ConcurrentMap

public interface ConcurrentMap
implements Map<K, V>

java.util.concurrent.ConcurrentMap<K, V>

Known indirect subclasses

ConcurrentHashMap<K, V>, ConcurrentNavigableMap<K, V>, ConcurrentSkipListMap<K, V>

ConcurrentHashMap<K, V>	A hash table supporting full concurrency of retrievals and high expected concurrency for updates.
ConcurrentNavigableMap<K, V>	A `ConcurrentMap` supporting `NavigableMap` operations, and recursively so for its navigable sub-maps.
ConcurrentSkipListMap<K, V>	A scalable concurrent `ConcurrentNavigableMap` implementation.

A Map providing thread safety and atomicity guarantees.

To maintain the specified guarantees, default implementations of methods including putIfAbsent(K, V) inherited from Map must be overridden by implementations of this interface. Similarly, implementations of the collections returned by methods Map.keySet(), Map.values(), and Map.entrySet() must override methods such as removeIf when necessary to preserve 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.

Summary

Public methods
`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.
`abstract V`	`putIfAbsent(K key, V value)` If the specified key is not already associated with a value, associates it with the given value.
`abstract boolean`	`remove(Object key, Object value)` Removes the entry for a key only if currently mapped to a given value.
`abstract boolean`	`replace(K key, V oldValue, V newValue)` Replaces the entry for a key only if currently mapped to a given value.
`abstract V`	`replace(K key, V value)` Replaces the entry for a key only if currently mapped to some 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.

Inherited methods

From interface java.util.Map

`abstract void`	`clear()` Removes all of the mappings from this map (optional operation).
`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.
`abstract boolean`	`containsKey(Object key)` Returns `true` if this map contains a mapping for the specified key.
`abstract boolean`	`containsValue(Object value)` Returns `true` if this map maps one or more keys to the specified value.
`static <K, V> Map<K, V>`	`copyOf(Map<? extends K, ? extends V> map)` Returns an unmodifiable Map containing the entries of the given Map.
`static <K, V> Entry<K, V>`	`entry(K k, V v)` Returns an unmodifiable `Entry` containing the given key and value.
`abstract Set<Entry<K, V>>`	`entrySet()` Returns a `Set` view of the mappings contained in this map.
`abstract boolean`	`equals(Object o)` Compares the specified object with this map for equality.
`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.
`abstract V`	`get(Object key)` Returns the value to which the specified key is mapped, or `null` if this map contains no mapping for the key.
`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.
`abstract int`	`hashCode()` Returns the hash code value for this map.
`abstract boolean`	`isEmpty()` Returns `true` if this map contains no key-value mappings.
`abstract Set<K>`	`keySet()` Returns a `Set` view of the keys contained in this map.
`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.
`static <K, V> Map<K, V>`	`of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5)` Returns an unmodifiable map containing five mappings.
`static <K, V> Map<K, V>`	`of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4)` Returns an unmodifiable map containing four mappings.
`static <K, V> Map<K, V>`	`of(K k1, V v1)` Returns an unmodifiable map containing a single mapping.
`static <K, V> Map<K, V>`	`of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5, K k6, V v6, K k7, V v7, K k8, V v8)` Returns an unmodifiable map containing eight mappings.
`static <K, V> Map<K, V>`	`of(K k1, V v1, K k2, V v2)` Returns an unmodifiable map containing two mappings.
`static <K, V> Map<K, V>`	`of(K k1, V v1, K k2, V v2, K k3, V v3)` Returns an unmodifiable map containing three mappings.
`static <K, V> Map<K, V>`	`of()` Returns an unmodifiable map containing zero mappings.
`static <K, V> Map<K, V>`	`of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5, K k6, V v6, K k7, V v7, K k8, V v8, K k9, V v9)` Returns an unmodifiable map containing nine mappings.
`static <K, V> Map<K, V>`	`of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5, K k6, V v6)` Returns an unmodifiable map containing six mappings.
`static <K, V> Map<K, V>`	`of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5, K k6, V v6, K k7, V v7)` Returns an unmodifiable map containing seven mappings.
`static <K, V> Map<K, V>`	`of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5, K k6, V v6, K k7, V v7, K k8, V v8, K k9, V v9, K k10, V v10)` Returns an unmodifiable map containing ten mappings.
`static <K, V> Map<K, V>`	`ofEntries(Entry...<? extends K, ? extends V> entries)` Returns an unmodifiable map containing keys and values extracted from the given entries.
`abstract V`	`put(K key, V value)` Associates the specified value with the specified key in this map (optional operation).
`abstract void`	`putAll(Map<? extends K, ? extends V> m)` Copies all of the mappings from the specified map to this map (optional operation).
`default V`	`putIfAbsent(K key, V value)` If the specified key is not already associated with a value (or is mapped to `null`) associates it with the given value and returns `null`, else returns the current value.
`default boolean`	`remove(Object key, Object value)` Removes the entry for the specified key only if it is currently mapped to the specified value.
`abstract V`	`remove(Object key)` Removes the mapping for a key from this map if it is present (optional operation).
`default boolean`	`replace(K key, V oldValue, V newValue)` Replaces the entry for the specified key only if currently mapped to the specified value.
`default V`	`replace(K key, V value)` Replaces the entry for the specified key only if it is currently mapped to some 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.
`abstract int`	`size()` Returns the number of key-value mappings in this map.
`abstract Collection<V>`	`values()` Returns a `Collection` view of the values contained in this map.

Public methods

compute

Added in API level 24

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

The remapping function should not modify this map during computation.

