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- 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> AMapproviding thread safety and atomicity guarantees.To maintain the specified guarantees, default implementations of methods including putIfAbsent(K, V)inherited fromMapmust be overridden by implementations of this interface. Similarly, implementations of the collections returned by methodsMap.keySet(),Map.values(), andMap.entrySet()must override methods such asremoveIfwhen necessary to preserve atomicity guarantees.Memory consistency effects: As with other concurrent collections, actions in a thread prior to placing an object into a ConcurrentMapas a key or value happen-before actions subsequent to the access or removal of that object from theConcurrentMapin another thread.This interface is a member of the Java Collections Framework. - Since:
- 1.5
 
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Method SummaryAll Methods Instance Methods Abstract Methods Default Methods Modifier and Type Method Description default Vcompute(K key, BiFunction<? super K,? super V,? extends V> remappingFunction)Attempts to compute a mapping for the specified key and its current mapped value (ornullif there is no current mapping).default VcomputeIfAbsent(K key, Function<? super K,? extends V> mappingFunction)If the specified key is not already associated with a value (or is mapped tonull), attempts to compute its value using the given mapping function and enters it into this map unlessnull.default VcomputeIfPresent(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 voidforEach(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 VgetOrDefault(Object key, V defaultValue)Returns the value to which the specified key is mapped, ordefaultValueif this map contains no mapping for the key.default Vmerge(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.VputIfAbsent(K key, V value)If the specified key is not already associated with a value, associates it with the given value.booleanremove(Object key, Object value)Removes the entry for a key only if currently mapped to a given value.Vreplace(K key, V value)Replaces the entry for a key only if currently mapped to some value.booleanreplace(K key, V oldValue, V newValue)Replaces the entry for a key only if currently mapped to a given value.default voidreplaceAll(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.
 
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Method Detail- 
getOrDefaultdefault V getOrDefault(Object key, V defaultValue) Returns the value to which the specified key is mapped, ordefaultValueif this map contains no mapping for the key.- Specified by:
- getOrDefaultin 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
 defaultValueif 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
 
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forEachdefault 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:
- forEachin 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
 IllegalStateExceptionthrown bygetKey()orgetValue()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
 
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putIfAbsentV 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 thismap:
 except that the action is performed atomically.if (!map.containsKey(key)) return map.put(key, value); else return map.get(key);- Specified by:
- putIfAbsentin 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
         nullif there was no mapping for the key. (Anullreturn can also indicate that the map previously associatednullwith the key, if the implementation supports null values.)
- Throws:
- UnsupportedOperationException- if the- putoperation 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
 
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removeboolean remove(Object key, Object value) Removes the entry for a key only if currently mapped to a given value. This is equivalent to, for thismap:
 except that the action is performed atomically.if (map.containsKey(key) && Objects.equals(map.get(key), value)) { map.remove(key); return true; } else { return false; }- Specified by:
- removein 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:
- trueif the value was removed
- Throws:
- UnsupportedOperationException- if the- removeoperation 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)
 
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replaceboolean 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 thismap:
 except that the action is performed atomically.if (map.containsKey(key) && Objects.equals(map.get(key), oldValue)) { map.put(key, newValue); return true; } else { return false; }- Specified by:
- replacein 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:
- trueif the value was replaced
- Throws:
- UnsupportedOperationException- if the- putoperation 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
 
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replaceV replace(K key, V value) Replaces the entry for a key only if currently mapped to some value. This is equivalent to, for thismap:
 except that the action is performed atomically.if (map.containsKey(key)) return map.put(key, value); else return null;- Specified by:
- replacein 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
         nullif there was no mapping for the key. (Anullreturn can also indicate that the map previously associatednullwith the key, if the implementation supports null values.)
- Throws:
- UnsupportedOperationException- if the- putoperation 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
 
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replaceAlldefault 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:
- replaceAllin interface- Map<K,V>
- Implementation Requirements:
- The default implementation is equivalent to, for this - map:
 The default implementation may retry these steps when multiple threads attempt updates including potentially calling the function repeatedly for a given key.- 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))); }- 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- setoperation 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
 
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computeIfAbsentdefault V computeIfAbsent(K key, Function<? super K,? extends V> mappingFunction) If the specified key is not already associated with a value (or is mapped tonull), attempts to compute its value using the given mapping function and enters it into this map unlessnull.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. - Specified by:
- computeIfAbsentin interface- Map<K,V>
- 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- key with which the specified value is to be associated
- mappingFunction- the mapping 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- putoperation 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
- IllegalArgumentException- if some property of the specified key or value prevents it from being stored in this map (optional)
- Since:
- 1.8
 
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computeIfPresentdefault 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. - Specified by:
- computeIfPresentin interface- Map<K,V>
- Implementation Requirements:
- The default implementation is equivalent to performing the following
 steps for this map:
 When multiple threads attempt updates, map operations and the remapping function may be called multiple times.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;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 remapping function to compute a value
- Returns:
- the new value associated with the specified key, or null if none
- Throws:
- UnsupportedOperationException- if the- putoperation 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
- IllegalArgumentException- if some property of the specified key or value prevents it from being stored in this map (optional)
- Since:
- 1.8
 
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computedefault 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 (ornullif there is no current mapping). For example, to either create or append aStringmsg to a value mapping:
 (Methodmap.compute(key, (k, v) -> (v == null) ? msg : v.concat(msg))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. - Specified by:
- computein interface- Map<K,V>
- Implementation Requirements:
- The default implementation is equivalent to performing the following
 steps for this map:
 When multiple threads attempt updates, map operations and the remapping function may be called multiple times.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; } }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 remapping function to compute a value
- Returns:
- the new value associated with the specified key, or null if none
- Throws:
- UnsupportedOperationException- if the- putoperation 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
- IllegalArgumentException- if some property of the specified key or value prevents it from being stored in this map (optional)
- Since:
- 1.8
 
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mergedefault 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 isnull. This method may be of use when combining multiple mapped values for a key. For example, to either create or append aString msgto 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. - Specified by:
- mergein interface- Map<K,V>
- Implementation Requirements:
- The default implementation is equivalent to performing the following
 steps for this map:
 When multiple threads attempt updates, map operations and the remapping function may be called multiple times.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; } }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 remapping 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- putoperation 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
- IllegalArgumentException- if some property of the specified key or value prevents it from being stored in this map (optional)
- Since:
- 1.8
 
 
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