- Type Parameters:
- K- the type of keys maintained by this map
- V- the type of mapped values
- All Implemented Interfaces:
- Serializable,- ConcurrentMap<K,,- V> - Map<K,- V> 
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 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 side-effect-free. 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 non-null result of applying a given function on each element; skipping further search when a result is found.
- reduce: Accumulates each element.  The supplied reduction
 function cannot rely on ordering (more formally, it should be
 both associative and commutative).  There are five variants:
 - Plain reductions. (There is not a form of this method for (key, value) function arguments since there is no corresponding return type.)
- Mapped reductions that accumulate the results of a given function applied to each element.
- Reductions to scalar doubles, longs, and ints, using a given basis value.
 
These bulk operations accept a parallelismThreshold
 argument. Methods proceed sequentially if the current map size is
 estimated to be less than the given threshold. Using a value of
 Long.MAX_VALUE suppresses all parallelism.  Using a value
 of 1 results in maximal parallelism by partitioning into
 enough subtasks to fully utilize the ForkJoinPool.commonPool() that is used for all parallel
 computations. Normally, you would initially choose one of these
 extreme values, and then measure performance of using 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.
- Since:
- 1.5
- See Also:
- 
Nested Class SummaryNested ClassesModifier and TypeClassDescriptionstatic final classA view of a ConcurrentHashMap as aSetof keys, in which additions may optionally be enabled by mapping to a common value.Nested classes/interfaces declared in class java.util.AbstractMapAbstractMap.SimpleEntry<K,V>, AbstractMap.SimpleImmutableEntry<K, V> 
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Constructor SummaryConstructorsConstructorDescriptionCreates a new, empty map with the default initial table size (16).ConcurrentHashMap(int initialCapacity) Creates a new, empty map with an initial table size accommodating the specified number of elements without the need to dynamically resize.ConcurrentHashMap(int initialCapacity, float loadFactor) Creates a new, empty map with an initial table size based on the given number of elements (initialCapacity) and initial table density (loadFactor).ConcurrentHashMap(int initialCapacity, float loadFactor, int concurrencyLevel) Creates a new, empty map with an initial table size based on the given number of elements (initialCapacity), initial table density (loadFactor), and number of concurrently updating threads (concurrencyLevel).ConcurrentHashMap(Map<? extends K, ? extends V> m) Creates a new map with the same mappings as the given map.
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Method SummaryModifier and TypeMethodDescriptionvoidclear()Removes all of the mappings from this map.Attempts to compute a mapping for the specified key and its current mapped value (ornullif there is no current mapping).computeIfAbsent(K key, Function<? super K, ? extends V> mappingFunction) If the specified key is not already associated with a value, attempts to compute its value using the given mapping function and enters it into this map unlessnull.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.booleanTests if some key maps into the specified value in this table.booleancontainsKey(Object key) Tests if the specified object is a key in this table.booleancontainsValue(Object value) Returnstrueif this map maps one or more keys to the specified value.elements()Returns an enumeration of the values in this table.entrySet()Returns aSetview of the mappings contained in this map.booleanCompares the specified object with this map for equality.voidforEach(long parallelismThreshold, BiConsumer<? super K, ? super V> action) Performs the given action for each (key, value).<U> voidforEach(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).voidforEachEntry(long parallelismThreshold, Consumer<? super Map.Entry<K, V>> action) Performs the given action for each entry.<U> voidforEachEntry(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.voidforEachKey(long parallelismThreshold, Consumer<? super K> action) Performs the given action for each key.<U> voidforEachKey(long parallelismThreshold, Function<? super K, ? extends U> transformer, Consumer<? super U> action) Performs the given action for each non-null transformation of each key.voidforEachValue(long parallelismThreshold, Consumer<? super V> action) Performs the given action for each value.<U> voidforEachValue(long parallelismThreshold, Function<? super V, ? extends U> transformer, Consumer<? super U> action) Performs the given action for each non-null transformation of each value.Returns the value to which the specified key is mapped, ornullif this map contains no mapping for the key.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.inthashCode()Returns the hash code value for thisMap, i.e., the sum of, for each key-value pair in the map,key.hashCode() ^ value.hashCode().booleanisEmpty()Returnstrueif this map contains no key-value mappings.keys()Returns an enumeration of the keys in this table.keySet()Returns aSetview of the keys contained in this map.Returns aSetview of the keys in this map, using the given common mapped value for any additions (i.e.,Collection.add(E)andCollection.addAll(Collection)).longReturns the number of mappings.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> Creates a newSetbacked by a ConcurrentHashMap from the given type toBoolean.TRUE.static <K> ConcurrentHashMap.KeySetView<K, Boolean> newKeySet(int initialCapacity) Creates a newSetbacked by a ConcurrentHashMap from the given type toBoolean.TRUE.Maps the specified key to the specified value in this table.voidCopies all of the mappings from the specified map to this one.putIfAbsent(K key, V value) If the specified key is not already associated with a value, associates it with the given value.<U> Ureduce(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.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> UreduceEntries(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.doublereduceEntriesToDouble(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.intreduceEntriesToInt(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.longreduceEntriesToLong(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.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> UreduceKeys(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.doublereduceKeysToDouble(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.intreduceKeysToInt(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.longreduceKeysToLong(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.doublereduceToDouble(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.intreduceToInt(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.longreduceToLong(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.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> UreduceValues(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.doublereduceValuesToDouble(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.intreduceValuesToInt(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.longreduceValuesToLong(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.Removes the key (and its corresponding value) from this map.booleanRemoves the entry for a key only if currently mapped to a given value.Replaces the entry for a key only if currently mapped to some value.booleanReplaces the entry for a key only if currently mapped to a given value.<U> Usearch(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> UsearchEntries(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> UsearchKeys(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> UsearchValues(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.intsize()Returns the number of key-value mappings in this map.toString()Returns a string representation of this map.values()Returns aCollectionview of the values contained in this map.Methods declared in class java.util.AbstractMapclone, isEmpty, sizeMethods declared in interface java.util.concurrent.ConcurrentMapforEach, replaceAll
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Constructor Details- 
ConcurrentHashMappublic ConcurrentHashMap()Creates a new, empty map with the default initial table size (16).
- 
ConcurrentHashMappublic ConcurrentHashMap(int initialCapacity) Creates a new, empty map with an initial table size accommodating the specified number of elements without the need to dynamically resize.- Parameters:
- initialCapacity- The implementation performs internal sizing to accommodate this many elements.
- Throws:
- IllegalArgumentException- if the initial capacity of elements is negative
 
