ConcurrentHashMap
public
class
ConcurrentHashMap
extends AbstractMap<K, V>
implements
ConcurrentMap<K, V>,
Serializable
java.lang.Object  
↳  java.util.AbstractMap<K, V>  
↳  java.util.concurrent.ConcurrentHashMap<K, V> 
A hash table supporting full concurrency of retrievals and
high expected concurrency for updates. This class obeys the
same functional specification as 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 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.
Summary
Nested classes  

class 
ConcurrentHashMap.KeySetView<K, V>
A view of a ConcurrentHashMap as a 
Public constructors  

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(Map<? extends K, ? extends V> m)
Creates a new map with the same mappings as the given map. 

ConcurrentHashMap(int initialCapacity, float loadFactor)
Creates a new, empty map with an initial table size based on
the given number of elements ( 

ConcurrentHashMap(int initialCapacity, float loadFactor, int concurrencyLevel)
Creates a new, empty map with an initial table size based on
the given number of elements ( 
Public methods  

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 (or 
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 unless 
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)
Returns 
Enumeration<V>

elements()
Returns an enumeration of the values in this table. 
Set<Entry<K, V>>

entrySet()
Returns a 
boolean

equals(Object o)
Compares the specified object with this map for equality. 
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. 
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). 
<U>
void

forEachEntry(long parallelismThreshold, Function<Entry<K, V>, ? extends U> transformer, Consumer<? super U> action)
Performs the given action for each nonnull transformation of each entry. 
void

forEachEntry(long parallelismThreshold, Consumer<? super Entry<K, V>> action)
Performs the given action for each entry. 
<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

forEachKey(long parallelismThreshold, Consumer<? super K> action)
Performs the given action for each key. 
<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. 
void

forEachValue(long parallelismThreshold, Consumer<? super V> action)
Performs the given action for each value. 
V

get(Object key)
Returns the value to which the specified key is mapped,
or 
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 this 
boolean

isEmpty()
Returns 
Set<K>

keySet()
Returns a 
KeySetView<K, V>

keySet(V mappedValue)
Returns a 
Enumeration<K>

keys()
Returns an enumeration of the keys in this table. 
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>
KeySetView<K, Boolean>

newKeySet()
Creates a new 
static
<K>
KeySetView<K, Boolean>

newKeySet(int initialCapacity)
Creates a new 
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. 
Entry<K, V>

reduceEntries(long parallelismThreshold, BiFunction<Entry<K, V>, Entry<K, V>, ? extends 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<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<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<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<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. 
boolean

replace(K key, V oldValue, V newValue)
Replaces 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. 
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. 
<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<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. 
int

size()
Returns the number of keyvalue mappings in this map. 
String

toString()
Returns a string representation of this map. 
Collection<V>

values()
Returns a 
Inherited methods  

Public constructors
ConcurrentHashMap
ConcurrentHashMap ()
Creates a new, empty map with the default initial table size (16).
ConcurrentHashMap
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 
int : The implementation performs internal
sizing to accommodate this many elements. 
Throws  

IllegalArgumentException 
if the initial capacity of elements is negative 
ConcurrentHashMap
ConcurrentHashMap (Map<? extends K, ? extends V> m)
Creates a new map with the same mappings as the given map.
Parameters  

m 
Map : the map

ConcurrentHashMap
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 
int : the initial capacity. The implementation
performs internal sizing to accommodate this many elements,
given the specified load factor. 
loadFactor 
float : 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 
ConcurrentHashMap
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
), table
density (loadFactor
), and number of concurrently
updating threads (concurrencyLevel
).
Parameters  

initialCapacity 
int : the initial capacity. The implementation
performs internal sizing to accommodate this many elements,
given the specified load factor. 
loadFactor 
float : the load factor (table density) for
establishing the initial table size 
concurrencyLevel 
int : 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 
Public methods
compute
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). 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.
Parameters  

key 
K : key with which the specified value is to be associated 
remappingFunction 
BiFunction : the function to compute a value 
Returns  

V 
the new value associated with the specified key, or null if none 
Throws  

NullPointerException 
if the specified key or remappingFunction is null 
IllegalStateException 
if the computation detectably attempts a recursive update to this map that would otherwise never complete 
RuntimeException 
or Error if the remappingFunction does so, in which case the mapping is unchanged 
computeIfAbsent
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 unless null
. 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.
Parameters  

key 
K : key with which the specified value is to be associated 
mappingFunction 
Function : the function to compute a value 
Returns  

