K - the type of keys maintained by this mapV - the type of mapped valuespublic 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
thread-safe, 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
happens-before relation with any (non-null) 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 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(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) 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 side-effect-free. Bulk operations on Map.Entry
objects do not support method setValue.
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 in-between
values that trade off overhead versus throughput.
The concurrency properties of bulk operations follow
from those of ConcurrentHashMap: Any non-null result returned
from get(key) and related access methods bears a
happens-before relation with the associated insertion or
update. The result of any bulk operation reflects the
composition of these per-element 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 non-scalar 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
key-value 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 non-null.
This class is a member of the Java Collections Framework.
| Modifier and Type | Class | Description |
|---|---|---|
static class |
ConcurrentHashMap.KeySetView<K,V> |
A view of a ConcurrentHashMap as a
Set of keys, in
which additions may optionally be enabled by mapping to a
common value. |
AbstractMap.SimpleEntry<K,V>, AbstractMap.SimpleImmutableEntry<K,V>| 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), 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.
|
| 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 (or
null if there is no current
mapping). |
V |
computeIfAbsent(K key,
Function<? super K,? extends V> mappingFunction) |
If the specified key is not already associated with a value,
attempts to compute its value using the given mapping function
and enters it into this map unless
null. |
V |
computeIfPresent(K key,
BiFunction<? super K,? super V,? extends V> remappingFunction) |
If the value for the specified key is present, attempts to
compute a new mapping given the key and its current mapped
value.
|
boolean |
contains(Object value) |
Legacy method testing 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
true 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 a
Set view of the mappings contained in this map. |
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 non-null 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 non-null 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 non-null 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 non-null transformation
of each value.
|
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. |
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
Map, i.e.,
the sum of, for each key-value pair in the map,
key.hashCode() ^ value.hashCode(). |
boolean |
isEmpty() |
Returns true if this map contains no key-value mappings.
|
Enumeration<K> |
keys() |
Returns an enumeration of the keys in this table.
|
ConcurrentHashMap.KeySetView<K,V> |
keySet() |
Returns a
Set view of the keys contained in this map. |
ConcurrentHashMap.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., Collection.add(E) and Collection.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
(non-null) value, associates it with the given value.
|
static <K> ConcurrentHashMap.KeySetView<K,Boolean> |
newKeySet() |
Creates a new
Set backed by a ConcurrentHashMap
from the given type to Boolean.TRUE. |
static <K> ConcurrentHashMap.KeySetView<K,Boolean> |
newKeySet(int initialCapacity) |
Creates a new
Set backed by a ConcurrentHashMap
from the given type to Boolean.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, associate 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.
|
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 non-null 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 non-null 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 non-null 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 non-null result from applying the given search
function on each value, or null if none.
|
int |
size() |
Returns the number of key-value mappings in this map.
|
String |
toString() |
Returns a string representation of this map.
|
Collection<V> |
values() |
Returns a
Collection view of the values contained in this map. |
clonepublic ConcurrentHashMap()
public ConcurrentHashMap(int initialCapacity)
initialCapacity - The implementation performs internal
sizing to accommodate this many elements.IllegalArgumentException - if the initial capacity of
elements is negativepublic ConcurrentHashMap(Map<? extends K,? extends V> m)
m - the mappublic ConcurrentHashMap(int initialCapacity,
float loadFactor)
initialCapacity) and
initial table density (loadFactor).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 sizeIllegalArgumentException - if the initial capacity of
elements is negative or the load factor is nonpositivepublic ConcurrentHashMap(int initialCapacity,
float loadFactor,
int concurrencyLevel)
initialCapacity), table
density (loadFactor), and number of concurrently
updating threads (concurrencyLevel).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.IllegalArgumentException - if the initial capacity is
negative or the load factor or concurrencyLevel are
nonpositivepublic int size()
public boolean isEmpty()
public V get(Object key)
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.)
get in interface Map<K,V>get in class AbstractMap<K,V>key - the key whose associated value is to be returnednull if this map contains no mapping for the keyNullPointerException - if the specified key is nullpublic boolean containsKey(Object key)
containsKey in interface Map<K,V>containsKey in class AbstractMap<K,V>key - possible keytrue if and only if the specified object
is a key in this table, as determined by the
equals method; false otherwiseNullPointerException - if the specified key is nullpublic boolean containsValue(Object value)
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.containsValue in interface Map<K,V>containsValue in class AbstractMap<K,V>value - value whose presence in this map is to be testedtrue if this map maps one or more keys to the
specified valueNullPointerException - if the specified value is nullpublic V put(K key, V value)
The value can be retrieved by calling the get method
with a key that is equal to the original key.