- 
ConcurrentHashMapCreates a new map with the same mappings as the given map.- Parameters:
- m- the map
 
- 
ConcurrentHashMappublic ConcurrentHashMap(int initialCapacity, float loadFactor) Creates a new, empty map with an initial table size based on the given number of elements (initialCapacity) and initial table density (loadFactor).- Parameters:
- initialCapacity- the initial capacity. The implementation performs internal sizing to accommodate this many elements, given the specified load factor.
- loadFactor- the load factor (table density) for establishing the initial table size
- Throws:
- IllegalArgumentException- if the initial capacity of elements is negative or the load factor is nonpositive
- Since:
- 1.6
 
- 
ConcurrentHashMappublic ConcurrentHashMap(int initialCapacity, float loadFactor, int concurrencyLevel) Creates a new, empty map with an initial table size based on the given number of elements (initialCapacity), initial table density (loadFactor), and number of concurrently updating threads (concurrencyLevel).- Parameters:
- initialCapacity- the initial capacity. The implementation performs internal sizing to accommodate this many elements, given the specified load factor.
- loadFactor- the load factor (table density) for establishing the initial table size
- concurrencyLevel- the estimated number of concurrently updating threads. The implementation may use this value as a sizing hint.
- Throws:
- IllegalArgumentException- if the initial capacity is negative or the load factor or concurrencyLevel are nonpositive
 
 
- 
- 
Method Details- 
sizepublic int size()Returns the number of key-value mappings in this map. If the map contains more thanInteger.MAX_VALUEelements, returnsInteger.MAX_VALUE.
- 
isEmptypublic boolean isEmpty()Returnstrueif this map contains no key-value mappings.
- 
getReturns the value to which the specified key is mapped, ornullif this map contains no mapping for the key.More formally, if this map contains a mapping from a key kto a valuevsuch thatkey.equals(k), then this method returnsv; otherwise it returnsnull. (There can be at most one such mapping.)- Specified by:
- getin interface- Map<K,- V> 
- Overrides:
- getin class- AbstractMap<K,- V> 
- Parameters:
- key- the key whose associated value is to be returned
- Returns:
- the value to which the specified key is mapped, or
         nullif this map contains no mapping for the key
- Throws:
- NullPointerException- if the specified key is null
 
