Module java.base
Package java.util.concurrent

Class ConcurrentHashMap<K,V>

  • 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>
    public class ConcurrentHashMap<K,V>
extends AbstractMap<K,V>
implements ConcurrentMap<K,V>, Serializable
    A hash table supporting full concurrency of retrievals and high expected concurrency for updates. This class obeys the same functional specification as Hashtable, and includes versions of methods corresponding to each method of Hashtable. However, even though all operations are 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:
    Serialized Form

    • Constructor Summary

      Constructors 
      Constructor Description
      ConcurrentHashMap()
      Creates a new, empty map with the default initial table size (16).
      ConcurrentHashMap​(int initialCapacity)
      Creates a new, empty map with an initial table size accommodating the specified number of elements without the need to dynamically resize.
      ConcurrentHashMap​(int initialCapacity, float loadFactor)
      Creates a new, empty map with an initial table size based on the given number of elements (initialCapacity) and initial table density (loadFactor).
      ConcurrentHashMap​(int initialCapacity, float loadFactor, int concurrencyLevel)
      Creates a new, empty map with an initial table size based on the given number of elements (initialCapacity), table density (loadFactor), and number of concurrently updating threads (concurrencyLevel).
      ConcurrentHashMap​(Map<? extends K,? extends V> m)
      Creates a new map with the same mappings as the given map.

