K - the type of keys maintained by this map
V - the type of mapped values
Map<K,V>
ConcurrentNavigableMap<K,V>
ConcurrentHashMap, ConcurrentSkipListMap
public interface ConcurrentMap<K,V> extends Map<K,V>
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 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.
Map.Entry<K,V>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.
V putIfAbsent(K key, V value)
If the specified key is not already associated with a value, associates 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 declared in interface java.util.Mapclear, containsKey, containsValue, entrySet, equals, get, hashCode, isEmpty, keySet, put, putAll, remove, size, values(
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.
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 returned
defaultValue - the default mapping of the key
defaultValue if this map contains no mapping for the key
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)
(
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.
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 entry
NullPointerException - if the specified action is null
(
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.
putIfAbsent in interface Map<K,V>
Map.
key - key with which the specified value is to be associated
value - value to be associated with the specified key
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.)
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
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.
remove in interface Map<K,V>
Map.
key - key with which the specified value is associated
value - value expected to be associated with the specified key
true if the value was removed
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)
(
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.
replace in interface Map<K,V>
Map.
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
true if the value was replaced
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
(
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.
replace in interface Map<K,V>
Map.
key - key with which the specified value is associated
value - value to be associated with the specified key
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.)
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
(
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.
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 entry
UnsupportedOperationException - if the set operation 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)
(
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.
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 associated
mappingFunction - the mapping function to compute a value
UnsupportedOperationException - 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 null
IllegalArgumentException - if some property of the specified key or value prevents it from being stored in this map (optional)
(
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.
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 associated
remappingFunction - the remapping function to compute a value
UnsupportedOperationException - 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 null
IllegalArgumentException - if some property of the specified key or value prevents it from being stored in this map (optional)
(
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.
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 associated
remappingFunction - the remapping function to compute a value
UnsupportedOperationException - 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 null
IllegalArgumentException - if some property of the specified key or value prevents it from being stored in this map (optional)
(
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.
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 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
UnsupportedOperationException - 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 null
IllegalArgumentException - if some property of the specified key or value prevents it from being stored in this map (optional)
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