V 
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 null 
IllegalStateException 
if the computation detectably attempts a recursive update to this map that would otherwise never complete 
RuntimeException 
or Error if the mappingFunction does so, in which case the mapping is left unestablished 
computeIfPresent
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. 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.
Parameters  

key 
K : key with which a value may be associated 
remappingFunction 
BiFunction : the function to compute a value 
Returns  

V 
the new value associated with the specified key, or null if none 
Throws  

NullPointerException 
if the specified key or remappingFunction is null 
IllegalStateException 
if the computation detectably attempts a recursive update to this map that would otherwise never complete 
RuntimeException 
or Error if the remappingFunction does so, in which case the mapping is unchanged 
contains
boolean contains (Object value)
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 class Hashtable
,
which supported this method prior to introduction of the
Java Collections Framework.
Parameters  

value 
Object : a value to search for 
Returns  

boolean 
true if and only if some key maps to the
value argument in this table as
determined by the equals method;
false otherwise 
Throws  

NullPointerException 
if the specified value is null 
containsKey
boolean containsKey (Object key)
Tests if the specified object is a key in this table.
Parameters  

key 
Object : possible key 
Returns  

boolean 
true if and only if the specified object
is a key in this table, as determined by the
equals method; false otherwise 
Throws  

NullPointerException 
if the specified key is null 
containsValue
boolean containsValue (Object value)
Returns true
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 method containsKey
.
Parameters  

value 
Object : value whose presence in this map is to be tested 
Returns  

boolean 
true if this map maps one or more keys to the
specified value 
Throws  

NullPointerException 
if the specified value is null 
elements
Enumeration<V> elements ()
Returns an enumeration of the values in this table.
Returns  

Enumeration<V> 
an enumeration of the values in this table 
See also:
entrySet
Set<Entry<K, V>> entrySet ()
Returns a Set
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 the Iterator.remove
, Set.remove
,
removeAll
, retainAll
, and clear
operations.
The view's iterators and spliterators are weakly consistent.
The view's spliterator
reports CONCURRENT
,
DISTINCT
, and NONNULL
.
Returns  

Set<Entry<K, V>> 
the set view 
equals
boolean equals (Object o)
Compares the specified object with this map for equality.
Returns true
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.
Parameters  

o 
Object : object to be compared for equality with this map 
Returns  

boolean 
true if the specified object is equal to this map

forEach
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 
BiConsumer : The action to be performed for each entry 
forEach
void forEach (long parallelismThreshold, BiConsumer<? super K, ? super V> action)
Performs the given action for each (key, value).
Parameters  

parallelismThreshold 
long : the (estimated) number of elements
needed for this operation to be executed in parallel 
action 
BiConsumer : the action 
forEach
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).
Parameters  

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

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

parallelismThreshold 
long : the (estimated) number of elements
needed for this operation to be executed in parallel 
action 
Consumer : the action 
forEachKey
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.
Parameters  

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

parallelismThreshold 
long : the (estimated) number of elements
needed for this operation to be executed in parallel 
action 
Consumer : the action 
forEachValue
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.
Parameters  

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

parallelismThreshold 
long : the (estimated) number of elements
needed for this operation to be executed in parallel 
action 
Consumer : the action 
get
V get (Object key)
Returns the value to which the specified key is mapped,
or null
if this map contains no mapping for the key.
More formally, if this map contains a mapping from a key
k
to a value v
such that key.equals(k)
,
then this method returns v
; otherwise it returns
null
. (There can be at most one such mapping.)
Parameters  

key 
Object : the key whose associated value is to be returned 
Returns  

V 
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 
getOrDefault
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.
Parameters  

key 
Object : the key whose associated value is to be returned 
defaultValue 
V : the value to return if this map contains
no mapping for the given key 
Returns  