    • Method Summary

      All Methods Static Methods Instance Methods Concrete Methods 
      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)
      Tests if some key maps into the specified value in this table.
      boolean containsKey​(Object key)
      Tests if the specified object is a key in this table.
      boolean containsValue​(Object value)
      Returns 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​(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().
      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, associates it with the given value.
      <U> U reduce​(long parallelismThreshold, BiFunction<? super K,? super V,? extends U> transformer, BiFunction<? super U,? super U,? extends U> reducer)
      Returns the result of accumulating the given transformation of all (key, value) pairs using the given reducer to combine values, or null if none.
      Map.Entry<K,V> reduceEntries​(long parallelismThreshold, BiFunction<Map.Entry<K,V>,Map.Entry<K,V>,? extends Map.Entry<K,V>> reducer)
      Returns the result of accumulating all entries using the given reducer to combine values, or null if none.
      <U> U reduceEntries​(long parallelismThreshold, Function<Map.Entry<K,V>,? extends U> transformer, BiFunction<? super U,? super U,? extends U> reducer)
      Returns the result of accumulating the given transformation of all entries using the given reducer to combine values, or null if none.
      double reduceEntriesToDouble​(long parallelismThreshold, ToDoubleFunction<Map.Entry<K,V>> transformer, double basis, DoubleBinaryOperator reducer)
      Returns the result of accumulating the given transformation of all entries using the given reducer to combine values, and the given basis as an identity value.
      int reduceEntriesToInt​(long parallelismThreshold, ToIntFunction<Map.Entry<K,V>> transformer, int basis, IntBinaryOperator reducer)
      Returns the result of accumulating the given transformation of all entries using the given reducer to combine values, and the given basis as an identity value.
      long reduceEntriesToLong​(long parallelismThreshold, ToLongFunction<Map.Entry<K,V>> transformer, long basis, LongBinaryOperator reducer)
      Returns the result of accumulating the given transformation of all entries using the given reducer to combine values, and the given basis as an identity value.
      K reduceKeys​(long parallelismThreshold, BiFunction<? super K,? super K,? extends K> reducer)
      Returns the result of accumulating all keys using the given reducer to combine values, or null if none.
      <U> U reduceKeys​(long parallelismThreshold, Function<? super K,? extends U> transformer, BiFunction<? super U,? super U,? extends U> reducer)
      Returns the result of accumulating the given transformation of all keys using the given reducer to combine values, or null if none.
      double reduceKeysToDouble​(long parallelismThreshold, ToDoubleFunction<? super K> transformer, double basis, DoubleBinaryOperator reducer)
      Returns the result of accumulating the given transformation of all keys using the given reducer to combine values, and the given basis as an identity value.
      int reduceKeysToInt​(long parallelismThreshold, ToIntFunction<? super K> transformer, int basis, IntBinaryOperator reducer)
      Returns the result of accumulating the given transformation of all keys using the given reducer to combine values, and the given basis as an identity value.
      long reduceKeysToLong​(long parallelismThreshold, ToLongFunction<? super K> transformer, long basis, LongBinaryOperator reducer)
      Returns the result of accumulating the given transformation of all keys using the given reducer to combine values, and the given basis as an identity value.
      double reduceToDouble​(long parallelismThreshold, ToDoubleBiFunction<? super K,? super V> transformer, double basis, DoubleBinaryOperator reducer)
      Returns the result of accumulating the given transformation of all (key, value) pairs using the given reducer to combine values, and the given basis as an identity value.
      int reduceToInt​(long parallelismThreshold, ToIntBiFunction<? super K,? super V> transformer, int basis, IntBinaryOperator reducer)
      Returns the result of accumulating the given transformation of all (key, value) pairs using the given reducer to combine values, and the given basis as an identity value.
      long reduceToLong​(long parallelismThreshold, ToLongBiFunction<? super K,? super V> transformer, long basis, LongBinaryOperator reducer)
      Returns the result of accumulating the given transformation of all (key, value) pairs using the given reducer to combine values, and the given basis as an identity value.
      V reduceValues​(long parallelismThreshold, BiFunction<? super V,? super V,? extends V> reducer)
      Returns the result of accumulating all values using the given reducer to combine values, or null if none.
      <U> U reduceValues​(long parallelismThreshold, Function<? super V,? extends U> transformer, BiFunction<? super U,? super U,? extends U> reducer)
      Returns the result of accumulating the given transformation of all values using the given reducer to combine values, or null if none.
      double reduceValuesToDouble​(long parallelismThreshold, ToDoubleFunction<? super V> transformer, double basis, DoubleBinaryOperator reducer)
      Returns the result of accumulating the given transformation of all values using the given reducer to combine values, and the given basis as an identity value.
      int reduceValuesToInt​(long parallelismThreshold, ToIntFunction<? super V> transformer, int basis, IntBinaryOperator reducer)
      Returns the result of accumulating the given transformation of all values using the given reducer to combine values, and the given basis as an identity value.
      long reduceValuesToLong​(long parallelismThreshold, ToLongFunction<? super V> transformer, long basis, LongBinaryOperator reducer)
      Returns the result of accumulating the given transformation of all values using the given reducer to combine values, and the given basis as an identity value.
      V remove​(Object key)
      Removes the key (and its corresponding value) from this map.
      boolean remove​(Object key, Object value)
      Removes the entry for a key only if currently mapped to a given value.
      V replace​(K key, V value)
      Replaces the entry for a key only if currently mapped to some value.
      boolean replace​(K key, V oldValue, V newValue)
      Replaces the entry for a key only if currently mapped to a given value.
      <U> U search​(long parallelismThreshold, BiFunction<? super K,? super V,? extends U> searchFunction)
      Returns a 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.
      String toString()
      Returns a string representation of this map.
      Collection<V> values()
      Returns a Collection view of the values contained in this map.

    • Constructor Detail

      • ConcurrentHashMap

        public ConcurrentHashMap()
        Creates a new, empty map with the default initial table size (16).

      • ConcurrentHashMap

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

      • ConcurrentHashMap

        public ConcurrentHashMap​(Map<? extends K,? extends V> m)
        Creates a new map with the same mappings as the given map.
        Parameters:
        m - the map

      • ConcurrentHashMap

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

      • ConcurrentHashMap

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

      • get

        public V get​(Object key)
        Returns the value to which the specified key is mapped, or null if this map contains no mapping for the key.

        More formally, if this map contains a mapping from a key k to a value v such that key.equals(k), then this method returns v; otherwise it returns null. (There can be at most one such mapping.)