- 
containsKeyTests if the specified object is a key in this table.- Specified by:
- containsKeyin interface- Map<K,- V> 
- Overrides:
- containsKeyin class- AbstractMap<K,- V> 
- Parameters:
- key- possible key
- Returns:
- trueif and only if the specified object is a key in this table, as determined by the- equalsmethod;- falseotherwise
- Throws:
- NullPointerException- if the specified key is null
 
- 
containsValueReturnstrueif this map maps one or more keys to the specified value. Note: This method may require a full traversal of the map, and is much slower than methodcontainsKey.- Specified by:
- containsValuein interface- Map<K,- V> 
- Overrides:
- containsValuein class- AbstractMap<K,- V> 
- Parameters:
- value- value whose presence in this map is to be tested
- Returns:
- trueif this map maps one or more keys to the specified value
- Throws:
- NullPointerException- if the specified value is null
 
- 
putMaps 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 getmethod with a key that is equal to the original key.- Specified by:
- putin interface- Map<K,- V> 
- Overrides:
- putin class- AbstractMap<K,- V> 
- Parameters:
- key- key with which the specified value is to be associated
- value- value to be associated with the specified key
- Returns:
- the previous value associated with key, ornullif there was no mapping forkey
- Throws:
- NullPointerException- if the specified key or value is null
 
- 
putAllCopies 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.
- 
removeRemoves the key (and its corresponding value) from this map. This method does nothing if the key is not in the map.- Specified by:
- removein interface- Map<K,- V> 
- Overrides:
- removein class- AbstractMap<K,- V> 
- Parameters:
- key- the key that needs to be removed
- Returns:
- the previous value associated with key, ornullif there was no mapping forkey
- Throws:
- NullPointerException- if the specified key is null
 
- 
clearpublic void clear()Removes all of the mappings from this map.
- 
keySetReturns aSetview 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 theIterator.remove,Set.remove,removeAll,retainAll, andclearoperations. It does not support theaddoraddAlloperations.The view's iterators and spliterators are weakly consistent. The view's spliteratorreportsSpliterator.CONCURRENT,Spliterator.DISTINCT, andSpliterator.NONNULL.
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valuesReturns aCollectionview 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 theIterator.remove,Collection.remove,removeAll,retainAll, andclearoperations. It does not support theaddoraddAlloperations.The view's iterators and spliterators are weakly consistent. The view's spliteratorreportsSpliterator.CONCURRENTandSpliterator.NONNULL.
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entrySetReturns aSetview 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 theIterator.remove,Set.remove,removeAll,retainAll, andclearoperations.The view's iterators and spliterators are weakly consistent. The view's spliteratorreportsSpliterator.CONCURRENT,Spliterator.DISTINCT, andSpliterator.NONNULL.
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hashCodepublic int hashCode()Returns the hash code value for thisMap, i.e., the sum of, for each key-value pair in the map,key.hashCode() ^ value.hashCode().
- 
toStringReturns a string representation of this map. The string representation consists of a list of key-value mappings (in no particular order) enclosed in braces ("{}"). 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.- Overrides:
- toStringin class- AbstractMap<K,- V> 
- Returns:
- a string representation of this map
 
- 
equalsCompares the specified object with this map for equality. Returnstrueif 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.
- 
putIfAbsentIf the specified key is not already associated with a value, associates it with the given value. This is equivalent to, for thismap:
 except that the action is performed atomically.if (!map.containsKey(key)) return map.put(key, value); else return map.get(key);- Specified by:
- putIfAbsentin interface- ConcurrentMap<K,- V> 
- Specified by:
- putIfAbsentin interface- Map<K,- V> 
- Parameters:
- key- key with which the specified value is to be associated
- value- value to be associated with the specified key
- Returns:
- the previous value associated with the specified key,
         or nullif there was no mapping for the key
- Throws:
- NullPointerException- if the specified key or value is null
 
- 
removeRemoves the entry for a key only if currently mapped to a given value. This is equivalent to, for thismap:
 except that the action is performed atomically.if (map.containsKey(key) && Objects.equals(map.get(key), value)) { map.remove(key); return true; } else { return false; }- Specified by:
- removein interface- ConcurrentMap<K,- V> 
- Specified by:
- removein interface- Map<K,- V> 
- Parameters:
- key- key with which the specified value is associated
- value- value expected to be associated with the specified key
- Returns:
- trueif the value was removed
- Throws:
- NullPointerException- if the specified key is null
 
