 Type Parameters:
K
 the type of keys maintained by this mapV
 the type of mapped values
 All Implemented Interfaces:
Serializable
,ConcurrentMap<K,V>
,Map<K,V>
public class ConcurrentHashMap<K,V> extends AbstractMap<K,V> implements ConcurrentMap<K,V>, Serializable
Hashtable
, and
includes versions of methods corresponding to each method of
Hashtable
. However, even though all operations are
threadsafe, retrieval operations do not entail locking,
and there is not any support for locking the entire table
in a way that prevents all access. This class is fully
interoperable with Hashtable
in programs that rely on its
thread safety but not on its synchronization details.
Retrieval operations (including get
) generally do not
block, so may overlap with update operations (including put
and remove
). Retrievals reflect the results of the most
recently completed update operations holding upon their
onset. (More formally, an update operation for a given key bears a
happensbefore relation with any (nonnull) retrieval for
that key reporting the updated value.) For aggregate operations
such as putAll
and clear
, concurrent retrievals may
reflect insertion or removal of only some entries. Similarly,
Iterators, Spliterators and Enumerations return elements reflecting the
state of the hash table at some point at or since the creation of the
iterator/enumeration. They do not throw ConcurrentModificationException
.
However, iterators are designed to be used by only one thread at a time.
Bear in mind that the results of aggregate status methods including
size
, isEmpty
, and containsValue
are typically
useful only when a map is not undergoing concurrent updates in other threads.
Otherwise the results of these methods reflect transient states
that may be adequate for monitoring or estimation purposes, but not
for program control.
The table is dynamically expanded when there are too many
collisions (i.e., keys that have distinct hash codes but fall into
the same slot modulo the table size), with the expected average
effect of maintaining roughly two bins per mapping (corresponding
to a 0.75 load factor threshold for resizing). There may be much
variance around this average as mappings are added and removed, but
overall, this maintains a commonly accepted time/space tradeoff for
hash tables. However, resizing this or any other kind of hash
table may be a relatively slow operation. When possible, it is a
good idea to provide a size estimate as an optional
initialCapacity
constructor argument. An additional optional
loadFactor
constructor argument provides a further means of
customizing initial table capacity by specifying the table density
to be used in calculating the amount of space to allocate for the
given number of elements. Also, for compatibility with previous
versions of this class, constructors may optionally specify an
expected concurrencyLevel
as an additional hint for
internal sizing. Note that using many keys with exactly the same
hashCode()
is a sure way to slow down performance of any
hash table. To ameliorate impact, when keys are Comparable
,
this class may use comparison order among keys to help break ties.
A Set
projection of a ConcurrentHashMap may be created
(using newKeySet()
or newKeySet(int)
), or viewed
(using keySet(Object)
when only keys are of interest, and the
mapped values are (perhaps transiently) not used or all take the
same mapping value.
A ConcurrentHashMap can be used as a scalable frequency map (a
form of histogram or multiset) by using LongAdder
values and initializing via
computeIfAbsent
. For example, to add a count
to a ConcurrentHashMap<String,LongAdder> freqs
, you can use
freqs.computeIfAbsent(key, k > new LongAdder()).increment();
This class and its views and iterators implement all of the
optional methods of the Map
and Iterator
interfaces.
Like Hashtable
but unlike HashMap
, this class
does not allow null
to be used as a key or value.
ConcurrentHashMaps support a set of sequential and parallel bulk
operations that, unlike most Stream
methods, are designed
to be safely, and often sensibly, applied even with maps that are
being concurrently updated by other threads; for example, when
computing a snapshot summary of the values in a shared registry.
There are three kinds of operation, each with four forms, accepting
functions with keys, values, entries, and (key, value) pairs as
arguments and/or return values. Because the elements of a
ConcurrentHashMap are not ordered in any particular way, and may be
processed in different orders in different parallel executions, the
correctness of supplied functions should not depend on any
ordering, or on any other objects or values that may transiently
change while computation is in progress; and except for forEach
actions, should ideally be sideeffectfree. Bulk operations on
Map.Entry
objects do not support method setValue
.
 forEach: Performs a given action on each element. A variant form applies a given transformation on each element before performing the action.
 search: Returns the first available nonnull result of applying a given function on each element; skipping further search when a result is found.
 reduce: Accumulates each element. The supplied reduction
function cannot rely on ordering (more formally, it should be
both associative and commutative). There are five variants:
 Plain reductions. (There is not a form of this method for (key, value) function arguments since there is no corresponding return type.)
 Mapped reductions that accumulate the results of a given function applied to each element.
 Reductions to scalar doubles, longs, and ints, using a given basis value.
These bulk operations accept a parallelismThreshold
argument. Methods proceed sequentially if the current map size is
estimated to be less than the given threshold. Using a value of
Long.MAX_VALUE
suppresses all parallelism. Using a value
of 1
results in maximal parallelism by partitioning into
enough subtasks to fully utilize the ForkJoinPool.commonPool()
that is used for all parallel
computations. Normally, you would initially choose one of these
extreme values, and then measure performance of using inbetween
values that trade off overhead versus throughput.
The concurrency properties of bulk operations follow
from those of ConcurrentHashMap: Any nonnull result returned
from get(key)
and related access methods bears a
happensbefore relation with the associated insertion or
update. The result of any bulk operation reflects the
composition of these perelement relations (but is not
necessarily atomic with respect to the map as a whole unless it
is somehow known to be quiescent). Conversely, because keys
and values in the map are never null, null serves as a reliable
atomic indicator of the current lack of any result. To
maintain this property, null serves as an implicit basis for
all nonscalar reduction operations. For the double, long, and
int versions, the basis should be one that, when combined with
any other value, returns that other value (more formally, it
should be the identity element for the reduction). Most common
reductions have these properties; for example, computing a sum
with basis 0 or a minimum with basis MAX_VALUE.
Search and transformation functions provided as arguments should similarly return null to indicate the lack of any result (in which case it is not used). In the case of mapped reductions, this also enables transformations to serve as filters, returning null (or, in the case of primitive specializations, the identity basis) if the element should not be combined. You can create compound transformations and filterings by composing them yourself under this "null means there is nothing there now" rule before using them in search or reduce operations.
Methods accepting and/or returning Entry arguments maintain
keyvalue associations. They may be useful for example when
finding the key for the greatest value. Note that "plain" Entry
arguments can be supplied using new
AbstractMap.SimpleEntry(k,v)
.
Bulk operations may complete abruptly, throwing an exception encountered in the application of a supplied function. Bear in mind when handling such exceptions that other concurrently executing functions could also have thrown exceptions, or would have done so if the first exception had not occurred.
Speedups for parallel compared to sequential forms are common but not guaranteed. Parallel operations involving brief functions on small maps may execute more slowly than sequential forms if the underlying work to parallelize the computation is more expensive than the computation itself. Similarly, parallelization may not lead to much actual parallelism if all processors are busy performing unrelated tasks.
All arguments to all task methods must be nonnull.
This class is a member of the Java Collections Framework.
 Since:
 1.5
 See Also:
 Serialized Form

