ConcurrentMap (Java Platform SE 8 ) (original) (raw)

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 aConcurrentMap as a key or valuehappen-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](../../../java/util/Map.html "interface in java.util")  
 `[Map.Entry](../../../java/util/Map.Entry.html "interface in java.util")<[K](../../../java/util/Map.Entry.html "type parameter in Map.Entry"),[V](../../../java/util/Map.Entry.html "type parameter in Map.Entry")>`

Method Summary

All Methods Instance Methods Abstract Methods Default Methods

Modifier and Type	Method	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, ordefaultValue 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](../../../java/util/Map.html "interface in java.util")  
   `[clear](../../../java/util/Map.html#clear--), [containsKey](../../../java/util/Map.html#containsKey-java.lang.Object-), [containsValue](../../../java/util/Map.html#containsValue-java.lang.Object-), [entrySet](../../../java/util/Map.html#entrySet--), [equals](../../../java/util/Map.html#equals-java.lang.Object-), [get](../../../java/util/Map.html#get-java.lang.Object-), [hashCode](../../../java/util/Map.html#hashCode--), [isEmpty](../../../java/util/Map.html#isEmpty--), [keySet](../../../java/util/Map.html#keySet--), [put](../../../java/util/Map.html#put-K-V-), [putAll](../../../java/util/Map.html#putAll-java.util.Map-), [remove](../../../java/util/Map.html#remove-java.lang.Object-), [size](../../../java/util/Map.html#size--), [values](../../../java/util/Map.html#values--)`

