ConcurrentMap (Java Platform SE 8 )
Keywords: java.util.concurrent.ConcurrentMap interface
Date: 2015-11-19

compact1, compact2, compact3
java.util.concurrent

## Interface ConcurrentMap<K,V>

- Type Parameters:
    `K` - the type of keys maintained by this map
    `V` - the type of mapped values
  All Superinterfaces:
    Map<K,V>
  All Known Subinterfaces:
    ConcurrentNavigableMap<K,V>
  All Known Implementing Classes:
    ConcurrentHashMap, ConcurrentSkipListMap

      public interface ConcurrentMap<K,V>
      extends Map<K,V>

  A `Map` providing thread safety and atomicity guarantees.

  Memory consistency effects: As with other concurrent collections, actions in a thread prior to placing an object into a `ConcurrentMap` as a key or value happen-before actions subsequent to the access or removal of that object from the `ConcurrentMap` in another thread.

  This interface is a member of the Java Collections Framework.

  Since:
    1.5

- - ### Nested Class Summary

    - ### Nested classes/interfaces inherited from interface java.util.Map

      `Map.Entry<K,V>`

  - ### Method Summary

    All Methods Instance Methods Abstract Methods Default Methods
    Modifier and Type | Method and Description
    --- | ---
    `default V` | `compute(K key, BiFunction<? super K,? super V,? extends V> remappingFunction)` Attempts to compute a mapping for the specified key and its current mapped value (or `null` if there is no current mapping).
    `default V` | `computeIfAbsent(K key, Function<? super K,? extends V> mappingFunction)` If the specified key is not already associated with a value (or is mapped to `null`), attempts to compute its value using the given mapping function and enters it into this map unless `null`.
    `default V` | `computeIfPresent(K key, BiFunction<? super K,? super V,? extends V> remappingFunction)` If the value for the specified key is present and non-null, attempts to compute a new mapping given the key and its current mapped value.
    `default void` | `forEach(BiConsumer<? super K,? super V> action)` Performs the given action for each entry in this map until all entries have been processed or the action throws an exception.
    `default V` | `getOrDefault(Object key, V defaultValue)` Returns the value to which the specified key is mapped, or `defaultValue` if this map contains no mapping for the key.
    `default V` | `merge(K key, V value, BiFunction<? super V,? super V,? extends V> remappingFunction)` If the specified key is not already associated with a value or is associated with null, associates it with the given non-null value.
    `V` | `putIfAbsent(K key, V value)` If the specified key is not already associated with a value, associate it with the given value.
    `boolean` | `remove(Object key, Object value)` Removes the entry for a key only if currently mapped to a given value.
    `V` | `replace(K key, V value)` Replaces the entry for a key only if currently mapped to some value.
    `boolean` | `replace(K key, V oldValue, V newValue)` Replaces the entry for a key only if currently mapped to a given value.
    `default void` | `replaceAll(BiFunction<? super K,? super V,? extends V> function)` Replaces each entry's value with the result of invoking the given function on that entry until all entries have been processed or the function throws an exception.

    - ### Methods inherited from interface java.util.Map

      `clear, containsKey, containsValue, entrySet, equals, get, hashCode, isEmpty, keySet, put, putAll, remove, size, values`

- - ### Method Detail

    - #### getOrDefault

          default V getOrDefault(Object key,
                                 V defaultValue)

      Returns the value to which the specified key is mapped, or `defaultValue` if this map contains no mapping for the key.
      Specified by:
        `getOrDefault` in interface `Map<K,V>`
      Implementation Note:
        This implementation assumes that the ConcurrentMap cannot contain null values and `get()` returning null unambiguously means the key is absent. Implementations which support null values must override this default implementation.
      Parameters:
        `key` - the key whose associated value is to be returned
        `defaultValue` - the default mapping of the key
      Returns:
        the value to which the specified key is mapped, or `defaultValue` if this map contains no mapping for the key
      Throws:
        `ClassCastException` - if the key is of an inappropriate type for this map (optional)
        `NullPointerException` - if the specified key is null and this map does not permit null keys (optional)
      Since:
        1.8

    - #### forEach

          default void forEach(BiConsumer<? super K,? super V> action)