Nested Class Summary
Nested Classes Modifier and Type Class Description static class
ConcurrentHashMap.KeySetView<K,V>
A view of a ConcurrentHashMap as aSet
of keys, in which additions may optionally be enabled by mapping to a common value.Nested classes/interfaces declared in class java.util.AbstractMap
AbstractMap.SimpleEntry<K,V>, AbstractMap.SimpleImmutableEntry<K,V>

Constructor Summary
Constructors Constructor Description ConcurrentHashMap()
Creates a new, empty map with the default initial table size (16).ConcurrentHashMap(int initialCapacity)
Creates a new, empty map with an initial table size accommodating the specified number of elements without the need to dynamically resize.ConcurrentHashMap(int initialCapacity, float loadFactor)
Creates a new, empty map with an initial table size based on the given number of elements (initialCapacity
) and initial table density (loadFactor
).ConcurrentHashMap(int initialCapacity, float loadFactor, int concurrencyLevel)
Creates a new, empty map with an initial table size based on the given number of elements (initialCapacity
), initial table density (loadFactor
), and number of concurrently updating threads (concurrencyLevel
).ConcurrentHashMap(Map<? extends K,? extends V> m)
Creates a new map with the same mappings as the given map. 
Method Summary
Modifier and Type Method Description void
clear()
Removes all of the mappings from this map.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 (ornull
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, attempts to compute its value using the given mapping function and enters it into this map unlessnull
.V
computeIfPresent(K key, BiFunction<? super K,? super V,? extends V> remappingFunction)
If the value for the specified key is present, attempts to compute a new mapping given the key and its current mapped value.boolean
contains(Object value)
Tests if some key maps into the specified value in this table.boolean
containsKey(Object key)
Tests if the specified object is a key in this table.boolean
containsValue(Object value)
Returnstrue
if this map maps one or more keys to the specified value.Enumeration<V>
elements()
Returns an enumeration of the values in this table.Set<Map.Entry<K,V>>
entrySet()
Returns aSet
view of the mappings contained in this map.boolean
equals(Object o)
Compares the specified object with this map for equality.void
forEach(long parallelismThreshold, BiConsumer<? super K,? super V> action)
Performs the given action for each (key, value).<U> void
forEach(long parallelismThreshold, BiFunction<? super K,? super V,? extends U> transformer, Consumer<? super U> action)
Performs the given action for each nonnull transformation of each (key, value).void
forEachEntry(long parallelismThreshold, Consumer<? super Map.Entry<K,V>> action)
Performs the given action for each entry.<U> void
forEachEntry(long parallelismThreshold, Function<Map.Entry<K,V>,? extends U> transformer, Consumer<? super U> action)
Performs the given action for each nonnull transformation of each entry.void
forEachKey(long parallelismThreshold, Consumer<? super K> action)
Performs the given action for each key.<U> void
forEachKey(long parallelismThreshold, Function<? super K,? extends U> transformer, Consumer<? super U> action)
Performs the given action for each nonnull transformation of each key.void
forEachValue(long parallelismThreshold, Consumer<? super V> action)
Performs the given action for each value.<U> void
forEachValue(long parallelismThreshold, Function<? super V,? extends U> transformer, Consumer<? super U> action)
Performs the given action for each nonnull transformation of each value.V
get(Object key)
Returns the value to which the specified key is mapped, ornull
if this map contains no mapping for the key.V
getOrDefault(Object key, V defaultValue)
Returns the value to which the specified key is mapped, or the given default value if this map contains no mapping for the key.int
hashCode()
Returns the hash code value for thisMap
, i.e., the sum of, for each keyvalue pair in the map,key.hashCode() ^ value.hashCode()
.Enumeration<K>
keys()
Returns an enumeration of the keys in this table.ConcurrentHashMap.KeySetView<K,V>
keySet()
Returns aSet
view of the keys contained in this map.ConcurrentHashMap.KeySetView<K,V>
keySet(V mappedValue)
Returns aSet
view of the keys in this map, using the given common mapped value for any additions (i.e.,Collection.add(E)
andCollection.addAll(Collection)
).long
mappingCount()
Returns the number of mappings.V
merge(K key, V value, BiFunction<? super V,? super V,? extends V> remappingFunction)
If the specified key is not already associated with a (nonnull) value, associates it with the given value.static <K> ConcurrentHashMap.KeySetView<K,Boolean>
newKeySet()
Creates a newSet
backed by a ConcurrentHashMap from the given type toBoolean.TRUE
.static <K> ConcurrentHashMap.KeySetView<K,Boolean>
newKeySet(int initialCapacity)
Creates a newSet
backed by a ConcurrentHashMap from the given type toBoolean.TRUE
.V
put(K key, V value)
Maps the specified key to the specified value in this table.void
putAll(Map<? extends K,? extends V> m)
Copies all of the mappings from the specified map to this one.V
putIfAbsent(K key, V value)
If the specified key is not already associated with a value, associates it with the given value.<U> U
reduce(long parallelismThreshold, BiFunction<? super K,? super V,? extends U> transformer, BiFunction<? super U,? super U,? extends U> reducer)
Returns the result of accumulating the given transformation of all (key, value) pairs using the given reducer to combine values, or null if none.Map.Entry<K,V>
reduceEntries(long parallelismThreshold, BiFunction<Map.Entry<K,V>,Map.Entry<K,V>,? extends Map.Entry<K,V>> reducer)
Returns the result of accumulating all entries using the given reducer to combine values, or null if none.<U> U