        Specified by:
        get in interface Map<K,V>
        Overrides:
        get in 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 null if this map contains no mapping for the key
        Throws:
        NullPointerException - if the specified key is null

      • containsKey

        public boolean containsKey​(Object key)
        Tests if the specified object is a key in this table.
        Specified by:
        containsKey in interface Map<K,V>
        Overrides:
        containsKey in class AbstractMap<K,V>
        Parameters:
        key - possible key
        Returns:
        true if and only if the specified object is a key in this table, as determined by the equals method; false otherwise
        Throws:
        NullPointerException - if the specified key is null

      • containsValue

        public boolean containsValue​(Object value)
        Returns true if this map maps one or more keys to the specified value. Note: This method may require a full traversal of the map, and is much slower than method containsKey.
        Specified by:
        containsValue in interface Map<K,V>
        Overrides:
        containsValue in class AbstractMap<K,V>
        Parameters:
        value - value whose presence in this map is to be tested
        Returns:
        true if this map maps one or more keys to the specified value
        Throws:
        NullPointerException - if the specified value is null

      • put

        public V put​(K key,
             V value)
        Maps the specified key to the specified value in this table. Neither the key nor the value can be null.

        The value can be retrieved by calling the get method with a key that is equal to the original key.

        Specified by:
        put in interface Map<K,V>
        Overrides:
        put in 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, or null if there was no mapping for key
        Throws:
        NullPointerException - if the specified key or value is null

      • putAll

        public void putAll​(Map<? extends K,? extends V> m)
        Copies all of the mappings from the specified map to this one. These mappings replace any mappings that this map had for any of the keys currently in the specified map.
        Specified by:
        putAll in interface Map<K,V>
        Overrides:
        putAll in class AbstractMap<K,V>
        Parameters:
        m - mappings to be stored in this map

      • remove

        public V remove​(Object key)
        Removes the key (and its corresponding value) from this map. This method does nothing if the key is not in the map.
        Specified by:
        remove in interface Map<K,V>
        Overrides:
        remove in class AbstractMap<K,V>
        Parameters:
        key - the key that needs to be removed
        Returns:
        the previous value associated with key, or null if there was no mapping for key
        Throws:
        NullPointerException - if the specified key is null

      • clear

        public void clear()
        Removes all of the mappings from this map.
        Specified by:
        clear in interface Map<K,V>
        Overrides:
        clear in class AbstractMap<K,V>

      • values

        public Collection<V> values()
        Returns a Collection view of the values contained in this map. The collection is backed by the map, so changes to the map are reflected in the collection, and 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.

        Specified by:
        values in interface Map<K,V>
        Overrides:
        values in class AbstractMap<K,V>
        Returns:
        the collection view

      • entrySet

        public Set<Map.Entry<K,V>> entrySet()
        Returns a Set view of the mappings contained in this map. The set is backed by the map, so changes to the map are reflected in the set, and 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.

        Specified by:
        entrySet in interface Map<K,V>
        Returns:
        the set view

      • toString

        public String toString()
        Returns 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:
        toString in class AbstractMap<K,V>
        Returns:
        a string representation of this map

      • equals

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

      • putIfAbsent

        public V putIfAbsent​(K key,
                     V value)
        If the specified key is not already associated with a value, associates it with the given value. This is equivalent to, for this map: 
         
 if (!map.containsKey(key))
   return map.put(key, value);
 else
   return map.get(key);
        except that the action is performed atomically.
        Specified by:
        putIfAbsent in interface ConcurrentMap<K,V>
        Specified by:
        putIfAbsent in 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 null if there was no mapping for the key
        Throws:
        NullPointerException - if the specified key or value is null

      • remove

        public boolean remove​(Object key,
                      Object value)
        Removes the entry for a key only if currently mapped to a given value. This is equivalent to, for this map: 
         
 if (map.containsKey(key)
     && Objects.equals(map.get(key), value)) {
   map.remove(key);
   return true;
 } else {
   return false;
 }
        except that the action is performed atomically.
        Specified by:
        remove in interface ConcurrentMap<K,V>
        Specified by:
        remove in 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:
        true if the value was removed
        Throws:
        NullPointerException - if the specified key is null