Method Detail

 * #### getOrDefault  
 default [V](../../../java/util/concurrent/ConcurrentMap.html "type parameter in ConcurrentMap") getOrDefault([Object](../../../java/lang/Object.html "class in java.lang") key,  
                        [V](../../../java/util/concurrent/ConcurrentMap.html "type parameter in ConcurrentMap") 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](../../../java/util/Map.html#getOrDefault-java.lang.Object-V-)` in interface `[Map](../../../java/util/Map.html "interface in java.util")<[K](../../../java/util/concurrent/ConcurrentMap.html "type parameter in ConcurrentMap"),[V](../../../java/util/concurrent/ConcurrentMap.html "type parameter in ConcurrentMap")>`  
 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](../../../java/lang/ClassCastException.html "class in java.lang")` \- if the key is of an inappropriate type for this map ([optional](../../../java/util/Collection.html#optional-restrictions))  
 `[NullPointerException](../../../java/lang/NullPointerException.html "class in java.lang")` \- if the specified key is null and this map does not permit null keys ([optional](../../../java/util/Collection.html#optional-restrictions))  
 Since:  
 1.8  
 * #### forEach  
 default void forEach([BiConsumer](../../../java/util/function/BiConsumer.html "interface in java.util.function")<? super [K](../../../java/util/concurrent/ConcurrentMap.html "type parameter in ConcurrentMap"),? super [V](../../../java/util/concurrent/ConcurrentMap.html "type parameter in ConcurrentMap")> 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](../../../java/util/Map.html#forEach-java.util.function.BiConsumer-)` in interface `[Map](../../../java/util/Map.html "interface in java.util")<[K](../../../java/util/concurrent/ConcurrentMap.html "type parameter in ConcurrentMap"),[V](../../../java/util/concurrent/ConcurrentMap.html "type parameter in ConcurrentMap")>`  
 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](../../../java/lang/NullPointerException.html "class in java.lang")` \- if the specified action is null  
 Since:  
 1.8  
 * #### putIfAbsent  
 [V](../../../java/util/concurrent/ConcurrentMap.html "type parameter in ConcurrentMap") putIfAbsent([K](../../../java/util/concurrent/ConcurrentMap.html "type parameter in ConcurrentMap") key,  
               [V](../../../java/util/concurrent/ConcurrentMap.html "type parameter in ConcurrentMap") 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](../../../java/util/Map.html#putIfAbsent-K-V-)` in interface `[Map](../../../java/util/Map.html "interface in java.util")<[K](../../../java/util/concurrent/ConcurrentMap.html "type parameter in ConcurrentMap"),[V](../../../java/util/concurrent/ConcurrentMap.html "type parameter in ConcurrentMap")>`  
 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](../../../java/lang/UnsupportedOperationException.html "class in java.lang")` \- if the `put` operation is not supported by this map  
 `[ClassCastException](../../../java/lang/ClassCastException.html "class in java.lang")` \- if the class of the specified key or value prevents it from being stored in this map  
 `[NullPointerException](../../../java/lang/NullPointerException.html "class in java.lang")` \- if the specified key or value is null, and this map does not permit null keys or values  
 `[IllegalArgumentException](../../../java/lang/IllegalArgumentException.html "class in java.lang")` \- if some property of the specified key or value prevents it from being stored in this map  
 * #### remove  
 boolean remove([Object](../../../java/lang/Object.html "class in java.lang") key,  
                [Object](../../../java/lang/Object.html "class in java.lang") 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](../../../java/util/Map.html#remove-java.lang.Object-java.lang.Object-)` in interface `[Map](../../../java/util/Map.html "interface in java.util")<[K](../../../java/util/concurrent/ConcurrentMap.html "type parameter in ConcurrentMap"),[V](../../../java/util/concurrent/ConcurrentMap.html "type parameter in ConcurrentMap")>`  
 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](../../../java/lang/UnsupportedOperationException.html "class in java.lang")` \- if the `remove` operation is not supported by this map  
 `[ClassCastException](../../../java/lang/ClassCastException.html "class in java.lang")` \- if the key or value is of an inappropriate type for this map ([optional](../Collection.html#optional-restrictions))  
 `[NullPointerException](../../../java/lang/NullPointerException.html "class in java.lang")` \- if the specified key or value is null, and this map does not permit null keys or values ([optional](../Collection.html#optional-restrictions))  
 * #### replace  
 boolean replace([K](../../../java/util/concurrent/ConcurrentMap.html "type parameter in ConcurrentMap") key,  
                 [V](../../../java/util/concurrent/ConcurrentMap.html "type parameter in ConcurrentMap") oldValue,  
                 [V](../../../java/util/concurrent/ConcurrentMap.html "type parameter in ConcurrentMap") 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](../../../java/util/Map.html#replace-K-V-V-)` in interface `[Map](../../../java/util/Map.html "interface in java.util")<[K](../../../java/util/concurrent/ConcurrentMap.html "type parameter in ConcurrentMap"),[V](../../../java/util/concurrent/ConcurrentMap.html "type parameter in ConcurrentMap")>`  
