K
- the type of keys maintained by this mapV
- the type of mapped valuespublic 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 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 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.
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)
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 entryNullPointerException
- if the specified action is nullV putIfAbsent(K key, V 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.
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)
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.
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)
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.
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)
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.
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)
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()) 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.
function
- the function to apply to each entryUnsupportedOperationException
- 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)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
.
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);
computeIfAbsent
in interface Map<K,V>
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.
key
- key with which the specified value is to be associatedmappingFunction
- the 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 nulldefault 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 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.
computeIfPresent
in interface Map<K,V>
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.
key
- key with which the specified value is to be associatedremappingFunction
- the 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 nulldefault 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). 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 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.
compute
in interface Map<K,V>
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.
key
- key with which the specified value is to be associatedremappingFunction
- the 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 nulldefault 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 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.
merge
in interface Map<K,V>
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.
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 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 null
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