      • replace

        public boolean replace​(K key,
                       V oldValue,
                       V newValue)
        Replaces the entry for a key only if currently mapped to a given value. This is equivalent to, for this map: 
         
 if (map.containsKey(key)
     && Objects.equals(map.get(key), oldValue)) {
   map.put(key, newValue);
   return true;
 } else {
   return false;
 }
        except that the action is performed atomically.
        Specified by:
        replace in interface ConcurrentMap<K,V>
        Specified by:
        replace in 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:
        true if the value was replaced
        Throws:
        NullPointerException - if any of the arguments are null

      • replace

        public V replace​(K key,
                 V value)
        Replaces the entry for a key only if currently mapped to some value. This is equivalent to, for this map: 
         
 if (map.containsKey(key))
   return map.put(key, value);
 else
   return null;
        except that the action is performed atomically.
        Specified by:
        replace in interface ConcurrentMap<K,V>
        Specified by:
        replace in 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 null if there was no mapping for the key
        Throws:
        NullPointerException - if the specified key or value is null

      • getOrDefault

        public 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.
        Specified by:
        getOrDefault in interface ConcurrentMap<K,V>
        Specified by:
        getOrDefault in 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

      • computeIfAbsent

        public V computeIfAbsent​(K key,
                         Function<? super K,? extends V> mappingFunction)
        If the specified key is not already associated with a value, attempts to compute its value using the given mapping function and enters it into this map unless null. The entire method invocation is performed atomically, so the function is applied at most once per key. Some attempted update operations on this map by other threads may be blocked while computation is in progress, so the computation should be short and simple, and must not attempt to update any other mappings of this map.
        Specified by:
        computeIfAbsent in interface ConcurrentMap<K,V>
        Specified by:
        computeIfAbsent in 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

      • computeIfPresent

        public V computeIfPresent​(K key,
                          BiFunction<? super K,? super V,? extends V> remappingFunction)
        If the value for the specified key is present, attempts to compute a new mapping given the key and its current mapped value. The entire method invocation is performed atomically. Some attempted update operations on this map by other threads may be blocked while computation is in progress, so the computation should be short and simple, and must not attempt to update any other mappings of this map.
        Specified by:
        computeIfPresent in interface ConcurrentMap<K,V>
        Specified by:
        computeIfPresent in 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

      • compute

        public V compute​(K key,
                 BiFunction<? super K,? super V,? extends V> remappingFunction)
        Attempts to compute a mapping for the specified key and its current mapped value (or null if there is no current mapping). The entire method invocation is performed atomically. Some attempted update operations on this map by other threads may be blocked while computation is in progress, so the computation should be short and simple, and must not attempt to update any other mappings of this Map.
        Specified by:
        compute in interface ConcurrentMap<K,V>
        Specified by:
        compute in 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

      • merge

        public V merge​(K key,
               V value,
               BiFunction<? super V,? super V,? extends V> remappingFunction)
        If the specified key is not already associated with a (non-null) value, associates it with the given value. Otherwise, replaces the value with the results of the given remapping function, or removes if null. The entire method invocation is performed atomically. Some attempted update operations on this map by other threads may be blocked while computation is in progress, so the computation should be short and simple, and must not attempt to update any other mappings of this Map.
        Specified by:
        merge in interface ConcurrentMap<K,V>
        Specified by:
        merge in 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

      • contains

        public boolean contains​(Object value)
        Tests if some key maps into the specified value in this table.

        Note that this method is identical in functionality to containsValue(Object), and exists solely to ensure full compatibility with class Hashtable, which supported this method prior to introduction of the Java Collections Framework.