 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](../../../java/lang/UnsupportedOperationException.html "class in java.lang")` \- if the `put` operation is not supported by this map  
 `[ClassCastException](../../../java/lang/ClassCastException.html "class in java.lang")` \- if the class of a specified key or value prevents it from being stored in this map  
 `[NullPointerException](../../../java/lang/NullPointerException.html "class in java.lang")` \- if a specified key or value is null, and this map does not permit null keys or values  
 `[IllegalArgumentException](../../../java/lang/IllegalArgumentException.html "class in java.lang")` \- if some property of a specified key or value prevents it from being stored in this map  
 * #### replace  
 [V](../../../java/util/concurrent/ConcurrentMap.html "type parameter in ConcurrentMap") replace([K](../../../java/util/concurrent/ConcurrentMap.html "type parameter in ConcurrentMap") key,  
           [V](../../../java/util/concurrent/ConcurrentMap.html "type parameter in ConcurrentMap") 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](../../../java/util/Map.html#replace-K-V-)` in interface `[Map](../../../java/util/Map.html "interface in java.util")<[K](../../../java/util/concurrent/ConcurrentMap.html "type parameter in ConcurrentMap"),[V](../../../java/util/concurrent/ConcurrentMap.html "type parameter in ConcurrentMap")>`  
 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](../../../java/lang/UnsupportedOperationException.html "class in java.lang")` \- if the `put` operation is not supported by this map  
 `[ClassCastException](../../../java/lang/ClassCastException.html "class in java.lang")` \- if the class of the specified key or value prevents it from being stored in this map  
 `[NullPointerException](../../../java/lang/NullPointerException.html "class in java.lang")` \- if the specified key or value is null, and this map does not permit null keys or values  
 `[IllegalArgumentException](../../../java/lang/IllegalArgumentException.html "class in java.lang")` \- if some property of the specified key or value prevents it from being stored in this map  
 * #### replaceAll  
 default void replaceAll([BiFunction](../../../java/util/function/BiFunction.html "interface in java.util.function")<? super [K](../../../java/util/concurrent/ConcurrentMap.html "type parameter in ConcurrentMap"),? super [V](../../../java/util/concurrent/ConcurrentMap.html "type parameter in ConcurrentMap"),? extends [V](../../../java/util/concurrent/ConcurrentMap.html "type parameter in ConcurrentMap")> 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](../../../java/util/Map.html#replaceAll-java.util.function.BiFunction-)` in interface `[Map](../../../java/util/Map.html "interface in java.util")<[K](../../../java/util/concurrent/ConcurrentMap.html "type parameter in ConcurrentMap"),[V](../../../java/util/concurrent/ConcurrentMap.html "type parameter in ConcurrentMap")>`  
 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](../../../java/lang/UnsupportedOperationException.html "class in java.lang")` \- if the `set` operation is not supported by this map's entry set iterator.  
 `[NullPointerException](../../../java/lang/NullPointerException.html "class in java.lang")` \- if function or a replacement value is null, and this map does not permit null keys or values ([optional](../../../java/util/Collection.html#optional-restrictions))  
 `[ClassCastException](../../../java/lang/ClassCastException.html "class in java.lang")` \- if a replacement value is of an inappropriate type for this map ([optional](../../../java/util/Collection.html#optional-restrictions))  
 `[IllegalArgumentException](../../../java/lang/IllegalArgumentException.html "class in java.lang")` \- if some property of a replacement value prevents it from being stored in this map ([optional](../../../java/util/Collection.html#optional-restrictions))  
 Since:  
 1.8  
 * #### computeIfAbsent  
 default [V](../../../java/util/concurrent/ConcurrentMap.html "type parameter in ConcurrentMap") computeIfAbsent([K](../../../java/util/concurrent/ConcurrentMap.html "type parameter in ConcurrentMap") key,  
                           [Function](../../../java/util/function/Function.html "interface in java.util.function")<? super [K](../../../java/util/concurrent/ConcurrentMap.html "type parameter in ConcurrentMap"),? extends [V](../../../java/util/concurrent/ConcurrentMap.html "type parameter in ConcurrentMap")> 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](../../../java/util/Map.html#computeIfAbsent-K-java.util.function.Function-)` in interface `[Map](../../../java/util/Map.html "interface in java.util")<[K](../../../java/util/concurrent/ConcurrentMap.html "type parameter in ConcurrentMap"),[V](../../../java/util/concurrent/ConcurrentMap.html "type parameter in ConcurrentMap")>`  
 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](../../../java/lang/UnsupportedOperationException.html "class in java.lang")` \- if the `put` operation is not supported by this map ([optional](../../../java/util/Collection.html#optional-restrictions))  
 `[ClassCastException](../../../java/lang/ClassCastException.html "class in java.lang")` \- if the class of the specified key or value prevents it from being stored in this map ([optional](../../../java/util/Collection.html#optional-restrictions))  