      Performs the given action for each entry in this map until all entries have been processed or the action throws an exception. Unless otherwise specified by the implementing class, actions are performed in the order of entry set iteration (if an iteration order is specified.) Exceptions thrown by the action are relayed to the caller.
      Specified by:
        `forEach` in interface `Map<K,V>`
      Implementation Requirements:
        The default implementation is equivalent to, for this `map`:

             for ((Map.Entry<K, V> entry : map.entrySet())
                 action.accept(entry.getKey(), entry.getValue());

      Implementation Note:
        The default implementation assumes that `IllegalStateException` thrown by `getKey()` or `getValue()` indicates that the entry has been removed and cannot be processed. Operation continues for subsequent entries.
      Parameters:
        `action` - The action to be performed for each entry
      Throws:
        `NullPointerException` - if the specified action is null
      Since:
        1.8

    - #### putIfAbsent

          V putIfAbsent(K key,
                        V value)

      If the specified key is not already associated with a value, associate it with the given value. This is equivalent to

           if (!map.containsKey(key))
             return map.put(key, value);
           else
             return map.get(key);

      except that the action is performed atomically.
      Specified by:
        `putIfAbsent` in interface `Map<K,V>`
      Implementation Note:
        This implementation intentionally re-abstracts the inappropriate default provided in `Map`.
      Parameters:
        `key` - key with which the specified value is to be associated
        `value` - value to be associated with the specified key
      Returns:
        the previous value associated with the specified key, or `null` if there was no mapping for the key. (A `null` return can also indicate that the map previously associated `null` with the key, if the implementation supports null values.)
      Throws:
        `UnsupportedOperationException` - if the `put` operation is not supported by this map
        `ClassCastException` - if the class of the specified key or value prevents it from being stored in this map
        `NullPointerException` - if the specified key or value is null, and this map does not permit null keys or values
        `IllegalArgumentException` - if some property of the specified key or value prevents it from being stored in this map

    - #### remove

          boolean remove(Object key,
                         Object value)

      Removes the entry for a key only if currently mapped to a given value. This is equivalent to

           if (map.containsKey(key) && Objects.equals(map.get(key), value)) {
             map.remove(key);
             return true;
           } else
             return false;

      except that the action is performed atomically.
      Specified by:
        `remove` in interface `Map<K,V>`
      Implementation Note:
        This implementation intentionally re-abstracts the inappropriate default provided in `Map`.
      Parameters:
        `key` - key with which the specified value is associated
        `value` - value expected to be associated with the specified key
      Returns:
        `true` if the value was removed
      Throws:
        `UnsupportedOperationException` - if the `remove` operation is not supported by this map
        `ClassCastException` - if the key or value is of an inappropriate type for this map (optional)
        `NullPointerException` - if the specified key or value is null, and this map does not permit null keys or values (optional)

    - #### replace

          boolean replace(K key,
                          V oldValue,
                          V newValue)

      Replaces the entry for a key only if currently mapped to a given value. This is equivalent to

           if (map.containsKey(key) && Objects.equals(map.get(key), oldValue)) {
             map.put(key, newValue);
             return true;
           } else
             return false;

      except that the action is performed atomically.
      Specified by:
        `replace` in interface `Map<K,V>`
      Implementation Note:
        This implementation intentionally re-abstracts the inappropriate default provided in `Map`.
      Parameters:
        `key` - key with which the specified value is associated
        `oldValue` - value expected to be associated with the specified key
        `newValue` - value to be associated with the specified key
      Returns:
        `true` if the value was replaced
      Throws:
        `UnsupportedOperationException` - if the `put` operation is not supported by this map
        `ClassCastException` - if the class of a specified key or value prevents it from being stored in this map
        `NullPointerException` - if a specified key or value is null, and this map does not permit null keys or values
        `IllegalArgumentException` - if some property of a specified key or value prevents it from being stored in this map

    - #### replace

          V replace(K key,
                    V value)

      Replaces the entry for a key only if currently mapped to some value. This is equivalent to

           if (map.containsKey(key)) {
             return map.put(key, value);
           } else
             return null;

      except that the action is performed atomically.
      Specified by:
        `replace` in interface `Map<K,V>`
      Implementation Note:
        This implementation intentionally re-abstracts the inappropriate default provided in `Map`.
      Parameters:
        `key` - key with which the specified value is associated
        `value` - value to be associated with the specified key
      Returns:
        the previous value associated with the specified key, or `null` if there was no mapping for the key. (A `null` return can also indicate that the map previously associated `null` with the key, if the implementation supports null values.)
      Throws:
        `UnsupportedOperationException` - if the `put` operation is not supported by this map
        `ClassCastException` - if the class of the specified key or value prevents it from being stored in this map
        `NullPointerException` - if the specified key or value is null, and this map does not permit null keys or values
        `IllegalArgumentException` - if some property of the specified key or value prevents it from being stored in this map