reduceEntries(long parallelismThreshold, Function<Map.Entry<K,V>,? extends U> transformer, BiFunction<? super U,? super U,? extends U> reducer)
Returns the result of accumulating the given transformation of all entries using the given reducer to combine values, or null if none.double
reduceEntriesToDouble(long parallelismThreshold, ToDoubleFunction<Map.Entry<K,V>> transformer, double basis, DoubleBinaryOperator reducer)
Returns the result of accumulating the given transformation of all entries using the given reducer to combine values, and the given basis as an identity value.int
reduceEntriesToInt(long parallelismThreshold, ToIntFunction<Map.Entry<K,V>> transformer, int basis, IntBinaryOperator reducer)
Returns the result of accumulating the given transformation of all entries using the given reducer to combine values, and the given basis as an identity value.long
reduceEntriesToLong(long parallelismThreshold, ToLongFunction<Map.Entry<K,V>> transformer, long basis, LongBinaryOperator reducer)
Returns the result of accumulating the given transformation of all entries using the given reducer to combine values, and the given basis as an identity value.K
reduceKeys(long parallelismThreshold, BiFunction<? super K,? super K,? extends K> reducer)
Returns the result of accumulating all keys using the given reducer to combine values, or null if none.<U> U
reduceKeys(long parallelismThreshold, Function<? super K,? extends U> transformer, BiFunction<? super U,? super U,? extends U> reducer)
Returns the result of accumulating the given transformation of all keys using the given reducer to combine values, or null if none.double
reduceKeysToDouble(long parallelismThreshold, ToDoubleFunction<? super K> transformer, double basis, DoubleBinaryOperator reducer)
Returns the result of accumulating the given transformation of all keys using the given reducer to combine values, and the given basis as an identity value.int
reduceKeysToInt(long parallelismThreshold, ToIntFunction<? super K> transformer, int basis, IntBinaryOperator reducer)
Returns the result of accumulating the given transformation of all keys using the given reducer to combine values, and the given basis as an identity value.long
reduceKeysToLong(long parallelismThreshold, ToLongFunction<? super K> transformer, long basis, LongBinaryOperator reducer)
Returns the result of accumulating the given transformation of all keys using the given reducer to combine values, and the given basis as an identity value.double
reduceToDouble(long parallelismThreshold, ToDoubleBiFunction<? super K,? super V> transformer, double basis, DoubleBinaryOperator reducer)
Returns the result of accumulating the given transformation of all (key, value) pairs using the given reducer to combine values, and the given basis as an identity value.int
reduceToInt(long parallelismThreshold, ToIntBiFunction<? super K,? super V> transformer, int basis, IntBinaryOperator reducer)
Returns the result of accumulating the given transformation of all (key, value) pairs using the given reducer to combine values, and the given basis as an identity value.long
reduceToLong(long parallelismThreshold, ToLongBiFunction<? super K,? super V> transformer, long basis, LongBinaryOperator reducer)
Returns the result of accumulating the given transformation of all (key, value) pairs using the given reducer to combine values, and the given basis as an identity value.V
reduceValues(long parallelismThreshold, BiFunction<? super V,? super V,? extends V> reducer)
Returns the result of accumulating all values using the given reducer to combine values, or null if none.<U> U
reduceValues(long parallelismThreshold, Function<? super V,? extends U> transformer, BiFunction<? super U,? super U,? extends U> reducer)
Returns the result of accumulating the given transformation of all values using the given reducer to combine values, or null if none.double
reduceValuesToDouble(long parallelismThreshold, ToDoubleFunction<? super V> transformer, double basis, DoubleBinaryOperator reducer)
Returns the result of accumulating the given transformation of all values using the given reducer to combine values, and the given basis as an identity value.int
reduceValuesToInt(long parallelismThreshold, ToIntFunction<? super V> transformer, int basis, IntBinaryOperator reducer)
Returns the result of accumulating the given transformation of all values using the given reducer to combine values, and the given basis as an identity value.long
reduceValuesToLong(long parallelismThreshold, ToLongFunction<? super V> transformer, long basis, LongBinaryOperator reducer)
Returns the result of accumulating the given transformation of all values using the given reducer to combine values, and the given basis as an identity value.V
remove(Object key)
Removes the key (and its corresponding value) from this map.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.<U> U
search(long parallelismThreshold, BiFunction<? super K,? super V,? extends U> searchFunction)
Returns a nonnull result from applying the given search function on each (key, value), or null if none.<U> U
searchEntries(long parallelismThreshold, Function<Map.Entry<K,V>,? extends U> searchFunction)
Returns a nonnull result from applying the given search function on each entry, or null if none.<U> U
searchKeys(long parallelismThreshold, Function<? super K,? extends U> searchFunction)
Returns a nonnull result from applying the given search function on each key, or null if none.<U> U
searchValues(long parallelismThreshold, Function<? super V,? extends U> searchFunction)
Returns a nonnull result from applying the given search function on each value, or null if none.String
toString()
Returns a string representation of this map.Collection<V>
values()
Returns aCollection
view of the values contained in this map.