- 
replaceReplaces the entry for a key only if currently mapped to a given value. This is equivalent to, for thismap:
 except that the action is performed atomically.if (map.containsKey(key) && Objects.equals(map.get(key), oldValue)) { map.put(key, newValue); return true; } else { return false; }- Specified by:
- replacein interface- ConcurrentMap<K,- V> 
- Specified by:
- replacein interface- Map<K,- V> 
- Parameters:
- key- key with which the specified value is associated
- oldValue- value expected to be associated with the specified key
- newValue- value to be associated with the specified key
- Returns:
- trueif the value was replaced
- Throws:
- NullPointerException- if any of the arguments are null
 
- 
replaceReplaces the entry for a key only if currently mapped to some value. This is equivalent to, for thismap:
 except that the action is performed atomically.if (map.containsKey(key)) return map.put(key, value); else return null;- Specified by:
- replacein interface- ConcurrentMap<K,- V> 
- Specified by:
- replacein interface- Map<K,- V> 
- Parameters:
- key- key with which the specified value is associated
- value- value to be associated with the specified key
- Returns:
- the previous value associated with the specified key,
         or nullif there was no mapping for the key
- Throws:
- NullPointerException- if the specified key or value is null
 
- 
getOrDefaultReturns the value to which the specified key is mapped, or the given default value if this map contains no mapping for the key.- Specified by:
- getOrDefaultin interface- ConcurrentMap<K,- V> 
- Specified by:
- getOrDefaultin interface- Map<K,- V> 
- Parameters:
- key- the key whose associated value is to be returned
- defaultValue- the value to return if this map contains no mapping for the given key
- Returns:
- the mapping for the key, if present; else the default value
- Throws:
- NullPointerException- if the specified key is null
 
- 
computeIfAbsentIf the specified key is not already associated with a value, attempts to compute its value using the given mapping function and enters it into this map unlessnull. The entire method invocation is performed atomically. The supplied function is invoked exactly once per invocation of this method if the key is absent, else not at all. 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.The mapping function must not modify this map during computation. - Specified by:
- computeIfAbsentin interface- ConcurrentMap<K,- V> 
- Specified by:
- computeIfAbsentin interface- Map<K,- V> 
- Parameters:
- key- key with which the specified value is to be associated
- mappingFunction- the function to compute a value
- Returns:
- the current (existing or computed) value associated with the specified key, or null if the computed value is null
- Throws:
- NullPointerException- if the specified key or mappingFunction is 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
 
- 
computeIfPresentIf 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. The supplied function is invoked exactly once per invocation of this method if the key is present, else not at all. 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.The remapping function must not modify this map during computation. - Specified by:
- computeIfPresentin interface- ConcurrentMap<K,- V> 
- Specified by:
- computeIfPresentin interface- Map<K,- V> 
- Parameters:
- key- key with which a value may be associated
- remappingFunction- the function to compute a value
- Returns:
- the new value associated with the specified key, or null if none
- Throws:
- NullPointerException- if the specified key or remappingFunction is 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
 
- 
computeAttempts to compute a mapping for the specified key and its current mapped value (ornullif there is no current mapping). The entire method invocation is performed atomically. The supplied function is invoked exactly once per invocation of this method. 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.The remapping function must not modify this map during computation. - Specified by:
- computein interface- ConcurrentMap<K,- V> 
- Specified by:
- computein interface- Map<K,- V> 
- Parameters:
- key- key with which the specified value is to be associated
- remappingFunction- the function to compute a value
- Returns:
- the new value associated with the specified key, or null if none
- Throws:
- NullPointerException- if the specified key or remappingFunction is 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
 
- 
mergeIf the specified key is not already associated with a (non-null) value, associates it with the given value. Otherwise, replaces the value with the results of the given remapping function, or removes ifnull. The entire method invocation is performed atomically. Some attempted update operations on this map by other threads may be blocked while computation is in progress, so the computation should be short and simple, and must not attempt to update any other mappings of this Map.- Specified by:
- mergein interface- ConcurrentMap<K,- V> 
- Specified by:
- mergein interface- Map<K,- V> 
- Parameters:
- key- key with which the specified value is to be associated
- value- the value to use if absent
- remappingFunction- the function to recompute a value if present
- Returns:
- the new value associated with the specified key, or null if none
- Throws:
- NullPointerException- if the specified key or the remappingFunction is null
- RuntimeException- or Error if the remappingFunction does so, in which case the mapping is unchanged
 