    - #### replaceAll

          default void replaceAll(BiFunction<? super K,? super V,? extends V> function)

      Replaces each entry's value with the result of invoking the given function on that entry until all entries have been processed or the function throws an exception. Exceptions thrown by the function are relayed to the caller.
      Specified by:
        `replaceAll` in interface `Map<K,V>`
      Implementation Requirements:

        The default implementation is equivalent to, for this `map`:

             for ((Map.Entry<K, V> entry : map.entrySet())
                 do {
                    K k = entry.getKey();
                    V v = entry.getValue();
                 } while(!replace(k, v, function.apply(k, v)));

        The default implementation may retry these steps when multiple threads attempt updates including potentially calling the function repeatedly for a given key.

        This implementation assumes that the ConcurrentMap cannot contain null values and `get()` returning null unambiguously means the key is absent. Implementations which support null values must override this default implementation.

      Parameters:
        `function` - the function to apply to each entry
      Throws:
        `UnsupportedOperationException` - if the `set` operation is not supported by this map's entry set iterator.
        `NullPointerException` - if function or a replacement value is null, and this map does not permit null keys or values (optional)
        `ClassCastException` - if a replacement value is of an inappropriate type for this map (optional)
        `IllegalArgumentException` - if some property of a replacement value prevents it from being stored in this map (optional)
      Since:
        1.8

    - #### computeIfAbsent

          default V computeIfAbsent(K key,
                                    Function<? super K,? extends V> mappingFunction)

      If the specified key is not already associated with a value (or is mapped to `null`), attempts to compute its value using the given mapping function and enters it into this map unless `null`.

      If the function returns `null` no mapping is recorded. If the function itself throws an (unchecked) exception, the exception is rethrown, and no mapping is recorded. The most common usage is to construct a new object serving as an initial mapped value or memoized result, as in:

           map.computeIfAbsent(key, k -> new Value(f(k)));

      Or to implement a multi-value map, `Map<K,Collection<V>>`, supporting multiple values per key:

           map.computeIfAbsent(key, k -> new HashSet<V>()).add(v);

      Specified by:
        `computeIfAbsent` in interface `Map<K,V>`
      Implementation Requirements:
        The default implementation is equivalent to the following steps for this `map`, then returning the current value or `null` if now absent:

             if (map.get(key) == null) {
                 V newValue = mappingFunction.apply(key);
                 if (newValue != null)
                     return map.putIfAbsent(key, newValue);
             }

        The default implementation may retry these steps when multiple threads attempt updates including potentially calling the mapping function multiple times.

        This implementation assumes that the ConcurrentMap cannot contain null values and `get()` returning null unambiguously means the key is absent. Implementations which support null values must override this default implementation.

      Parameters:
        `key` - key with which the specified value is to be associated
        `mappingFunction` - the function to compute a value
      Returns:
        the current (existing or computed) value associated with the specified key, or null if the computed value is null
      Throws:
        `UnsupportedOperationException` - if the `put` operation is not supported by this map (optional)
        `ClassCastException` - if the class of the specified key or value prevents it from being stored in this map (optional)
        `NullPointerException` - if the specified key is null and this map does not support null keys, or the mappingFunction is null
      Since:
        1.8

    - #### computeIfPresent

          default V computeIfPresent(K key,
                                     BiFunction<? super K,? super V,? extends V> remappingFunction)

      If the value for the specified key is present and non-null, attempts to compute a new mapping given the key and its current mapped value.

      If the function returns `null`, the mapping is removed. If the function itself throws an (unchecked) exception, the exception is rethrown, and the current mapping is left unchanged.

      Specified by:
        `computeIfPresent` in interface `Map<K,V>`
      Implementation Requirements:
        The default implementation is equivalent to performing the following steps for this `map`, then returning the current value or `null` if now absent. :

             if (map.get(key) != null) {
                 V oldValue = map.get(key);
                 V newValue = remappingFunction.apply(key, oldValue);
                 if (newValue != null)
                     map.replace(key, oldValue, newValue);
                 else
                     map.remove(key, oldValue);
             }

        The default implementation may retry these steps when multiple threads attempt updates including potentially calling the remapping function multiple times.

