K
- the type of keys maintained by this mapV
- the type of mapped valuesMap<K,V>
ConcurrentNavigableMap<K,V>
ConcurrentHashMap
, ConcurrentSkipListMap
public interface ConcurrentMap<K,V> extends Map<K,V>
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.
Modifier and Type | Method | Description |
---|---|---|
default V |
compute |
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 |
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 |
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 |
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 |
Returns the value to which the specified key is mapped, or defaultValue if this map contains no mapping for the key. |
default V |
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. |
V |
putIfAbsent |
If the specified key is not already associated with a value, associates it with the given value. |
boolean |
remove |
Removes the entry for a key only if currently mapped to a given value. |
V |
replace |
Replaces the entry for a key only if currently mapped to some value. |
boolean |
replace |
Replaces the entry for a key only if currently mapped to a given value. |
default void |
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. |
default V getOrDefault(Object key, V defaultValue)
defaultValue
if this map contains no mapping for the key.getOrDefault
in interface Map<K,V>
get()
returning null unambiguously means the key is absent. Implementations which support null values must override this default implementation.key
- the key whose associated value is to be returneddefaultValue
- the default mapping of the keydefaultValue
if this map contains no mapping for the keyClassCastException
- 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)default void forEach(BiConsumer<? super K,? super V> action)
forEach
in interface Map<K,V>
map
:
for (Map.Entry<K,V> entry : map.entrySet()) {
action.accept(entry.getKey(), entry.getValue());
}
IllegalStateException
thrown by getKey()
or getValue()
indicates that the entry has been removed and cannot be processed. Operation continues for subsequent entries.action
- The action to be performed for each entryNullPointerException
- if the specified action is nullV putIfAbsent(K key, V value)
map
:
if (!map.containsKey(key))
return map.put(key, value);
else
return map.get(key);
except that the action is performed atomically.putIfAbsent
in interface Map<K,V>
Map
.key
- key with which the specified value is to be associatedvalue
- value to be associated with the specified keynull
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.)UnsupportedOperationException
- if the put
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 mapboolean remove(Object key, Object value)
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.remove
in interface Map<K,V>
Map
.key
- key with which the specified value is associatedvalue
- value expected to be associated with the specified keytrue
if the value was removedUnsupportedOperationException
- if the remove
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)boolean replace(K key, V oldValue, V newValue)
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.replace
in interface Map<K,V>
Map
.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 keytrue
if the value was replacedUnsupportedOperationException
- if the put
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 mapV replace(K key, V value)
map
:
if (map.containsKey(key))
return map.put(key, value);
else
return null;
except that the action is performed atomically.replace
in interface Map<K,V>
Map
.key
- key with which the specified value is associatedvalue
- value to be associated with the specified keynull
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.)UnsupportedOperationException
- if the put
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 mapdefault void replaceAll(BiFunction<? super K,? super V,? extends V> function)
replaceAll
in interface Map<K,V>
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.
function
- the function to apply to each entryUnsupportedOperationException
- if the set
operation is not supported by this map's entry set iterator.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)default V computeIfAbsent(K key, Function<? super K,? extends V> mappingFunction)
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.
computeIfAbsent
in interface Map<K,V>
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.
key
- key with which the specified value is to be associatedmappingFunction
- the mapping function to compute a valueUnsupportedOperationException
- if the put
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)default V computeIfPresent(K key, BiFunction<? super K,? super V,? extends V> remappingFunction)
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.
computeIfPresent
in interface Map<K,V>
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.
key
- key with which the specified value is to be associatedremappingFunction
- the remapping function to compute a valueUnsupportedOperationException
- if the put
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)default V compute(K key, BiFunction<? super K,? super V,? extends V> remappingFunction)
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.
compute
in interface Map<K,V>
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.
key
- key with which the specified value is to be associatedremappingFunction
- the remapping function to compute a valueUnsupportedOperationException
- if the put
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)default V merge(K key, V value, BiFunction<? super V,? super V,? extends V> remappingFunction)
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.
merge
in interface Map<K,V>
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.
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 presentUnsupportedOperationException
- if the put
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)
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Documentation extracted from Debian's OpenJDK Development Kit package.
Licensed under the GNU General Public License, version 2, with the Classpath Exception.
Various third party code in OpenJDK is licensed under different licenses (see Debian package).
Java and OpenJDK are trademarks or registered trademarks of Oracle and/or its affiliates.
https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/util/concurrent/ConcurrentMap.html