put in interface Map<K,V>put in class AbstractMap<K,V>key - key with which the specified value is to be associatedvalue - value to be associated with the specified keykey, or
null if there was no mapping for keyNullPointerException - if the specified key or value is nullpublic void putAll(Map<? extends K,? extends V> m)
public V remove(Object key)
remove in interface Map<K,V>remove in class AbstractMap<K,V>key - the key that needs to be removedkey, or
null if there was no mapping for keyNullPointerException - if the specified key is nullpublic void clear()
public ConcurrentHashMap.KeySetView<K,V> keySet()
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 vice-versa. 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 view's iterators and spliterators are weakly consistent.
The view's spliterator reports Spliterator.CONCURRENT,
Spliterator.DISTINCT, and Spliterator.NONNULL.
public Collection<V> values()
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 vice-versa. 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 Spliterator.CONCURRENT
and Spliterator.NONNULL.
public Set<Map.Entry<K,V>> entrySet()
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 vice-versa. 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 Spliterator.CONCURRENT,
Spliterator.DISTINCT, and Spliterator.NONNULL.
public int hashCode()
Map, i.e.,
the sum of, for each key-value pair in the map,
key.hashCode() ^ value.hashCode().hashCode in interface Map<K,V>hashCode in class AbstractMap<K,V>Map.Entry.hashCode(),
Object.equals(Object),
Set.equals(Object)public String toString()
{}"). Adjacent
mappings are separated by the characters ", " (comma
and space). Each key-value mapping is rendered as the key
followed by an equals sign ("=") followed by the
associated value.toString in class AbstractMap<K,V>public boolean equals(Object o)
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.equals in interface Map<K,V>equals in class AbstractMap<K,V>o - object to be compared for equality with this maptrue if the specified object is equal to this mapObject.hashCode(),
HashMappublic V putIfAbsent(K key, V value)
if (!map.containsKey(key))
return map.put(key, value);
else
return map.get(key);
except that the action is performed atomically.putIfAbsent in interface ConcurrentMap<K,V>putIfAbsent in interface Map<K,V>key - key with which the specified value is to be associatedvalue - value to be associated with the specified keynull if there was no mapping for the keyNullPointerException - if the specified key or value is nullpublic boolean remove(Object key, Object value)
if (map.containsKey(key) && Objects.equals(map.get(key), value)) {
map.remove(key);
return true;
} else
return false;
except that the action is performed atomically.remove in interface ConcurrentMap<K,V>remove in interface Map<K,V>key - key with which the specified value is associatedvalue - value expected to be associated with the specified keytrue if the value was removedNullPointerException - if the specified key is nullpublic boolean replace(K key, V oldValue, V newValue)
if (map.containsKey(key) && Objects.equals(map.get(key), oldValue)) {
map.put(key, newValue);
return true;
} else
return false;
except that the action is performed atomically.replace in interface ConcurrentMap<K,V>replace in interface Map<K,V>key - key with which the specified value is associatedoldValue - value expected to be associated with the specified keynewValue - value to be associated with the specified keytrue if the value was replacedNullPointerException - if any of the arguments are nullpublic V replace(K key, V value)
if (map.containsKey(key)) {
return map.put(key, value);
} else
return null;
except that the action is performed atomically.replace in interface ConcurrentMap<K,V>replace in interface Map<K,V>key - key with which the specified value is associatedvalue - value to be associated with the specified keynull if there was no mapping for the keyNullPointerException - if the specified key or value is nullpublic V getOrDefault(Object key, V defaultValue)
getOrDefault in interface ConcurrentMap<K,V>getOrDefault in interface Map<K,V>key - the key whose associated value is to be returneddefaultValue - the value to return if this map contains
no mapping for the given keyNullPointerException - if the specified key is nullpublic void forEach(BiConsumer<? super K,? super V> action)
ConcurrentMappublic void replaceAll(BiFunction<? super K,? super V,? extends V> function)
ConcurrentMapreplaceAll in interface ConcurrentMap<K,V>replaceAll in interface Map<K,V>function - the function to apply to each entrypublic V computeIfAbsent(K key, Function<? super K,? extends V> mappingFunction)
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.computeIfAbsent in interface ConcurrentMap<K,V>computeIfAbsent in interface Map<K,V>key - key with which the specified value is to be associatedmappingFunction - the function to compute a valueNullPointerException - 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 unestablishedpublic V computeIfPresent(K key, BiFunction<? super K,? super V,? extends V> remappingFunction)
