Map

A Map is a collection of key-value pairs. Each key is unique; each key maps to exactly one value. Use HashMap as the default implementation.

Common operations

Map<String, Integer> ages = new HashMap<>();
ages.put("Ada", 36);
ages.put("Grace", 85);
ages.get("Ada");                 // 36
ages.getOrDefault("Unknown", 0); // 0
ages.containsKey("Grace");       // true
ages.remove("Ada");
ages.size();                     // 1

// Iterate:
ages.forEach((k, v) -> System.out.println(k + " -> " + v));

Implementations

Immutable map (Java 9+)

Map<String, Integer> m = Map.of("a", 1, "b", 2, "c", 3);
Map<String, Integer> big = Map.ofEntries(
    Map.entry("a", 1),
    Map.entry("b", 2)
);

Map is not a Collection

Map does not extend Collection. Its entries, keys, and values are accessed via entrySet(), keySet(), values() — each returns a view that is a Collection.