- 
containsTests if some key maps into the specified value in this table.Note that this method is identical in functionality to containsValue(Object), and exists solely to ensure full compatibility with classHashtable, which supported this method prior to introduction of the Java Collections Framework.- Parameters:
- value- a value to search for
- Returns:
- trueif and only if some key maps to the- valueargument in this table as determined by the- equalsmethod;- falseotherwise
- Throws:
- NullPointerException- if the specified value is null
 
- 
keysReturns an enumeration of the keys in this table.- Returns:
- an enumeration of the keys in this table
- See Also:
 
- 
elementsReturns an enumeration of the values in this table.- Returns:
- an enumeration of the values in this table
- See Also:
 
- 
mappingCountpublic long mappingCount()Returns the number of mappings. This method should be used instead ofsize()because a ConcurrentHashMap may contain more mappings than can be represented as an int. The value returned is an estimate; the actual count may differ if there are concurrent insertions or removals.- Returns:
- the number of mappings
- Since:
- 1.8
 
- 
newKeySetCreates a newSetbacked by a ConcurrentHashMap from the given type toBoolean.TRUE.- Type Parameters:
- K- the element type of the returned set
- Returns:
- the new set
- Since:
- 1.8
 
- 
newKeySetCreates a newSetbacked by a ConcurrentHashMap from the given type toBoolean.TRUE.- Type Parameters:
- K- the element type of the returned set
- Parameters:
- initialCapacity- The implementation performs internal sizing to accommodate this many elements.
- Returns:
- the new set
- Throws:
- IllegalArgumentException- if the initial capacity of elements is negative
- Since:
- 1.8
 
- 
keySetReturns aSetview of the keys in this map, using the given common mapped value for any additions (i.e.,Collection.add(E)andCollection.addAll(Collection)). This is of course only appropriate if it is acceptable to use the same value for all additions from this view.- Parameters:
- mappedValue- the mapped value to use for any additions
- Returns:
- the set view
- Throws:
- NullPointerException- if the mappedValue is null
 
- 
forEachPerforms the given action for each (key, value).- Parameters:
- parallelismThreshold- the (estimated) number of elements needed for this operation to be executed in parallel
- action- the action
- Since:
- 1.8
 
- 
forEachpublic <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).- Type Parameters:
- U- the return type of the transformer
- Parameters:
- parallelismThreshold- the (estimated) number of elements needed for this operation to be executed in parallel
- transformer- a function returning the transformation for an element, or null if there is no transformation (in which case the action is not applied)
- action- the action
- Since:
- 1.8
 
- 
searchpublic <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. Upon success, further element processing is suppressed and the results of any other parallel invocations of the search function are ignored.- Type Parameters:
- U- the return type of the search function
- Parameters:
- parallelismThreshold- the (estimated) number of elements needed for this operation to be executed in parallel
- searchFunction- a function returning a non-null result on success, else null
- Returns:
- a non-null result from applying the given search function on each (key, value), or null if none
- Since:
- 1.8
 
- 
reducepublic <U> U reduce(long parallelismThreshold, BiFunction<? super K, ? super V, ? extends U> transformer, BiFunction<? super U, ? super U, ? extends U> reducer) Returns the result of accumulating the given transformation of all (key, value) pairs using the given reducer to combine values, or null if none.- Type Parameters:
- U- the return type of the transformer
- Parameters:
- parallelismThreshold- the (estimated) number of elements needed for this operation to be executed in parallel
- transformer- a function returning the transformation for an element, or null if there is no transformation (in which case it is not combined)
- reducer- a commutative associative combining function
- Returns:
- the result of accumulating the given transformation of all (key, value) pairs
- Since:
- 1.8
 
- 
reduceToDoublepublic double reduceToDouble(long parallelismThreshold, ToDoubleBiFunction<? super K, ? super V> transformer, double basis, DoubleBinaryOperator reducer) Returns the result of accumulating the given transformation of all (key, value) pairs using the given reducer to combine values, and the given basis as an identity value.- Parameters:
- parallelismThreshold- the (estimated) number of elements needed for this operation to be executed in parallel
- transformer- a function returning the transformation for an element
- basis- the identity (initial default value) for the reduction
- reducer- a commutative associative combining function
- Returns:
- the result of accumulating the given transformation of all (key, value) pairs
- Since:
- 1.8
 