Constructor Details

ConcurrentHashMap
public ConcurrentHashMap()Creates a new, empty map with the default initial table size (16). 
ConcurrentHashMap
public ConcurrentHashMap(int initialCapacity)Creates a new, empty map with an initial table size accommodating the specified number of elements without the need to dynamically resize. Parameters:
initialCapacity
 The implementation performs internal sizing to accommodate this many elements. Throws:
IllegalArgumentException
 if the initial capacity of elements is negative

ConcurrentHashMap
Creates a new map with the same mappings as the given map. Parameters:
m
 the map

ConcurrentHashMap
public ConcurrentHashMap(int initialCapacity, float loadFactor)Creates a new, empty map with an initial table size based on the given number of elements (initialCapacity
) and initial table density (loadFactor
). Parameters:
initialCapacity
 the initial capacity. The implementation performs internal sizing to accommodate this many elements, given the specified load factor.loadFactor
 the load factor (table density) for establishing the initial table size Throws:
IllegalArgumentException
 if the initial capacity of elements is negative or the load factor is nonpositive Since:
 1.6

ConcurrentHashMap
public ConcurrentHashMap(int initialCapacity, float loadFactor, int concurrencyLevel)Creates a new, empty map with an initial table size based on the given number of elements (initialCapacity
), initial table density (loadFactor
), and number of concurrently updating threads (concurrencyLevel
). Parameters:
initialCapacity
 the initial capacity. The implementation performs internal sizing to accommodate this many elements, given the specified load factor.loadFactor
 the load factor (table density) for establishing the initial table sizeconcurrencyLevel
 the estimated number of concurrently updating threads. The implementation may use this value as a sizing hint. Throws:
IllegalArgumentException
 if the initial capacity is negative or the load factor or concurrencyLevel are nonpositive


Method Details

get
Returns the value to which the specified key is mapped, ornull
if this map contains no mapping for the key.More formally, if this map contains a mapping from a key
k
to a valuev
such thatkey.equals(k)
, then this method returnsv
; otherwise it returnsnull
. (There can be at most one such mapping.) Specified by:
get
in interfaceMap<K,V>
 Overrides:
get
in classAbstractMap<K,V>
 Parameters:
key
 the key whose associated value is to be returned Returns:
 the value to which the specified key is mapped, or
null
if this map contains no mapping for the key  Throws:
NullPointerException
 if the specified key is null

containsKey
Tests if the specified object is a key in this table. Specified by:
containsKey
in interfaceMap<K,V>
 Overrides:
containsKey
in classAbstractMap<K,V>
 Parameters:
key
 possible key Returns:
true
if and only if the specified object is a key in this table, as determined by theequals
method;false
otherwise Throws:
NullPointerException
 if the specified key is null

containsValue
Returnstrue
if this map maps one or more keys to the specified value. Note: This method may require a full traversal of the map, and is much slower than methodcontainsKey
. Specified by:
containsValue
in interfaceMap<K,V>
 Overrides:
containsValue
in classAbstractMap<K,V>
 Parameters:
value
 value whose presence in this map is to be tested Returns:
true
if this map maps one or more keys to the specified value Throws:
NullPointerException
 if the specified value is null

put
Maps the specified key to the specified value in this table. Neither the key nor the value can be null.The value can be retrieved by calling the
get
method with a key that is equal to the original key. Specified by:
put
in interfaceMap<K,V>
 Overrides:
put
in classAbstractMap<K,V>
 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
key
, ornull
if there was no mapping forkey
 Throws:
NullPointerException
 if the specified key or value is null

putAll
Copies all of the mappings from the specified map to this one. These mappings replace any mappings that this map had for any of the keys currently in the specified map. 
remove
Removes the key (and its corresponding value) from this map. This method does nothing if the key is not in the map. Specified by:
remove
in interfaceMap<K,V>
 Overrides:
remove
in classAbstractMap<K,V>
 Parameters:
key
 the key that needs to be removed Returns:
 the previous value associated with
key
, ornull
if there was no mapping forkey
 Throws:
NullPointerException
 if the specified key is null

clear
public void clear()Removes all of the mappings from this map. 
keySet
Returns aSet
view of the keys contained in this map. The set is backed by the map, so changes to the map are reflected in the set, and viceversa. The set supports element removal, which removes the corresponding mapping from this map, via theIterator.remove
,Set.remove
,removeAll
,retainAll
, andclear
operations. It does not support theadd
oraddAll
operations.The view's iterators and spliterators are weakly consistent.
The view's
spliterator
reportsSpliterator.CONCURRENT
,Spliterator.DISTINCT
, andSpliterator.NONNULL
. 
values
Returns aCollection
view of the values contained in this map. The collection is backed by the map, so changes to the map are reflected in the collection, and viceversa. The collection supports element removal, which removes the corresponding mapping from this map, via theIterator.remove
,Collection.remove
,removeAll
,retainAll
, andclear
operations. It does not support theadd
oraddAll
operations.The view's iterators and spliterators are weakly consistent.
The view's
spliterator
reportsSpliterator.CONCURRENT
andSpliterator.NONNULL
. 
entrySet
Returns aSet
view of the mappings contained in this map. The set is backed by the map, so changes to the map are reflected in the set, and viceversa. The set supports element removal, which removes the corresponding mapping from the map, via theIterator.remove
,Set.remove
,removeAll
,retainAll
, andclear
operations.The view's iterators and spliterators are weakly consistent.
The view's
spliterator
reportsSpliterator.CONCURRENT
,Spliterator.DISTINCT
, andSpliterator.NONNULL
. 
hashCode
public int hashCode()Returns the hash code value for thisMap
, i.e., the sum of, for each keyvalue pair in the map,key.hashCode() ^ value.hashCode()
. Specified by:
hashCode
in interfaceMap<K,V>
 Overrides:
hashCode
in classAbstractMap<K,V>
 Returns:
 the hash code value for this map
 See Also:
Map.Entry.hashCode()
,Object.equals(Object)
,Set.equals(Object)