 `[NullPointerException](../../../java/lang/NullPointerException.html "class in java.lang")` \- 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](../../../java/util/concurrent/ConcurrentMap.html "type parameter in ConcurrentMap") computeIfPresent([K](../../../java/util/concurrent/ConcurrentMap.html "type parameter in ConcurrentMap") key,  
                            [BiFunction](../../../java/util/function/BiFunction.html "interface in java.util.function")<? super [K](../../../java/util/concurrent/ConcurrentMap.html "type parameter in ConcurrentMap"),? super [V](../../../java/util/concurrent/ConcurrentMap.html "type parameter in ConcurrentMap"),? extends [V](../../../java/util/concurrent/ConcurrentMap.html "type parameter in ConcurrentMap")> 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](../../../java/util/Map.html#computeIfPresent-K-java.util.function.BiFunction-)` in interface `[Map](../../../java/util/Map.html "interface in java.util")<[K](../../../java/util/concurrent/ConcurrentMap.html "type parameter in ConcurrentMap"),[V](../../../java/util/concurrent/ConcurrentMap.html "type parameter in ConcurrentMap")>`  
 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](../../../java/lang/UnsupportedOperationException.html "class in java.lang")` \- if the `put` operation is not supported by this map ([optional](../../../java/util/Collection.html#optional-restrictions))  
 `[ClassCastException](../../../java/lang/ClassCastException.html "class in java.lang")` \- if the class of the specified key or value prevents it from being stored in this map ([optional](../../../java/util/Collection.html#optional-restrictions))  
 `[NullPointerException](../../../java/lang/NullPointerException.html "class in java.lang")` \- 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](../../../java/util/concurrent/ConcurrentMap.html "type parameter in ConcurrentMap") compute([K](../../../java/util/concurrent/ConcurrentMap.html "type parameter in ConcurrentMap") key,  
                   [BiFunction](../../../java/util/function/BiFunction.html "interface in java.util.function")<? super [K](../../../java/util/concurrent/ConcurrentMap.html "type parameter in ConcurrentMap"),? super [V](../../../java/util/concurrent/ConcurrentMap.html "type parameter in ConcurrentMap"),? extends [V](../../../java/util/concurrent/ConcurrentMap.html "type parameter in ConcurrentMap")> 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()](../../../java/util/Map.html#merge-K-V-java.util.function.BiFunction-) 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](../../../java/util/Map.html#compute-K-java.util.function.BiFunction-)` in interface `[Map](../../../java/util/Map.html "interface in java.util")<[K](../../../java/util/concurrent/ConcurrentMap.html "type parameter in ConcurrentMap"),[V](../../../java/util/concurrent/ConcurrentMap.html "type parameter in ConcurrentMap")>`  
 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](../../../java/lang/UnsupportedOperationException.html "class in java.lang")` \- if the `put` operation is not supported by this map ([optional](../../../java/util/Collection.html#optional-restrictions))  
 `[ClassCastException](../../../java/lang/ClassCastException.html "class in java.lang")` \- if the class of the specified key or value prevents it from being stored in this map ([optional](../../../java/util/Collection.html#optional-restrictions))  
 `[NullPointerException](../../../java/lang/NullPointerException.html "class in java.lang")` \- 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](../../../java/util/concurrent/ConcurrentMap.html "type parameter in ConcurrentMap") merge([K](../../../java/util/concurrent/ConcurrentMap.html "type parameter in ConcurrentMap") key,  
                 [V](../../../java/util/concurrent/ConcurrentMap.html "type parameter in ConcurrentMap") value,  
                 [BiFunction](../../../java/util/function/BiFunction.html "interface in java.util.function")<? super [V](../../../java/util/concurrent/ConcurrentMap.html "type parameter in ConcurrentMap"),? super [V](../../../java/util/concurrent/ConcurrentMap.html "type parameter in ConcurrentMap"),? extends [V](../../../java/util/concurrent/ConcurrentMap.html "type parameter in ConcurrentMap")> 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](../../../java/util/Map.html#merge-K-V-java.util.function.BiFunction-)` in interface `[Map](../../../java/util/Map.html "interface in java.util")<[K](../../../java/util/concurrent/ConcurrentMap.html "type parameter in ConcurrentMap"),[V](../../../java/util/concurrent/ConcurrentMap.html "type parameter in ConcurrentMap")>`  
 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](../../../java/lang/UnsupportedOperationException.html "class in java.lang")` \- if the `put` operation is not supported by this map ([optional](../../../java/util/Collection.html#optional-restrictions))  
 `[ClassCastException](../../../java/lang/ClassCastException.html "class in java.lang")` \- if the class of the specified key or value prevents it from being stored in this map ([optional](../../../java/util/Collection.html#optional-restrictions))  
 `[NullPointerException](../../../java/lang/NullPointerException.html "class in java.lang")` \- 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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ConcurrentMap (Java Platform SE 8 ) (original) (raw)

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