V 
the mapping for the key, if present; else the default value 
Throws  

NullPointerException 
if the specified key is null 
hashCode
int hashCode ()
Returns the hash code value for this Map
, i.e.,
the sum of, for each keyvalue pair in the map,
key.hashCode() ^ value.hashCode()
.
Returns  

int 
the hash code value for this map 
isEmpty
boolean isEmpty ()
Returns true
if this map contains no keyvalue mappings.
Returns  

boolean 
true if this map contains no keyvalue mappings

keySet
Set<K> keySet ()
Returns a Set
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 the Iterator.remove
, Set.remove
,
removeAll
, retainAll
, and clear
operations. It does not support the add
or
addAll
operations.
The set returned by this method is guaranteed to an instance of
ConcurrentHashMap.KeySetView
.
The view's iterators and spliterators are weakly consistent.
The view's spliterator
reports CONCURRENT
,
DISTINCT
, and NONNULL
.
Returns  

Set<K> 
the set view 
keySet
KeySetView<K, V> keySet (V mappedValue)
Returns a Set
view of the keys in this map, using the
given common mapped value for any additions (i.e., add(E)
and 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 
V : the mapped value to use for any additions 
Returns  

KeySetView<K, V> 
the set view 
Throws  

NullPointerException 
if the mappedValue is null 
keys
Enumeration<K> keys ()
Returns an enumeration of the keys in this table.
Returns  

Enumeration<K> 
an enumeration of the keys in this table 
See also:
mappingCount
long mappingCount ()
Returns the number of mappings. This method should be used
instead of 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  

long 
the number of mappings 
merge
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.
Otherwise, replaces the value with the results of the given
remapping function, or removes if null
. 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.
Parameters  

key 
K : key with which the specified value is to be associated 
value 
V : the value to use if absent 
remappingFunction 
BiFunction : the function to recompute a value if present 
Returns  

V 
the new value associated with the specified key, or null if none 
Throws  

NullPointerException 
if the specified key or the remappingFunction is null 
RuntimeException 
or Error if the remappingFunction does so, in which case the mapping is unchanged 
newKeySet
KeySetView<K, Boolean> newKeySet ()
Creates a new Set
backed by a ConcurrentHashMap
from the given type to Boolean.TRUE
.
Returns  

KeySetView<K, Boolean> 
the new set 
newKeySet
KeySetView<K, Boolean> newKeySet (int initialCapacity)
Creates a new Set
backed by a ConcurrentHashMap
from the given type to Boolean.TRUE
.
Parameters  

initialCapacity 
int : The implementation performs internal
sizing to accommodate this many elements. 
Returns  

KeySetView<K, Boolean> 
the new set 
Throws  

IllegalArgumentException 
if the initial capacity of elements is negative 
put
V put (K key, V value)
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.
Parameters  

key 
K : key with which the specified value is to be associated 
value 
V : value to be associated with the specified key 
Returns  

V 
the previous value associated with key , or
null if there was no mapping for key 
Throws  

NullPointerException 
if the specified key or value is null 
putAll
void putAll (Map<? extends K, ? extends V> m)
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.
Parameters  

m 
Map : mappings to be stored in this map

putIfAbsent
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 
K : key with which the specified value is to be associated 
value 
V : value to be associated with the specified key 
Returns  

V 
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 
reduce
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.
Parameters  

parallelismThreshold 
long : the (estimated) number of elements
needed for this operation to be executed in parallel 
transformer 
BiFunction : a function returning the transformation
for an element, or null if there is no transformation (in
which case it is not combined) 
reducer 
BiFunction : a commutative associative combining function 
Returns  

U 
the result of accumulating the given transformation of all (key, value) pairs 
reduceEntries
Entry<K, V> reduceEntries (long parallelismThreshold, BiFunction<Entry<K, V>, Entry<K, V>, ? extends Entry<K, V>> reducer)
Returns the result of accumulating all entries using the given reducer to combine values, or null if none.
Parameters  

parallelismThreshold 
long : the (estimated) number of elements
needed for this operation to be executed in parallel 
reducer 
BiFunction : a commutative associative combining function 
Returns  

Entry<K, V> 
the result of accumulating all entries 
reduceEntries
U reduceEntries (long parallelismThreshold, Function<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.
Parameters  

parallelismThreshold 
long : the (estimated) number of elements
needed for this operation to be executed in parallel 
transformer 
Function : a function returning the transformation
for an element, or null if there is no transformation (in
which case it is not combined) 
reducer 
BiFunction : a commutative associative combining function 
Returns  