toString
Returns a string representation of this map. The string representation consists of a list of keyvalue mappings (in no particular order) enclosed in braces ("{}
"). Adjacent mappings are separated by the characters", "
(comma and space). Each keyvalue mapping is rendered as the key followed by an equals sign ("=
") followed by the associated value. Overrides:
toString
in classAbstractMap<K,V>
 Returns:
 a string representation of this map

equals
Compares the specified object with this map for equality. Returnstrue
if the given object is a map with the same mappings as this map. This operation may return misleading results if either map is concurrently modified during execution of this method. Specified by:
equals
in interfaceMap<K,V>
 Overrides:
equals
in classAbstractMap<K,V>
 Parameters:
o
 object to be compared for equality with this map Returns:
true
if the specified object is equal to this map See Also:
Object.hashCode()
,HashMap

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 interfaceConcurrentMap<K,V>
 Specified by:
putIfAbsent
in interfaceMap<K,V>
 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  Throws:
NullPointerException
 if the specified key or value is null

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 interfaceConcurrentMap<K,V>
 Specified by:
remove
in interfaceMap<K,V>
 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:
NullPointerException
 if the specified key is null

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 interfaceConcurrentMap<K,V>
 Specified by:
replace
in interfaceMap<K,V>
 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:
NullPointerException
 if any of the arguments are null

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 interfaceConcurrentMap<K,V>
 Specified by:
replace
in interfaceMap<K,V>
 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  Throws:
NullPointerException
 if the specified key or value is null

getOrDefault
Returns the value to which the specified key is mapped, or the given default value if this map contains no mapping for the key. Specified by:
getOrDefault
in interfaceConcurrentMap<K,V>
 Specified by:
getOrDefault
in interfaceMap<K,V>
 Parameters:
key
 the key whose associated value is to be returneddefaultValue
 the value to return if this map contains no mapping for the given key Returns:
 the mapping for the key, if present; else the default value
 Throws:
NullPointerException
 if the specified key is null

computeIfAbsent
If the specified key is not already associated with a value, attempts to compute its value using the given mapping function and enters it into this map unlessnull
. The entire method invocation is performed atomically, so the function is applied at most once per key. Some attempted update operations on this map by other threads may be blocked while computation is in progress, so the computation should be short and simple, and must not attempt to update any other mappings of this map. Specified by:
computeIfAbsent
in interfaceConcurrentMap<K,V>
 Specified by:
computeIfAbsent
in interfaceMap<K,V>
 Parameters:
key
 key with which the specified value is to be associatedmappingFunction
 the 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:
NullPointerException
 if the specified key or mappingFunction is nullIllegalStateException
 if the computation detectably attempts a recursive update to this map that would otherwise never completeRuntimeException
 or Error if the mappingFunction does so, in which case the mapping is left unestablished

computeIfPresent
If the value for the specified key is present, attempts to compute a new mapping given the key and its current mapped value. The entire method invocation is performed atomically. Some attempted update operations on this map by other threads may be blocked while computation is in progress, so the computation should be short and simple, and must not attempt to update any other mappings of this map. Specified by:
computeIfPresent
in interfaceConcurrentMap<K,V>
 Specified by:
computeIfPresent
in interfaceMap<K,V>
 Parameters:
key
 key with which a value may be associatedremappingFunction
 the function to compute a value Returns:
 the new value associated with the specified key, or null if none
 Throws:
NullPointerException
 if the specified key or remappingFunction is nullIllegalStateException
 if the computation detectably attempts a recursive update to this map that would otherwise never completeRuntimeException
 or Error if the remappingFunction does so, in which case the mapping is unchanged

compute
Attempts to compute a mapping for the specified key and its current mapped value (ornull
if there is no current mapping). The entire method invocation is performed atomically. Some attempted update operations on this map by other threads may be blocked while computation is in progress, so the computation should be short and simple, and must not attempt to update any other mappings of this Map. Specified by:
compute
in interfaceConcurrentMap<K,V>
 Specified by:
compute
in interfaceMap<K,V>
 Parameters:
key
 key with which the specified value is to be associatedremappingFunction
 the function to compute a value Returns:
 the new value associated with the specified key, or null if none
 Throws:
NullPointerException
 if the specified key or remappingFunction is nullIllegalStateException
 if the computation detectably attempts a recursive update to this map that would otherwise never completeRuntimeException
 or Error if the remappingFunction does so, in which case the mapping is unchanged

merge
If the specified key is not already associated with a (nonnull) value, associates it with the given value. Otherwise, replaces the value with the results of the given remapping function, or removes ifnull
. The entire method invocation is performed atomically. Some attempted update operations on this map by other threads may be blocked while computation is in progress, so the computation should be short and simple, and must not attempt to update any other mappings of this Map. Specified by:
merge
in interfaceConcurrentMap<K,V>
 Specified by:
merge
in interfaceMap<K,V>
 Parameters:
key
 key with which the specified value is to be associatedvalue
 the value to use if absentremappingFunction
 the function to recompute a value if present Returns:
 the new value associated with the specified key, or null if none
 Throws:
NullPointerException
 if the specified key or the remappingFunction is nullRuntimeException
 or Error if the remappingFunction does so, in which case the mapping is unchanged

contains
Tests if some key maps into the specified value in this table.Note that this method is identical in functionality to
containsValue(Object)
, and exists solely to ensure full compatibility with classHashtable
, which supported this method prior to introduction of the Java Collections Framework. Parameters:
value
 a value to search for Returns:
true
if and only if some key maps to thevalue
argument in this table as determined by theequals
method;false
otherwise Throws:
NullPointerException
 if the specified value is null