Implementation Requirements:

The default implementation is equivalent to performing the following steps for this map:
```
 for (;;) { V oldValue = map.get(key); V newValue = remappingFunction.apply(key, oldValue); if (newValue != null) { if ((oldValue != null) ? map.replace(key, oldValue, newValue) : map.putIfAbsent(key, newValue) == null) return newValue; } else if (oldValue == null || map.remove(key, oldValue)) { return null; } }
```
When multiple threads attempt updates, map operations and the remapping function may be called 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`	`K`: key with which the specified value is to be associated
`remappingFunction`	`BiFunction`: the remapping function to compute a value

Returns
`V`	the new value associated with the specified key, or null if none

Throws
`UnsupportedOperationException`
`ClassCastException`
`NullPointerException`
`IllegalArgumentException`

computeIfAbsent

Added in API level 24

 public 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 mapping function returns null, no mapping is recorded. If the mapping 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);

The mapping function should not modify this map during computation.

Implementation Requirements:

The default implementation is equivalent to the following steps for this map:
```
 V oldValue, newValue; return ((oldValue = map.get(key)) == null && (newValue = mappingFunction.apply(key)) != null && (oldValue = map.putIfAbsent(key, newValue)) == null) ? newValue : oldValue;
```
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`	`K`: key with which the specified value is to be associated
`mappingFunction`	`Function`: the mapping function to compute a value

Returns
`V`	the current (existing or computed) value associated with the specified key, or null if the computed value is null

Throws
`UnsupportedOperationException`
`ClassCastException`
`NullPointerException`
`IllegalArgumentException`

computeIfPresent

Added in API level 24

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

The remapping function should not modify this map during computation.

Implementation Requirements:

The default implementation is equivalent to performing the following steps for this map:
```
 for (V oldValue; (oldValue = map.get(key)) != null; ) { V newValue = remappingFunction.apply(key, oldValue); if ((newValue == null) ? map.remove(key, oldValue) : map.replace(key, oldValue, newValue)) return newValue; } return null;
```
When multiple threads attempt updates, map operations and the remapping function may be called 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`	`K`: key with which the specified value is to be associated
`remappingFunction`	`BiFunction`: the remapping function to compute a value

Returns
`V`	the new value associated with the specified key, or null if none

Throws
`UnsupportedOperationException`
`ClassCastException`
`NullPointerException`
`IllegalArgumentException`

forEach

Added in API level 24

 public 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.

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`	`BiConsumer`: The action to be performed for each entry

Throws
`NullPointerException`

getOrDefault

Added in API level 24

 public 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.

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`	`Object`: the key whose associated value is to be returned
`defaultValue`	`V`: the default mapping of the key

Returns
`V`	the value to which the specified key is mapped, or `defaultValue` if this map contains no mapping for the key

Throws
`ClassCastException`
`NullPointerException`

merge

Added in API level 24

 public 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)

The remapping function should not modify this map during computation.

Implementation Requirements:

The default implementation is equivalent to performing the following steps for this map:
```
 for (;;) { V oldValue = map.get(key); if (oldValue != null) { V newValue = remappingFunction.apply(oldValue, value); if (newValue != null) { if (map.replace(key, oldValue, newValue)) return newValue; } else if (map.remove(key, oldValue)) { return null; } } else if (map.putIfAbsent(key, value) == null) { return value; } }
```
When multiple threads attempt updates, map operations and the remapping function may be called 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`	`K`: key with which the resulting value is to be associated
`value`	`V`: 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`	`BiFunction`: the remapping function to recompute a value if present

Returns
`V`	the new value associated with the specified key, or null if no value is associated with the key

Throws
`UnsupportedOperationException`
`ClassCastException`
`NullPointerException`
`IllegalArgumentException`

putIfAbsent

Added in API level 1

 public abstract V putIfAbsent (K key, V value)

If the specified key is not already associated with a value, associates it with the given value. This is equivalent to, for this map:

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

except that the action is performed atomically.

Implementation Note:

This implementation intentionally re-abstracts the inappropriate default provided in Map.

Parameters
`key`	`K`: key with which the specified value is to be associated
`value`	`V`: value to be associated with the specified key

Returns
`V`	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

Added in API level 1

 public abstract boolean remove (Object key, Object value)

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

 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.

Implementation Note:

This implementation intentionally re-abstracts the inappropriate default provided in Map.

Parameters
`key`	`Object`: key with which the specified value is associated
`value`	`Object`: value expected to be associated with the specified key

Returns
`boolean`	`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

Added in API level 1

 public abstract 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, for this map:

 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.

Implementation Note:

This implementation intentionally re-abstracts the inappropriate default provided in Map.

Parameters
`key`	`K`: key with which the specified value is associated
`oldValue`	`V`: value expected to be associated with the specified key
`newValue`	`V`: value to be associated with the specified key

Returns
`boolean`	`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

Added in API level 1

 public abstract V replace (K key, V value)

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

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

except that the action is performed atomically.

Implementation Note:

This implementation intentionally re-abstracts the inappropriate default provided in Map.

Parameters
`key`	`K`: key with which the specified value is associated
`value`	`V`: value to be associated with the specified key

Returns
`V`	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

Added in API level 24

 public 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.

Implementation Requirements:

The default implementation is equivalent to, for this map:
```
 for (Map.Entry<K,V> entry : map.entrySet()) { K k; V v; do { k = entry.getKey(); v = entry.getValue(); } while (!map.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`	`BiFunction`: the function to apply to each entry

Throws
`UnsupportedOperationException`
`NullPointerException`
`ClassCastException`
`IllegalArgumentException`