Known Indirect Subclasses
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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.
Public Method Summary
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). |
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 . |
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.
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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.
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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. |
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.
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abstract V |
putIfAbsent(K key, V value)
If the specified key is not already associated
with a value, associates it with the given value.
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abstract boolean | |
abstract boolean |
replace(K key, V oldValue, V newValue)
Replaces the entry for a key only if currently mapped to a given value.
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abstract V |
replace(K key, V value)
Replaces the entry for a key only if currently mapped to some value.
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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.
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Inherited Method Summary
Public Methods
public 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 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.
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
public 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 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.
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
public 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.
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
public 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.
Parameters
action | The action to be performed for each entry |
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Throws
NullPointerException |
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public 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.
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
defaultValue
if this map contains no mapping for the key
public 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 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.
Parameters
key | key with which the resulting value is to be associated |
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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
public abstract V 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 this map
:
if (!map.containsKey(key))
return map.put(key, value);
else
return map.get(key);
except that the action is performed atomically.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
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 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 |
public abstract boolean remove (Object key, Object value)
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.Parameters
key | key with which the specified value is associated |
---|---|
value | value expected to be associated with the specified key |
Returns
true
if the value was removed
Throws
UnsupportedOperationException | if the remove operation
is not supported by this map |
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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) |
public abstract 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, 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.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
true
if the value was replaced
Throws
UnsupportedOperationException | if the put operation
is not supported by this map |
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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 |
public abstract V replace (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.Parameters
key | key with which the specified value is associated |
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value | 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 the put operation
is not supported by this map |
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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 |
public 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.
Parameters
function | the function to apply to each entry |
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