computeIfPresent in interface ConcurrentMap<K,V>computeIfPresent in interface Map<K,V>key - key with which a value may be associatedremappingFunction - the function to compute a valueNullPointerException - 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 unchangedpublic V compute(K key, BiFunction<? super K,? super V,? extends V> remappingFunction)
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.compute in interface ConcurrentMap<K,V>compute in interface Map<K,V>key - key with which the specified value is to be associatedremappingFunction - the function to compute a valueNullPointerException - 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 unchangedpublic V merge(K key, V value, BiFunction<? super V,? super V,? extends V> remappingFunction)
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.merge in interface ConcurrentMap<K,V>merge in interface Map<K,V>key - key with which the specified value is to be associatedvalue - the value to use if absentremappingFunction - the function to recompute a value if presentNullPointerException - if the specified key or the
remappingFunction is nullRuntimeException - or Error if the remappingFunction does so,
in which case the mapping is unchangedpublic boolean contains(Object value)
containsValue(Object), and exists solely to ensure
full compatibility with class Hashtable,
which supported this method prior to introduction of the
Java Collections framework.value - a value to search fortrue if and only if some key maps to the
value argument in this table as
determined by the equals method;
false otherwiseNullPointerException - if the specified value is nullpublic Enumeration<K> keys()
keySet()public Enumeration<V> elements()
values()public long mappingCount()
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.public static <K> ConcurrentHashMap.KeySetView<K,Boolean> newKeySet()
Set backed by a ConcurrentHashMap
from the given type to Boolean.TRUE.K - the element type of the returned setpublic static <K> ConcurrentHashMap.KeySetView<K,Boolean> newKeySet(int initialCapacity)
Set backed by a ConcurrentHashMap
from the given type to Boolean.TRUE.K - the element type of the returned setinitialCapacity - The implementation performs internal
sizing to accommodate this many elements.IllegalArgumentException - if the initial capacity of
elements is negativepublic ConcurrentHashMap.KeySetView<K,V> keySet(V mappedValue)
Set view of the keys in this map, using the
given common mapped value for any additions (i.e., Collection.add(E) and Collection.addAll(Collection)).
This is of course only appropriate if it is acceptable to use
the same value for all additions from this view.mappedValue - the mapped value to use for any additionsNullPointerException - if the mappedValue is nullpublic void forEach(long parallelismThreshold,
BiConsumer<? super K,? super V> action)
parallelismThreshold - the (estimated) number of elements
needed for this operation to be executed in parallelaction - the actionpublic <U> void forEach(long parallelismThreshold,
BiFunction<? super K,? super V,? extends U> transformer,
Consumer<? super U> action)
U - the return type of the transformerparallelismThreshold - 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 actionpublic <U> U search(long parallelismThreshold,
BiFunction<? super K,? super V,? extends U> searchFunction)
U - the return type of the search functionparallelismThreshold - the (estimated) number of elements
needed for this operation to be executed in parallelsearchFunction - a function returning a non-null
result on success, else nullpublic <U> U reduce(long parallelismThreshold,
BiFunction<? super K,? super V,? extends U> transformer,
BiFunction<? super U,? super U,? extends U> reducer)
U - the return type of the transformerparallelismThreshold - 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 functionpublic double reduceToDouble(long parallelismThreshold,
ToDoubleBiFunction<? super K,? super V> transformer,
double basis,
DoubleBinaryOperator reducer)
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 functionpublic long reduceToLong(long parallelismThreshold,
ToLongBiFunction<? super K,? super V> transformer,
long basis,
LongBinaryOperator reducer)
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 functionpublic int reduceToInt(long parallelismThreshold,
ToIntBiFunction<? super K,? super V> transformer,
int basis,
IntBinaryOperator reducer)
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 functionpublic void forEachKey(long parallelismThreshold,
Consumer<? super K> action)
parallelismThreshold - the (estimated) number of elements
needed for this operation to be executed in parallelaction - the actionpublic <U> void forEachKey(long parallelismThreshold,
Function<? super K,? extends U> transformer,
Consumer<? super U> action)
U - the return type of the transformerparallelismThreshold - 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 actionpublic <U> U searchKeys(long parallelismThreshold,
Function<? super K,? extends U> searchFunction)
U - the return type of the search functionparallelismThreshold - the (estimated) number of elements
needed for this operation to be executed in parallelsearchFunction - a function returning a non-null
