java.util · java.base · since Java 1.2

Map

declaration

public interface Map<K, V>

Key → value associations. The modern default methods (getOrDefault, computeIfAbsent, merge) eliminated most manual null-dancing.

Key methods

`V get(Object key) / V put(K key, V value)`	Core lookup/store; get returns null on miss, put returns the previous value or null.
`V getOrDefault(Object key, V default)`	Miss-safe read — no null check needed.
`V putIfAbsent(K, V)`	Conditional insert — returns the existing value if present, null if it inserted.
`V computeIfAbsent(K, Function<K,V>)`	Get-or-create in one call — the standard multimap/cache idiom; the function is not invoked when a mapping already exists.
`V computeIfPresent(K, BiFunction<K,V,V>)`	Update-if-present; returning null from the function removes the entry.
`V compute(K, BiFunction<K,V,V>)`	Unconditional read-modify-write — the function sees null for a missing key, and a null result removes the entry.
`V merge(K, V, BiFunction<V,V,V>)`	Insert-or-combine — counting and summing in one line; a null result removes the entry.
`void forEach(BiConsumer<K,V>)`	Traverse key+value pairs without an explicit entrySet() loop.
`void replaceAll(BiFunction<K,V,V>)`	In-place transform of every value.
`Set<K> keySet() / Collection<V> values() / Set<Map.Entry<K,V>> entrySet()`	Live views over the map — removal through any of them writes through to the map.
`static <K,V> Map<K,V> of(K,V,…) / ofEntries(Entry…) / entry(K,V)`	Immutable literals — of() covers up to 10 pairs, ofEntries() any number, entry() builds one Map.Entry.
`static <K,V> Map<K,V> copyOf(Map)`	Immutable defensive copy.