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Added in API level 1

ConcurrentMap

public interface ConcurrentMap
implements Map<K, V>

java.util.concurrent.ConcurrentMap<K, V>
Known Indirect Subclasses


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 keySet(), values(), and 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.

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

Public methods

compute

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

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

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

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

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

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 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 = 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
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
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
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
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

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
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