keys
Returns an enumeration of the keys in this table. Returns:
 an enumeration of the keys in this table
 See Also:
keySet()

elements
Returns an enumeration of the values in this table. Returns:
 an enumeration of the values in this table
 See Also:
values()

mappingCount
public long mappingCount()Returns the number of mappings. This method should be used instead ofMap.size()
because a ConcurrentHashMap may contain more mappings than can be represented as an int. The value returned is an estimate; the actual count may differ if there are concurrent insertions or removals. Returns:
 the number of mappings
 Since:
 1.8

newKeySet
Creates a newSet
backed by a ConcurrentHashMap from the given type toBoolean.TRUE
. Type Parameters:
K
 the element type of the returned set Returns:
 the new set
 Since:
 1.8

newKeySet
Creates a newSet
backed by a ConcurrentHashMap from the given type toBoolean.TRUE
. Type Parameters:
K
 the element type of the returned set Parameters:
initialCapacity
 The implementation performs internal sizing to accommodate this many elements. Returns:
 the new set
 Throws:
IllegalArgumentException
 if the initial capacity of elements is negative Since:
 1.8

keySet
Returns aSet
view of the keys in this map, using the given common mapped value for any additions (i.e.,Collection.add(E)
andCollection.addAll(Collection)
). This is of course only appropriate if it is acceptable to use the same value for all additions from this view. Parameters:
mappedValue
 the mapped value to use for any additions Returns:
 the set view
 Throws:
NullPointerException
 if the mappedValue is null

forEach
Performs the given action for each (key, value). Parameters:
parallelismThreshold
 the (estimated) number of elements needed for this operation to be executed in parallelaction
 the action Since:
 1.8

forEach
public <U> void forEach(long parallelismThreshold, BiFunction<? super K,? super V,? extends U> transformer, Consumer<? super U> action)Performs the given action for each nonnull transformation of each (key, value). Type Parameters:
U
 the return type of the transformer Parameters:
parallelismThreshold
 the (estimated) number of elements needed for this operation to be executed in paralleltransformer
 a function returning the transformation for an element, or null if there is no transformation (in which case the action is not applied)action
 the action Since:
 1.8

search
public <U> U search(long parallelismThreshold, BiFunction<? super K,? super V,? extends U> searchFunction)Returns a nonnull result from applying the given search function on each (key, value), or null if none. Upon success, further element processing is suppressed and the results of any other parallel invocations of the search function are ignored. Type Parameters:
U
 the return type of the search function Parameters:
parallelismThreshold
 the (estimated) number of elements needed for this operation to be executed in parallelsearchFunction
 a function returning a nonnull result on success, else null Returns:
 a nonnull result from applying the given search function on each (key, value), or null if none
 Since:
 1.8

reduce
public <U> U reduce(long parallelismThreshold, BiFunction<? super K,? super V,? extends U> transformer, BiFunction<? super U,? super U,? extends U> reducer)Returns the result of accumulating the given transformation of all (key, value) pairs using the given reducer to combine values, or null if none. Type Parameters:
U
 the return type of the transformer Parameters:
parallelismThreshold
 the (estimated) number of elements needed for this operation to be executed in paralleltransformer
 a function returning the transformation for an element, or null if there is no transformation (in which case it is not combined)reducer
 a commutative associative combining function Returns:
 the result of accumulating the given transformation of all (key, value) pairs
 Since:
 1.8

reduceToDouble
public double reduceToDouble(long parallelismThreshold, ToDoubleBiFunction<? super K,? super V> transformer, double basis, DoubleBinaryOperator reducer)Returns the result of accumulating the given transformation of all (key, value) pairs using the given reducer to combine values, and the given basis as an identity value. Parameters:
parallelismThreshold
 the (estimated) number of elements needed for this operation to be executed in paralleltransformer
 a function returning the transformation for an elementbasis
 the identity (initial default value) for the reductionreducer
 a commutative associative combining function Returns:
 the result of accumulating the given transformation of all (key, value) pairs
 Since:
 1.8

reduceToLong
public long reduceToLong(long parallelismThreshold, ToLongBiFunction<? super K,? super V> transformer, long basis, LongBinaryOperator reducer)Returns the result of accumulating the given transformation of all (key, value) pairs using the given reducer to combine values, and the given basis as an identity value. Parameters:
parallelismThreshold
 the (estimated) number of elements needed for this operation to be executed in paralleltransformer
 a function returning the transformation for an elementbasis
 the identity (initial default value) for the reductionreducer
 a commutative associative combining function Returns:
 the result of accumulating the given transformation of all (key, value) pairs
 Since:
 1.8

reduceToInt
public int reduceToInt(long parallelismThreshold, ToIntBiFunction<? super K,? super V> transformer, int basis, IntBinaryOperator reducer)Returns the result of accumulating the given transformation of all (key, value) pairs using the given reducer to combine values, and the given basis as an identity value. Parameters:
parallelismThreshold
 the (estimated) number of elements needed for this operation to be executed in paralleltransformer
 a function returning the transformation for an elementbasis
 the identity (initial default value) for the reductionreducer
 a commutative associative combining function Returns:
 the result of accumulating the given transformation of all (key, value) pairs
 Since:
 1.8

forEachKey
Performs the given action for each key. Parameters:
parallelismThreshold
 the (estimated) number of elements needed for this operation to be executed in parallelaction
 the action Since:
 1.8

forEachKey
public <U> void forEachKey(long parallelismThreshold, Function<? super K,? extends U> transformer, Consumer<? super U> action)Performs the given action for each nonnull transformation of each key. Type Parameters:
U
 the return type of the transformer Parameters:
parallelismThreshold
 the (estimated) number of elements needed for this operation to be executed in paralleltransformer
 a function returning the transformation for an element, or null if there is no transformation (in which case the action is not applied)action
 the action Since:
 1.8