U 
the result of accumulating the given transformation of all entries 
reduceEntriesToDouble
double reduceEntriesToDouble (long parallelismThreshold, ToDoubleFunction<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 
long : the (estimated) number of elements
needed for this operation to be executed in parallel 
transformer 
ToDoubleFunction : a function returning the transformation
for an element 
basis 
double : the identity (initial default value) for the reduction 
reducer 
DoubleBinaryOperator : a commutative associative combining function 
Returns  

double 
the result of accumulating the given transformation of all entries 
reduceEntriesToInt
int reduceEntriesToInt (long parallelismThreshold, ToIntFunction<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 
long : the (estimated) number of elements
needed for this operation to be executed in parallel 
transformer 
ToIntFunction : a function returning the transformation
for an element 
basis 
int : the identity (initial default value) for the reduction 
reducer 
IntBinaryOperator : a commutative associative combining function 
Returns  

int 
the result of accumulating the given transformation of all entries 
reduceEntriesToLong
long reduceEntriesToLong (long parallelismThreshold, ToLongFunction<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 
long : the (estimated) number of elements
needed for this operation to be executed in parallel 
transformer 
ToLongFunction : a function returning the transformation
for an element 
basis 
long : the identity (initial default value) for the reduction 
reducer 
LongBinaryOperator : a commutative associative combining function 
Returns  

long 
the result of accumulating the given transformation of all entries 
reduceKeys
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 
long : the (estimated) number of elements
needed for this operation to be executed in parallel 
reducer 
BiFunction : a commutative associative combining function 
Returns  

K 
the result of accumulating all keys using the given reducer to combine values, or null if none 
reduceKeys
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.
Parameters  

parallelismThreshold 
long : the (estimated) number of elements
needed for this operation to be executed in parallel 
transformer 
Function : a function returning the transformation
for an element, or null if there is no transformation (in
which case it is not combined) 
reducer 
BiFunction : a commutative associative combining function 
Returns  

U 
the result of accumulating the given transformation of all keys 
reduceKeysToDouble
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 
long : the (estimated) number of elements
needed for this operation to be executed in parallel 
transformer 
ToDoubleFunction : a function returning the transformation
for an element 
basis 
double : the identity (initial default value) for the reduction 
reducer 
DoubleBinaryOperator : a commutative associative combining function 
Returns  

double 
the result of accumulating the given transformation of all keys 
reduceKeysToInt
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 
long : the (estimated) number of elements
needed for this operation to be executed in parallel 
transformer 
ToIntFunction : a function returning the transformation
for an element 
basis 
int : the identity (initial default value) for the reduction 
reducer 
IntBinaryOperator : a commutative associative combining function 
Returns  

int 
the result of accumulating the given transformation of all keys 
reduceKeysToLong
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 
long : the (estimated) number of elements
needed for this operation to be executed in parallel 
transformer 
ToLongFunction : a function returning the transformation
for an element 
basis 
long : the identity (initial default value) for the reduction 
reducer 
LongBinaryOperator : a commutative associative combining function 
Returns  

long 
the result of accumulating the given transformation of all keys 
reduceToDouble
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 
long : the (estimated) number of elements
needed for this operation to be executed in parallel 
transformer 
ToDoubleBiFunction : a function returning the transformation
for an element 
basis 
double : the identity (initial default value) for the reduction 
reducer 
DoubleBinaryOperator : a commutative associative combining function 
Returns  

double 
the result of accumulating the given transformation of all (key, value) pairs 
reduceToInt
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 
long : the (estimated) number of elements
needed for this operation to be executed in parallel 
transformer 
ToIntBiFunction : a function returning the transformation
for an element 
basis 
int : the identity (initial default value) for the reduction 
reducer 
IntBinaryOperator : a commutative associative combining function 
Returns  

int 
the result of accumulating the given transformation of all (key, value) pairs 
reduceToLong
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 
long : the (estimated) number of elements
needed for this operation to be executed in parallel 
transformer 
ToLongBiFunction : a function returning the transformation
for an element 
basis 
long : the identity (initial default value) for the reduction 
reducer 
LongBinaryOperator : a commutative associative combining function 
Returns  

long 
the result of accumulating the given transformation of all (key, value) pairs 
reduceValues
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 
long : the (estimated) number of elements
needed for this operation to be executed in parallel 
reducer 
BiFunction : a commutative associative combining function 
Returns  

