java.util.concurrent · java.base · since Java 1.5

ConcurrentHashMap

declaration

public class ConcurrentHashMap<K, V> extends AbstractMap<K, V>
        implements ConcurrentMap<K, V>, Serializable

The concurrent Map: lock-free reads, per-bin write locking, and atomic compound operations (computeIfAbsent, merge) that replace client-side locking entirely.

Key methods

`V computeIfAbsent(K, Function<K,V>)`	Atomic get-or-create (the mapping function runs under the bin lock — keep it short, no map reentry).
`V merge(K, V, BiFunction<V,V,V>)`	Atomic insert-or-combine.
`V putIfAbsent(K, V) / boolean replace(K, V old, V new)`	Atomic conditional writes.
`void forEach(long parallelismThreshold, BiConsumer)`	Bulk parallel operations.
`static <K> KeySetView<K,Boolean> newKeySet()`	A concurrent Set.