- 
reduceToLongpublic long reduceToLong(long parallelismThreshold, ToLongBiFunction<? super K, ? super V> transformer, long basis, LongBinaryOperator reducer) Returns the result of accumulating the given transformation of all (key, value) pairs using the given reducer to combine values, and the given basis as an identity value.- Parameters:
- parallelismThreshold- the (estimated) number of elements needed for this operation to be executed in parallel
- transformer- a function returning the transformation for an element
- basis- the identity (initial default value) for the reduction
- reducer- a commutative associative combining function
- Returns:
- the result of accumulating the given transformation of all (key, value) pairs
- Since:
- 1.8
 
- 
reduceToIntpublic int reduceToInt(long parallelismThreshold, ToIntBiFunction<? super K, ? super V> transformer, int basis, IntBinaryOperator reducer) Returns the result of accumulating the given transformation of all (key, value) pairs using the given reducer to combine values, and the given basis as an identity value.- Parameters:
- parallelismThreshold- the (estimated) number of elements needed for this operation to be executed in parallel
- transformer- a function returning the transformation for an element
- basis- the identity (initial default value) for the reduction
- reducer- a commutative associative combining function
- Returns:
- the result of accumulating the given transformation of all (key, value) pairs
- Since:
- 1.8
 
- 
forEachKeyPerforms the given action for each key.- Parameters:
- parallelismThreshold- the (estimated) number of elements needed for this operation to be executed in parallel
- action- the action
- Since:
- 1.8
 
- 
forEachKeypublic <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.- Type Parameters:
- U- the return type of the transformer
- Parameters:
- parallelismThreshold- the (estimated) number of elements needed for this operation to be executed in parallel
- transformer- a function returning the transformation for an element, or null if there is no transformation (in which case the action is not applied)
- action- the action
- Since:
- 1.8
 
- 
searchKeysReturns a non-null result from applying the given search function on each key, or null if none. Upon success, further element processing is suppressed and the results of any other parallel invocations of the search function are ignored.- Type Parameters:
- U- the return type of the search function
- Parameters:
- parallelismThreshold- the (estimated) number of elements needed for this operation to be executed in parallel
- searchFunction- a function returning a non-null result on success, else null
- Returns:
- a non-null result from applying the given search function on each key, or null if none
- Since:
- 1.8
 
- 
reduceKeysReturns the result of accumulating all keys using the given reducer to combine values, or null if none.- Parameters:
- parallelismThreshold- the (estimated) number of elements needed for this operation to be executed in parallel
- reducer- a commutative associative combining function
- Returns:
- the result of accumulating all keys using the given reducer to combine values, or null if none
- Since:
- 1.8
 
- 
reduceKeyspublic <U> U reduceKeys(long parallelismThreshold, Function<? super K, ? extends U> transformer, BiFunction<? super U, ? super U, ? extends U> reducer) Returns the result of accumulating the given transformation of all keys using the given reducer to combine values, or null if none.- Type Parameters:
- U- the return type of the transformer
- Parameters:
- parallelismThreshold- the (estimated) number of elements needed for this operation to be executed in parallel
- transformer- a function returning the transformation for an element, or null if there is no transformation (in which case it is not combined)
- reducer- a commutative associative combining function
- Returns:
- the result of accumulating the given transformation of all keys
- Since:
- 1.8
 
- 
reduceKeysToDoublepublic double reduceKeysToDouble(long parallelismThreshold, ToDoubleFunction<? super K> transformer, double basis, DoubleBinaryOperator reducer) Returns the result of accumulating the given transformation of all keys using the given reducer to combine values, and the given basis as an identity value.- Parameters:
- parallelismThreshold- the (estimated) number of elements needed for this operation to be executed in parallel
- transformer- a function returning the transformation for an element
- basis- the identity (initial default value) for the reduction
- reducer- a commutative associative combining function
- Returns:
- the result of accumulating the given transformation of all keys
- Since:
- 1.8
 
- 
reduceKeysToLongpublic long reduceKeysToLong(long parallelismThreshold, ToLongFunction<? super K> transformer, long basis, LongBinaryOperator reducer) Returns the result of accumulating the given transformation of all keys using the given reducer to combine values, and the given basis as an identity value.- Parameters:
- parallelismThreshold- the (estimated) number of elements needed for this operation to be executed in parallel
- transformer- a function returning the transformation for an element
- basis- the identity (initial default value) for the reduction
- reducer- a commutative associative combining function
- Returns:
- the result of accumulating the given transformation of all keys
- Since:
- 1.8
 
