- Type Parameters:
K
- the type of keys maintained by this mapV
- the type of mapped values
- All Superinterfaces:
Map<K,
V>
- All Known Subinterfaces:
ConcurrentNavigableMap<K,
V>
- All Known Implementing Classes:
ConcurrentHashMap
,ConcurrentSkipListMap
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.
- Since:
- 1.5
-
Nested Class Summary
-
Method Summary
Modifier and TypeMethodDescriptiondefault V
Attempts to compute a mapping for the specified key and its current mapped value, ornull
if there is no current mapping (optional operation).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 tonull
), attempts to compute its value using the given mapping function and enters it into this map unlessnull
(optional operation).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 (optional operation).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
If the specified key is not already associated with a value or is associated with null, associates it with the given non-null value (optional operation).putIfAbsent
(K key, V value) If the specified key is not already associated with a value, associates it with the given value.boolean
Removes the entry for a key only if currently mapped to a given value.Replaces the entry for a key only if currently mapped to some value.boolean
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 (optional operation).
-
Method Details
-
getOrDefault
Returns the value to which the specified key is mapped, ordefaultValue
if this map contains no mapping for the key.- Specified by:
getOrDefault
in interfaceMap<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 returneddefaultValue
- the default mapping of the key- Returns:
- the value to which the specified key is mapped, or
defaultValue
if this map contains no mapping for the key - Throws:
ClassCastException
- if the key is of an inappropriate type for this map (optional)NullPointerException
- if the specified key is null and this map does not permit null keys (optional)- Since:
- 1.8
-
forEach
Performs the given action for each entry in this map until all entries have been processed or the action throws an exception. Unless otherwise specified by the implementing class, actions are performed in the order of entry set iteration (if an iteration order is specified.) Exceptions thrown by the action are relayed to the caller.- Specified by:
forEach
in interfaceMap<K,
V> - Implementation Requirements:
- The default implementation is equivalent to, for this
map
:for (Map.Entry<K,V> entry : map.entrySet()) { action.accept(entry.getKey(), entry.getValue()); }
- Implementation Note:
- The default implementation assumes that
IllegalStateException
thrown 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
-
putIfAbsent
If the specified key is not already associated with a value, associates it with the given value. This is equivalent to, for thismap
:if (!map.containsKey(key)) return map.put(key, value); else return map.get(key);
- Specified by:
putIfAbsent
in interfaceMap<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 associatedvalue
- 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. (Anull
return can also indicate that the map previously associatednull
with the key, if the implementation supports null values.) - Throws:
UnsupportedOperationException
- if theput
operation is not supported by this mapClassCastException
- if the class of the specified key or value prevents it from being stored in this mapNullPointerException
- if the specified key or value is null, and this map does not permit null keys or valuesIllegalArgumentException
- if some property of the specified key or value prevents it from being stored in this map
-
remove
Removes the entry for a key only if currently mapped to a given value. This is equivalent to, for thismap
:if (map.containsKey(key) && Objects.equals(map.get(key), value)) { map.remove(key); return true; } else { return false; }
- Specified by:
remove
in interfaceMap<K,
V> - Implementation Note:
- This implementation intentionally re-abstracts the
inappropriate default provided in
Map
. - Parameters:
key
- key with which the specified value is associatedvalue
- value expected to be associated with the specified key- Returns:
true
if the value was removed- Throws:
UnsupportedOperationException
- if theremove
operation is not supported by this mapClassCastException
- 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
Replaces the entry for a key only if currently mapped to a given value. This is equivalent to, for thismap
:if (map.containsKey(key) && Objects.equals(map.get(key), oldValue)) { map.put(key, newValue); return true; } else { return false; }
- Specified by:
replace
in interfaceMap<K,
V> - Implementation Note:
- This implementation intentionally re-abstracts the
inappropriate default provided in
Map
. - Parameters:
key
- key with which the specified value is associatedoldValue
- value expected to be associated with the specified keynewValue
- value to be associated with the specified key- Returns:
true
if the value was replaced- Throws:
UnsupportedOperationException
- if theput
operation is not supported by this mapClassCastException
- if the class of a specified key or value prevents it from being stored in this mapNullPointerException
- if a specified key or value is null, and this map does not permit null keys or valuesIllegalArgumentException
- if some property of a specified key or value prevents it from being stored in this map
-
replace
Replaces the entry for a key only if currently mapped to some value. This is equivalent to, for thismap