searchKeys
public <U> U searchKeys(long parallelismThreshold, Function<? super K,? extends U> searchFunction)Returns a nonnull result from applying the given search function on each key, or null if none. Upon success, further element processing is suppressed and the results of any other parallel invocations of the search function are ignored. Type Parameters:
U
 the return type of the search function Parameters:
parallelismThreshold
 the (estimated) number of elements needed for this operation to be executed in parallelsearchFunction
 a function returning a nonnull result on success, else null Returns:
 a nonnull result from applying the given search function on each key, or null if none
 Since:
 1.8

reduceKeys
public K reduceKeys(long parallelismThreshold, BiFunction<? super K,? super K,? extends K> reducer)Returns the result of accumulating all keys using the given reducer to combine values, or null if none. Parameters:
parallelismThreshold
 the (estimated) number of elements needed for this operation to be executed in parallelreducer
 a commutative associative combining function Returns:
 the result of accumulating all keys using the given reducer to combine values, or null if none
 Since:
 1.8

reduceKeys
public <U> U reduceKeys(long parallelismThreshold, Function<? super K,? extends U> transformer, BiFunction<? super U,? super U,? extends U> reducer)Returns the result of accumulating the given transformation of all keys using the given reducer to combine values, or null if none. Type Parameters:
U
 the return type of the transformer Parameters:
parallelismThreshold
 the (estimated) number of elements needed for this operation to be executed in paralleltransformer
 a function returning the transformation for an element, or null if there is no transformation (in which case it is not combined)reducer
 a commutative associative combining function Returns:
 the result of accumulating the given transformation of all keys
 Since:
 1.8

reduceKeysToDouble
public double reduceKeysToDouble(long parallelismThreshold, ToDoubleFunction<? super K> transformer, double basis, DoubleBinaryOperator reducer)Returns the result of accumulating the given transformation of all keys using the given reducer to combine values, and the given basis as an identity value. Parameters:
parallelismThreshold
 the (estimated) number of elements needed for this operation to be executed in paralleltransformer
 a function returning the transformation for an elementbasis
 the identity (initial default value) for the reductionreducer
 a commutative associative combining function Returns:
 the result of accumulating the given transformation of all keys
 Since:
 1.8

reduceKeysToLong
public long reduceKeysToLong(long parallelismThreshold, ToLongFunction<? super K> transformer, long basis, LongBinaryOperator reducer)Returns the result of accumulating the given transformation of all keys using the given reducer to combine values, and the given basis as an identity value. Parameters:
parallelismThreshold
 the (estimated) number of elements needed for this operation to be executed in paralleltransformer
 a function returning the transformation for an elementbasis
 the identity (initial default value) for the reductionreducer
 a commutative associative combining function Returns:
 the result of accumulating the given transformation of all keys
 Since:
 1.8

reduceKeysToInt
public int reduceKeysToInt(long parallelismThreshold, ToIntFunction<? super K> transformer, int basis, IntBinaryOperator reducer)Returns the result of accumulating the given transformation of all keys using the given reducer to combine values, and the given basis as an identity value. Parameters:
parallelismThreshold
 the (estimated) number of elements needed for this operation to be executed in paralleltransformer
 a function returning the transformation for an elementbasis
 the identity (initial default value) for the reductionreducer
 a commutative associative combining function Returns:
 the result of accumulating the given transformation of all keys
 Since:
 1.8

forEachValue
Performs the given action for each value. Parameters:
parallelismThreshold
 the (estimated) number of elements needed for this operation to be executed in parallelaction
 the action Since:
 1.8

forEachValue
public <U> void forEachValue(long parallelismThreshold, Function<? super V,? extends U> transformer, Consumer<? super U> action)Performs the given action for each nonnull transformation of each value. Type Parameters:
U
 the return type of the transformer Parameters:
parallelismThreshold
 the (estimated) number of elements needed for this operation to be executed in paralleltransformer
 a function returning the transformation for an element, or null if there is no transformation (in which case the action is not applied)action
 the action Since:
 1.8

searchValues
public <U> U searchValues(long parallelismThreshold, Function<? super V,? extends U> searchFunction)Returns a nonnull result from applying the given search function on each value, or null if none. Upon success, further element processing is suppressed and the results of any other parallel invocations of the search function are ignored. Type Parameters:
U
 the return type of the search function Parameters:
parallelismThreshold
 the (estimated) number of elements needed for this operation to be executed in parallelsearchFunction
 a function returning a nonnull result on success, else null Returns:
 a nonnull result from applying the given search function on each value, or null if none
 Since:
 1.8

reduceValues
public V reduceValues(long parallelismThreshold, BiFunction<? super V,? super V,? extends V> reducer)Returns the result of accumulating all values using the given reducer to combine values, or null if none. Parameters:
parallelismThreshold
 the (estimated) number of elements needed for this operation to be executed in parallelreducer
 a commutative associative combining function Returns:
 the result of accumulating all values
 Since:
 1.8

reduceValues
public <U> U reduceValues(long parallelismThreshold, Function<? super V,? extends U> transformer, BiFunction<? super U,? super U,? extends U> reducer)Returns the result of accumulating the given transformation of all values using the given reducer to combine values, or null if none. Type Parameters:
U
 the return type of the transformer Parameters:
parallelismThreshold
 the (estimated) number of elements needed for this operation to be executed in paralleltransformer
 a function returning the transformation for an element, or null if there is no transformation (in which case it is not combined)reducer
 a commutative associative combining function Returns:
 the result of accumulating the given transformation of all values
 Since:
 1.8