- 
reduceKeysToIntpublic int reduceKeysToInt(long parallelismThreshold, ToIntFunction<? super K> transformer, int basis, IntBinaryOperator reducer) Returns the result of accumulating the given transformation of all keys using the given reducer to combine values, and the given basis as an identity value.- Parameters:
- parallelismThreshold- the (estimated) number of elements needed for this operation to be executed in parallel
- transformer- a function returning the transformation for an element
- basis- the identity (initial default value) for the reduction
- reducer- a commutative associative combining function
- Returns:
- the result of accumulating the given transformation of all keys
- Since:
- 1.8
 
- 
forEachValuePerforms the given action for each value.- Parameters:
- parallelismThreshold- the (estimated) number of elements needed for this operation to be executed in parallel
- action- the action
- Since:
- 1.8
 
- 
forEachValuepublic <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.- Type Parameters:
- U- the return type of the transformer
- Parameters:
- parallelismThreshold- the (estimated) number of elements needed for this operation to be executed in parallel
- transformer- a function returning the transformation for an element, or null if there is no transformation (in which case the action is not applied)
- action- the action
- Since:
- 1.8
 
- 
searchValuespublic <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. Upon success, further element processing is suppressed and the results of any other parallel invocations of the search function are ignored.- Type Parameters:
- U- the return type of the search function
- Parameters:
- parallelismThreshold- the (estimated) number of elements needed for this operation to be executed in parallel
- searchFunction- a function returning a non-null result on success, else null
- Returns:
- a non-null result from applying the given search function on each value, or null if none
- Since:
- 1.8
 
- 
reduceValuespublic V reduceValues(long parallelismThreshold, BiFunction<? super V, ? super V, ? extends V> reducer) Returns the result of accumulating all values using the given reducer to combine values, or null if none.- Parameters:
- parallelismThreshold- the (estimated) number of elements needed for this operation to be executed in parallel
- reducer- a commutative associative combining function
- Returns:
- the result of accumulating all values
- Since:
- 1.8
 
- 
reduceValuespublic <U> U reduceValues(long parallelismThreshold, Function<? super V, ? extends U> transformer, BiFunction<? super U, ? super U, ? extends U> reducer) Returns the result of accumulating the given transformation of all values using the given reducer to combine values, or null if none.- Type Parameters:
- U- the return type of the transformer
- Parameters:
- parallelismThreshold- the (estimated) number of elements needed for this operation to be executed in parallel
- transformer- a function returning the transformation for an element, or null if there is no transformation (in which case it is not combined)
- reducer- a commutative associative combining function
- Returns:
- the result of accumulating the given transformation of all values
- Since:
- 1.8
 
- 
reduceValuesToDoublepublic double reduceValuesToDouble(long parallelismThreshold, ToDoubleFunction<? super V> transformer, double basis, DoubleBinaryOperator reducer) Returns the result of accumulating the given transformation of all values using the given reducer to combine values, and the given basis as an identity value.- Parameters:
- parallelismThreshold- the (estimated) number of elements needed for this operation to be executed in parallel
- transformer- a function returning the transformation for an element
- basis- the identity (initial default value) for the reduction
- reducer- a commutative associative combining function
- Returns:
- the result of accumulating the given transformation of all values
- Since:
- 1.8
 
- 
reduceValuesToLongpublic long reduceValuesToLong(long parallelismThreshold, ToLongFunction<? super V> transformer, long basis, LongBinaryOperator reducer) Returns the result of accumulating the given transformation of all values using the given reducer to combine values, and the given basis as an identity value.- Parameters:
- parallelismThreshold- the (estimated) number of elements needed for this operation to be executed in parallel
- transformer- a function returning the transformation for an element
- basis- the identity (initial default value) for the reduction
- reducer- a commutative associative combining function
- Returns:
- the result of accumulating the given transformation of all values
- Since:
- 1.8
 
