ConcurrentMap is an interface in Java's java.util.concurrent package that provides a thread-safe version of the Map interface. It extends the Map interface and adds methods that allow for safe, concurrent access to the map.

The ConcurrentMap interface provides the following additional methods:

• putIfAbsent(K key, V value): Inserts the specified key-value pair into the map if the key is not already associated with a value.
• remove(Object key, Object value): Removes the mapping for the specified key if it is currently mapped to the specified value.
• replace(K key, V oldValue, V newValue): Replaces the value for the specified key if it is currently mapped to the specified old value.

The ConcurrentMap interface also defines methods that operate on the map atomically, such as putIfAbsent, replace, and remove. These methods allow for safe, concurrent updates to the map without the need for external synchronization.

The ConcurrentHashMap class is a concrete implementation of the ConcurrentMap interface in Java. It provides a scalable, concurrent implementation of a hash table that allows multiple threads to access and modify the map concurrently. The ConcurrentHashMap class uses a technique called lock striping to divide the map into segments, each of which can be locked independently, allowing multiple threads to modify different segments of the map concurrently.