reduceValuesToDouble
public double reduceValuesToDouble(long parallelismThreshold, ToDoubleFunction<? super V> transformer, double basis, DoubleBinaryOperator reducer)Returns the result of accumulating the given transformation of all values using the given reducer to combine values, and the given basis as an identity value. Parameters:
parallelismThreshold
 the (estimated) number of elements needed for this operation to be executed in paralleltransformer
 a function returning the transformation for an elementbasis
 the identity (initial default value) for the reductionreducer
 a commutative associative combining function Returns:
 the result of accumulating the given transformation of all values
 Since:
 1.8

reduceValuesToLong
public long reduceValuesToLong(long parallelismThreshold, ToLongFunction<? super V> transformer, long basis, LongBinaryOperator reducer)Returns the result of accumulating the given transformation of all values using the given reducer to combine values, and the given basis as an identity value. Parameters:
parallelismThreshold
 the (estimated) number of elements needed for this operation to be executed in paralleltransformer
 a function returning the transformation for an elementbasis
 the identity (initial default value) for the reductionreducer
 a commutative associative combining function Returns:
 the result of accumulating the given transformation of all values
 Since:
 1.8

reduceValuesToInt
public int reduceValuesToInt(long parallelismThreshold, ToIntFunction<? super V> transformer, int basis, IntBinaryOperator reducer)Returns the result of accumulating the given transformation of all values using the given reducer to combine values, and the given basis as an identity value. Parameters:
parallelismThreshold
 the (estimated) number of elements needed for this operation to be executed in paralleltransformer
 a function returning the transformation for an elementbasis
 the identity (initial default value) for the reductionreducer
 a commutative associative combining function Returns:
 the result of accumulating the given transformation of all values
 Since:
 1.8

forEachEntry
Performs the given action for each entry. Parameters:
parallelismThreshold
 the (estimated) number of elements needed for this operation to be executed in parallelaction
 the action Since:
 1.8

forEachEntry
public <U> void forEachEntry(long parallelismThreshold, Function<Map.Entry<K,V>,? extends U> transformer, Consumer<? super U> action)Performs the given action for each nonnull transformation of each entry. Type Parameters:
U
 the return type of the transformer Parameters:
parallelismThreshold
 the (estimated) number of elements needed for this operation to be executed in paralleltransformer
 a function returning the transformation for an element, or null if there is no transformation (in which case the action is not applied)action
 the action Since:
 1.8

searchEntries
public <U> U searchEntries(long parallelismThreshold, Function<Map.Entry<K,V>,? extends U> searchFunction)Returns a nonnull result from applying the given search function on each entry, or null if none. Upon success, further element processing is suppressed and the results of any other parallel invocations of the search function are ignored. Type Parameters:
U
 the return type of the search function Parameters:
parallelismThreshold
 the (estimated) number of elements needed for this operation to be executed in parallelsearchFunction
 a function returning a nonnull result on success, else null Returns:
 a nonnull result from applying the given search function on each entry, or null if none
 Since:
 1.8

reduceEntries
public Map.Entry<K,V> reduceEntries(long parallelismThreshold, BiFunction<Map.Entry<K,V>,Map.Entry<K,V>,? extends Map.Entry<K,V>> reducer)Returns the result of accumulating all entries using the given reducer to combine values, or null if none. Parameters:
parallelismThreshold
 the (estimated) number of elements needed for this operation to be executed in parallelreducer
 a commutative associative combining function Returns:
 the result of accumulating all entries
 Since:
 1.8

reduceEntries
public <U> U reduceEntries(long parallelismThreshold, Function<Map.Entry<K,V>,? extends U> transformer, BiFunction<? super U,? super U,? extends U> reducer)Returns the result of accumulating the given transformation of all entries using the given reducer to combine values, or null if none. Type Parameters:
U
 the return type of the transformer Parameters:
parallelismThreshold
 the (estimated) number of elements needed for this operation to be executed in paralleltransformer
 a function returning the transformation for an element, or null if there is no transformation (in which case it is not combined)reducer
 a commutative associative combining function Returns:
 the result of accumulating the given transformation of all entries
 Since:
 1.8

reduceEntriesToDouble
public double reduceEntriesToDouble(long parallelismThreshold, ToDoubleFunction<Map.Entry<K,V>> transformer, double basis, DoubleBinaryOperator reducer)Returns the result of accumulating the given transformation of all entries using the given reducer to combine values, and the given basis as an identity value. Parameters:
parallelismThreshold
 the (estimated) number of elements needed for this operation to be executed in paralleltransformer
 a function returning the transformation for an elementbasis
 the identity (initial default value) for the reductionreducer
 a commutative associative combining function Returns:
 the result of accumulating the given transformation of all entries
 Since:
 1.8

reduceEntriesToLong
public long reduceEntriesToLong(long parallelismThreshold, ToLongFunction<Map.Entry<K,V>> transformer, long basis, LongBinaryOperator reducer)Returns the result of accumulating the given transformation of all entries using the given reducer to combine values, and the given basis as an identity value. Parameters:
parallelismThreshold
 the (estimated) number of elements needed for this operation to be executed in paralleltransformer
 a function returning the transformation for an elementbasis
 the identity (initial default value) for the reductionreducer
 a commutative associative combining function Returns:
 the result of accumulating the given transformation of all entries
 Since:
 1.8

reduceEntriesToInt
public int reduceEntriesToInt(long parallelismThreshold, ToIntFunction<Map.Entry<K,V>> transformer, int basis, IntBinaryOperator reducer)Returns the result of accumulating the given transformation of all entries using the given reducer to combine values, and the given basis as an identity value. Parameters:
parallelismThreshold
 the (estimated) number of elements needed for this operation to be executed in paralleltransformer
 a function returning the transformation for an elementbasis
 the identity (initial default value) for the reductionreducer
 a commutative associative combining function Returns:
 the result of accumulating the given transformation of all entries
 Since:
 1.8