- 
reduceValuesToIntpublic int reduceValuesToInt(long parallelismThreshold, ToIntFunction<? super V> transformer, int basis, IntBinaryOperator reducer) Returns the result of accumulating the given transformation of all values using the given reducer to combine values, and the given basis as an identity value.- Parameters:
- parallelismThreshold- the (estimated) number of elements needed for this operation to be executed in parallel
- transformer- a function returning the transformation for an element
- basis- the identity (initial default value) for the reduction
- reducer- a commutative associative combining function
- Returns:
- the result of accumulating the given transformation of all values
- Since:
- 1.8
 
- 
forEachEntryPerforms the given action for each entry.- Parameters:
- parallelismThreshold- the (estimated) number of elements needed for this operation to be executed in parallel
- action- the action
- Since:
- 1.8
 
- 
forEachEntrypublic <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.- Type Parameters:
- U- the return type of the transformer
- Parameters:
- parallelismThreshold- the (estimated) number of elements needed for this operation to be executed in parallel
- transformer- a function returning the transformation for an element, or null if there is no transformation (in which case the action is not applied)
- action- the action
- Since:
- 1.8
 
- 
searchEntriespublic <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. Upon success, further element processing is suppressed and the results of any other parallel invocations of the search function are ignored.- Type Parameters:
- U- the return type of the search function
- Parameters:
- parallelismThreshold- the (estimated) number of elements needed for this operation to be executed in parallel
- searchFunction- a function returning a non-null result on success, else null
- Returns:
- a non-null result from applying the given search function on each entry, or null if none
- Since:
- 1.8
 
- 
reduceEntriespublic Map.Entry<K,V> reduceEntries(long parallelismThreshold, BiFunction<Map.Entry<K, V>, Map.Entry<K, V>, ? extends Map.Entry<K, V>> reducer) Returns the result of accumulating all entries using the given reducer to combine values, or null if none.- Parameters:
- parallelismThreshold- the (estimated) number of elements needed for this operation to be executed in parallel
- reducer- a commutative associative combining function
- Returns:
- the result of accumulating all entries
- Since:
- 1.8
 
- 
reduceEntriespublic <U> U reduceEntries(long parallelismThreshold, Function<Map.Entry<K, V>, ? extends U> transformer, BiFunction<? super U, ? super U, ? extends U> reducer) Returns the result of accumulating the given transformation of all entries using the given reducer to combine values, or null if none.- Type Parameters:
- U- the return type of the transformer
- Parameters:
- parallelismThreshold- the (estimated) number of elements needed for this operation to be executed in parallel
- transformer- a function returning the transformation for an element, or null if there is no transformation (in which case it is not combined)
- reducer- a commutative associative combining function
- Returns:
- the result of accumulating the given transformation of all entries
- Since:
- 1.8
 
- 
reduceEntriesToDoublepublic double reduceEntriesToDouble(long parallelismThreshold, ToDoubleFunction<Map.Entry<K, V>> transformer, double basis, DoubleBinaryOperator reducer) Returns the result of accumulating the given transformation of all entries using the given reducer to combine values, and the given basis as an identity value.- Parameters:
- parallelismThreshold- the (estimated) number of elements needed for this operation to be executed in parallel
- transformer- a function returning the transformation for an element
- basis- the identity (initial default value) for the reduction
- reducer- a commutative associative combining function
- Returns:
- the result of accumulating the given transformation of all entries
- Since:
- 1.8
 
- 
reduceEntriesToLongpublic long reduceEntriesToLong(long parallelismThreshold, ToLongFunction<Map.Entry<K, V>> transformer, long basis, LongBinaryOperator reducer) Returns the result of accumulating the given transformation of all entries using the given reducer to combine values, and the given basis as an identity value.- Parameters:
- parallelismThreshold- the (estimated) number of elements needed for this operation to be executed in parallel
- transformer- a function returning the transformation for an element
- basis- the identity (initial default value) for the reduction
- reducer- a commutative associative combining function
- Returns:
- the result of accumulating the given transformation of all entries
- Since:
- 1.8
 
- 
reduceEntriesToIntpublic int reduceEntriesToInt(long parallelismThreshold, ToIntFunction<Map.Entry<K, V>> transformer, int basis, IntBinaryOperator reducer) Returns the result of accumulating the given transformation of all entries using the given reducer to combine values, and the given basis as an identity value.- Parameters:
- parallelismThreshold- the (estimated) number of elements needed for this operation to be executed in parallel
- transformer- a function returning the transformation for an element
- basis- the identity (initial default value) for the reduction
- reducer- a commutative associative combining function
- Returns:
- the result of accumulating the given transformation of all entries
- Since:
- 1.8
 
 
-