// TreeMap — sorted by key; LinkedHashMap — insertion order

Need ordered + indexed + duplicates  → ArrayList
Need uniqueness, fast lookup          → HashSet
Need key→value, fast lookup           → HashMap
Need sorted order                     → TreeSet / TreeMap
Need FIFO processing                  → ArrayDeque / LinkedList (Queue)
Need thread-safety                    → ConcurrentHashMap, etc.

ArrayList: get O(1), add-end O(1), contains/remove O(n)
HashMap/HashSet: get/put/contains O(1) average
TreeMap/TreeSet: O(log n) (sorted)
LinkedList: add/remove-ends O(1), get O(n)