:if (map.containsKey(key)) return map.put(key, value); else return null;
- Specified by:
replace
in interfaceMap<K,
V> - Implementation Note:
- This implementation intentionally re-abstracts the
inappropriate default provided in
Map
. - Parameters:
key
- key with which the specified value is associatedvalue
- 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. (Anull
return can also indicate that the map previously associatednull
with the key, if the implementation supports null values.) - Throws:
UnsupportedOperationException
- if theput
operation is not supported by this mapClassCastException
- if the class of the specified key or value prevents it from being stored in this mapNullPointerException
- if the specified key or value is null, and this map does not permit null keys or valuesIllegalArgumentException
- if some property of the specified key or value prevents it from being stored in this map
-
replaceAll
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 (optional operation). Exceptions thrown by the function are relayed to the caller.- Specified by:
replaceAll
in interfaceMap<K,
V> - 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))); }
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 thereplaceAll
operation is not supported by this map (optional)NullPointerException
- if the specified function is null, or if a replacement value is null and this map does not permit null values (optional)ClassCastException
- if the class of a replacement value prevents it from being stored in this map (optional)IllegalArgumentException
- if some property of a replacement value prevents it from being stored in this map (optional)- Since:
- 1.8
-
computeIfAbsent
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
(optional operation).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:
computeIfAbsent
in interfaceMap<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 associatedmappingFunction
- 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 thecomputeIfAbsent
operation is not supported by this map (optional)ClassCastException
- if the class of the specified key or value prevents it from being stored in this map (optional)NullPointerException
- if the specified key is null and this map does not support null keys, or the mappingFunction is nullIllegalArgumentException
- if some property of the specified key or value prevents it from being stored in this map (optional)- Since:
- 1.8
-
computeIfPresent
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 (optional operation).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:
computeIfPresent
in interfaceMap<K,
V> - 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;
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 associatedremappingFunction
- the remapping function to compute a value- Returns:
- the new value associated with the specified key, or null if none
- Throws:
UnsupportedOperationException
- if thecomputeIfPresent
operation is not supported by this map (optional)ClassCastException
- if the class of the specified key or value prevents it from being stored in this map (optional)NullPointerException
- if the specified key is null and this map does not support null keys, or the remappingFunction is nullIllegalArgumentException
- if some property of the specified key or value prevents it from being stored in this map (optional)- Since:
- 1.8
-
compute
Attempts to compute a mapping for the specified key and its current mapped value, ornull
if there is no current mapping (optional operation). For example, to either create or append aString
msg to a value mapping:map.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:
compute
in interfaceMap<K,
V> - 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; } }
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 associatedremappingFunction
- the remapping function to compute a value- Returns:
- the new value associated with the specified key, or null if none
- Throws:
UnsupportedOperationException
- if thecompute
operation is not supported by this map (optional)ClassCastException
- if the class of the specified key or value prevents it from being stored in this map (optional)NullPointerException
- if the specified key is null and this map does not support null keys, or the remappingFunction is nullIllegalArgumentException
- if some property of the specified key or value prevents it from being stored in this map (optional)- Since:
- 1.8
-
merge
If the specified key is not already associated with a value or is associated with null, associates it with the given non-null value (optional operation). 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 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.
- Specified by:
merge
in interfaceMap<K,
V> - 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; } }
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 associatedvalue
- 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 keyremappingFunction
- 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 themerge
operation is not supported by this map (optional)ClassCastException
- if the class of the specified key or value prevents it from being stored in this map (optional)NullPointerException
- if the specified key is null and this map does not support null keys or the value or remappingFunction is nullIllegalArgumentException
- if some property of the specified key or value prevents it from being stored in this map (optional)- Since:
- 1.8
-