V 
the result of accumulating all values 
reduceValues
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.
Parameters  

parallelismThreshold 
long : the (estimated) number of elements
needed for this operation to be executed in parallel 
transformer 
Function : a function returning the transformation
for an element, or null if there is no transformation (in
which case it is not combined) 
reducer 
BiFunction : a commutative associative combining function 
Returns  

U 
the result of accumulating the given transformation of all values 
reduceValuesToDouble
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 
long : the (estimated) number of elements
needed for this operation to be executed in parallel 
transformer 
ToDoubleFunction : a function returning the transformation
for an element 
basis 
double : the identity (initial default value) for the reduction 
reducer 
DoubleBinaryOperator : a commutative associative combining function 
Returns  

double 
the result of accumulating the given transformation of all values 
reduceValuesToInt
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 
long : the (estimated) number of elements
needed for this operation to be executed in parallel 
transformer 
ToIntFunction : a function returning the transformation
for an element 
basis 
int : the identity (initial default value) for the reduction 
reducer 
IntBinaryOperator : a commutative associative combining function 
Returns  

int 
the result of accumulating the given transformation of all values 
reduceValuesToLong
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 
long : the (estimated) number of elements
needed for this operation to be executed in parallel 
transformer 
ToLongFunction : a function returning the transformation
for an element 
basis 
long : the identity (initial default value) for the reduction 
reducer 
LongBinaryOperator : a commutative associative combining function 
Returns  

long 
the result of accumulating the given transformation of all values 
remove
V remove (Object key)
Removes the key (and its corresponding value) from this map. This method does nothing if the key is not in the map.
Parameters  

key 
Object : the key that needs to be removed 
Returns  

V 
the previous value associated with key , or
null if there was no mapping for key 
Throws  

NullPointerException 
if the specified key is null 
remove
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 
Object : key with which the specified value is associated 
value 
Object : value expected to be associated with the specified key 
Returns  

boolean 
true if the value was removed 
Throws  

NullPointerException 
if the specified key is null 
replace
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 
K : key with which the specified value is associated 
oldValue 
V : value expected to be associated with the specified key 
newValue 
V : value to be associated with the specified key 
Returns  

boolean 
true if the value was replaced 
Throws  

NullPointerException 
if any of the arguments are null 
replace
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 
K : key with which the specified value is associated 
value 
V : value to be associated with the specified key 
Returns  

V 
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 
replaceAll
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 
BiFunction : the function to apply to each entry 
search
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.
Parameters  

parallelismThreshold 
long : the (estimated) number of elements
needed for this operation to be executed in parallel 
searchFunction 
BiFunction : a function returning a nonnull
result on success, else null 
Returns  

U 
a nonnull result from applying the given search function on each (key, value), or null if none 
searchEntries
U searchEntries (long parallelismThreshold, Function<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.
Parameters  

parallelismThreshold 
long : the (estimated) number of elements
needed for this operation to be executed in parallel 
searchFunction 
Function : a function returning a nonnull
result on success, else null 
Returns  

U 
a nonnull result from applying the given search function on each entry, or null if none 
searchKeys
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.
Parameters  

parallelismThreshold 
long : the (estimated) number of elements
needed for this operation to be executed in parallel 
searchFunction 
Function : a function returning a nonnull
result on success, else null 
Returns  

U 
a nonnull result from applying the given search function on each key, or null if none 
searchValues
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.
Parameters  

parallelismThreshold 
long : the (estimated) number of elements
needed for this operation to be executed in parallel 
searchFunction 
Function : a function returning a nonnull
result on success, else null 
Returns  

U 
a nonnull result from applying the given search function on each value, or null if none 
size
int size ()
Returns the number of keyvalue mappings in this map. If the
map contains more than Integer.MAX_VALUE
elements, returns
Integer.MAX_VALUE
.
Returns  

int 
the number of keyvalue mappings in this map 
toString
String 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.
Returns  

String 
a string representation of this map 
values
Collection<V> values ()
Returns a Collection
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 the Iterator.remove
,
Collection.remove
, removeAll
,
retainAll
, and clear
operations. It does not
support the add
or addAll
operations.
The view's iterators and spliterators are weakly consistent.
The view's spliterator
reports CONCURRENT
and NONNULL
.
Returns  

Collection<V> 
the collection view 