        This implementation assumes that the ConcurrentMap cannot contain null values and `get()` returning null unambiguously means the key is absent. Implementations which support null values must override this default implementation.

      Parameters:
        `key` - key with which the specified value is to be associated
        `remappingFunction` - the function to compute a value
      Returns:
        the new value associated with the specified key, or null if none
      Throws:
        `UnsupportedOperationException` - if the `put` operation is not supported by this map (optional)
        `ClassCastException` - if the class of the specified key or value prevents it from being stored in this map (optional)
        `NullPointerException` - if the specified key is null and this map does not support null keys, or the remappingFunction is null
      Since:
        1.8

    - #### compute

          default V compute(K key,
                            BiFunction<? super K,? super V,? extends V> remappingFunction)

      Attempts to compute a mapping for the specified key and its current mapped value (or `null` if there is no current mapping). For example, to either create or append a `String` msg to a value mapping:

           map.compute(key, (k, v) -> (v == null) ? msg : v.concat(msg))

      (Method `merge()` is often simpler to use for such purposes.)

      If the function returns `null`, the mapping is removed (or remains absent if initially absent). If the function itself throws an (unchecked) exception, the exception is rethrown, and the current mapping is left unchanged.

      Specified by:
        `compute` in interface `Map<K,V>`
      Implementation Requirements:
        The default implementation is equivalent to performing the following steps for this `map`, then returning the current value or `null` if absent:

             V oldValue = map.get(key);
             V newValue = remappingFunction.apply(key, oldValue);
             if (oldValue != null ) {
                if (newValue != null)
                   map.replace(key, oldValue, newValue);
                else
                   map.remove(key, oldValue);
             } else {
                if (newValue != null)
                   map.putIfAbsent(key, newValue);
                else
                   return null;
             }

        The default implementation may retry these steps when multiple threads attempt updates including potentially calling the remapping function multiple times.

        This implementation assumes that the ConcurrentMap cannot contain null values and `get()` returning null unambiguously means the key is absent. Implementations which support null values must override this default implementation.

      Parameters:
        `key` - key with which the specified value is to be associated
        `remappingFunction` - the function to compute a value
      Returns:
        the new value associated with the specified key, or null if none
      Throws:
        `UnsupportedOperationException` - if the `put` operation is not supported by this map (optional)
        `ClassCastException` - if the class of the specified key or value prevents it from being stored in this map (optional)
        `NullPointerException` - if the specified key is null and this map does not support null keys, or the remappingFunction is null
      Since:
        1.8

    - #### merge

          default V merge(K key,
                          V value,
                          BiFunction<? super V,? super V,? extends V> remappingFunction)

      If the specified key is not already associated with a value or is associated with null, associates it with the given non-null value. Otherwise, replaces the associated value with the results of the given remapping function, or removes if the result is `null`. This method may be of use when combining multiple mapped values for a key. For example, to either create or append a `String msg` to a value mapping:

           map.merge(key, msg, String::concat)

      If the function returns `null` the mapping is removed. If the function itself throws an (unchecked) exception, the exception is rethrown, and the current mapping is left unchanged.

      Specified by:
        `merge` in interface `Map<K,V>`
      Implementation Requirements:
        The default implementation is equivalent to performing the following steps for this `map`, then returning the current value or `null` if absent:

             V oldValue = map.get(key);
             V newValue = (oldValue == null) ? value :
                          remappingFunction.apply(oldValue, value);
             if (newValue == null)
                 map.remove(key);
             else
                 map.put(key, newValue);

        The default implementation may retry these steps when multiple threads attempt updates including potentially calling the remapping function multiple times.

        This implementation assumes that the ConcurrentMap cannot contain null values and `get()` returning null unambiguously means the key is absent. Implementations which support null values must override this default implementation.

      Parameters:
        `key` - key with which the resulting value is to be associated
        `value` - the non-null value to be merged with the existing value associated with the key or, if no existing value or a null value is associated with the key, to be associated with the key
        `remappingFunction` - the function to recompute a value if present
      Returns:
        the new value associated with the specified key, or null if no value is associated with the key
      Throws:
        `UnsupportedOperationException` - if the `put` operation is not supported by this map (optional)
        `ClassCastException` - if the class of the specified key or value prevents it from being stored in this map (optional)
        `NullPointerException` - if the specified key is null and this map does not support null keys or the value or remappingFunction is null
      Since:
        1.8

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