result on success, else nullpublic K reduceKeys(long parallelismThreshold, BiFunction<? super K,? super K,? extends K> reducer)
parallelismThreshold - the (estimated) number of elements
needed for this operation to be executed in parallelreducer - a commutative associative combining functionpublic <U> U reduceKeys(long parallelismThreshold,
Function<? super K,? extends U> transformer,
BiFunction<? super U,? super U,? extends U> reducer)
U - the return type of the transformerparallelismThreshold - 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 functionpublic double reduceKeysToDouble(long parallelismThreshold,
ToDoubleFunction<? super K> transformer,
double basis,
DoubleBinaryOperator reducer)
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 functionpublic long reduceKeysToLong(long parallelismThreshold,
ToLongFunction<? super K> transformer,
long basis,
LongBinaryOperator reducer)
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 functionpublic int reduceKeysToInt(long parallelismThreshold,
ToIntFunction<? super K> transformer,
int basis,
IntBinaryOperator reducer)
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 functionpublic void forEachValue(long parallelismThreshold,
Consumer<? super V> action)
parallelismThreshold - the (estimated) number of elements
needed for this operation to be executed in parallelaction - the actionpublic <U> void forEachValue(long parallelismThreshold,
Function<? super V,? extends U> transformer,
Consumer<? super U> action)
U - the return type of the transformerparallelismThreshold - 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 actionpublic <U> U searchValues(long parallelismThreshold,
Function<? super V,? extends U> searchFunction)
U - the return type of the search functionparallelismThreshold - the (estimated) number of elements
needed for this operation to be executed in parallelsearchFunction - a function returning a non-null
result on success, else nullpublic V reduceValues(long parallelismThreshold, BiFunction<? super V,? super V,? extends V> reducer)
parallelismThreshold - the (estimated) number of elements
needed for this operation to be executed in parallelreducer - a commutative associative combining functionpublic <U> U reduceValues(long parallelismThreshold,
Function<? super V,? extends U> transformer,
BiFunction<? super U,? super U,? extends U> reducer)
U - the return type of the transformerparallelismThreshold - 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 functionpublic double reduceValuesToDouble(long parallelismThreshold,
ToDoubleFunction<? super V> transformer,
double basis,
DoubleBinaryOperator reducer)
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 functionpublic long reduceValuesToLong(long parallelismThreshold,
ToLongFunction<? super V> transformer,
long basis,
LongBinaryOperator reducer)
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 functionpublic int reduceValuesToInt(long parallelismThreshold,
ToIntFunction<? super V> transformer,
int basis,
IntBinaryOperator reducer)
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 functionpublic void forEachEntry(long parallelismThreshold,
Consumer<? super Map.Entry<K,V>> action)
parallelismThreshold - the (estimated) number of elements
needed for this operation to be executed in parallelaction - the actionpublic <U> void forEachEntry(long parallelismThreshold,
Function<Map.Entry<K,V>,? extends U> transformer,
Consumer<? super U> action)
U - the return type of the transformerparallelismThreshold - 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 actionpublic <U> U searchEntries(long parallelismThreshold,
Function<Map.Entry<K,V>,? extends U> searchFunction)
U - the return type of the search functionparallelismThreshold - the (estimated) number of elements
needed for this operation to be executed in parallelsearchFunction - a function returning a non-null
result on success, else nullpublic Map.Entry<K,V> reduceEntries(long parallelismThreshold, BiFunction<Map.Entry<K,V>,Map.Entry<K,V>,? extends Map.Entry<K,V>> reducer)
parallelismThreshold - the (estimated) number of elements
needed for this operation to be executed in parallelreducer - a commutative associative combining functionpublic <U> U reduceEntries(long parallelismThreshold,
Function<Map.Entry<K,V>,? extends U> transformer,
BiFunction<? super U,? super U,? extends U> reducer)
U - the return type of the transformerparallelismThreshold - 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 functionpublic double reduceEntriesToDouble(long parallelismThreshold,
ToDoubleFunction<Map.Entry<K,V>> transformer,
double basis,
DoubleBinaryOperator reducer)
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 functionpublic long reduceEntriesToLong(long parallelismThreshold,
ToLongFunction<Map.Entry<K,V>> transformer,
long basis,
LongBinaryOperator reducer)
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 functionpublic int reduceEntriesToInt(long parallelismThreshold,
ToIntFunction<Map.Entry<K,V>> transformer,
int basis,
IntBinaryOperator reducer)
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 Submit a bug or feature
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