        Parameters:
        value - a value to search for
        Returns:
        true if and only if some key maps to the value argument in this table as determined by the equals method; false otherwise
        Throws:
        NullPointerException - if the specified value is null

      • keys

        public Enumeration<K> keys()
        Returns an enumeration of the keys in this table.
        Returns:
        an enumeration of the keys in this table
        See Also:
        keySet()

      • elements

        public Enumeration<V> elements()
        Returns an enumeration of the values in this table.
        Returns:
        an enumeration of the values in this table
        See Also:
        values()

      • mappingCount

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

      • newKeySet

        public static <K> ConcurrentHashMap.KeySetView<K,Boolean> newKeySet()
        Creates a new Set backed by a ConcurrentHashMap from the given type to Boolean.TRUE.
        Type Parameters:
        K - the element type of the returned set
        Returns:
        the new set
        Since:
        1.8

      • newKeySet

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

      • forEach

        public void forEach​(long parallelismThreshold,
                    BiConsumer<? super K,? super V> action)
        Performs 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

      • forEach

        public <U> void forEach​(long parallelismThreshold,
                        BiFunction<? super K,? super V,? extends U> transformer,
                        Consumer<? super U> action)
        Performs the given action for each 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

      • search

        public <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

      • reduce

        public <U> U reduce​(long parallelismThreshold,
                    BiFunction<? super K,? super V,? extends U> transformer,
                    BiFunction<? super U,? super U,? extends U> reducer)
        Returns the result of accumulating the given transformation of all (key, value) pairs using the given reducer to combine values, or null if none.
        Type Parameters:
        U - the return type of the transformer
        Parameters:
        parallelismThreshold - the (estimated) number of elements needed for this operation to be executed in 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

      • reduceToDouble

        public double reduceToDouble​(long parallelismThreshold,
                             ToDoubleBiFunction<? super K,? super V> transformer,
                             double basis,
                             DoubleBinaryOperator reducer)
        Returns the result of accumulating the given transformation of all (key, value) pairs using the given reducer to combine values, and the given basis as an identity value.
        Parameters:
        parallelismThreshold - the (estimated) number of elements needed for this operation to be executed in 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

      • reduceToLong

        public long reduceToLong​(long parallelismThreshold,
                         ToLongBiFunction<? super K,? super V> transformer,
                         long basis,
                         LongBinaryOperator reducer)
        Returns the result of accumulating the given transformation of all (key, value) pairs using the given reducer to combine values, and the given basis as an identity value.
        Parameters:
        parallelismThreshold - the (estimated) number of elements needed for this operation to be executed in 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

      • reduceToInt

        public int reduceToInt​(long parallelismThreshold,
                       ToIntBiFunction<? super K,? super V> transformer,
                       int basis,
                       IntBinaryOperator reducer)
        Returns the result of accumulating the given transformation of all (key, value) pairs using the given reducer to combine values, and the given basis as an identity value.
        Parameters:
        parallelismThreshold - the (estimated) number of elements needed for this operation to be executed in 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

      • forEachKey

        public void forEachKey​(long parallelismThreshold,
                       Consumer<? super K> action)
        Performs 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

      • forEachKey

        public <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

      • searchKeys

        public <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. 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

      • reduceKeys

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

      • reduceKeys

        public <U> U reduceKeys​(long parallelismThreshold,
                        Function<? super K,? extends U> transformer,
                        BiFunction<? super U,? super U,? extends U> reducer)
        Returns the result of accumulating the given transformation of all keys using the given reducer to combine values, or null if none.
        Type Parameters:
        U - the return type of the transformer
        Parameters:
        parallelismThreshold - the (estimated) number of elements needed for this operation to be executed in 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

      • reduceKeysToDouble

        public double reduceKeysToDouble​(long parallelismThreshold,
                                 ToDoubleFunction<? super K> transformer,
                                 double basis,
                                 DoubleBinaryOperator reducer)
        Returns the result of accumulating the given transformation of all keys using the given reducer to combine values, and the given basis as an identity value.
        Parameters:
        parallelismThreshold - the (estimated) number of elements needed for this operation to be executed in 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

      • reduceKeysToLong

        public long reduceKeysToLong​(long parallelismThreshold,
                             ToLongFunction<? super K> transformer,
                             long basis,
                             LongBinaryOperator reducer)
        Returns the result of accumulating the given transformation of all keys using the given reducer to combine values, and the given basis as an identity value.
        Parameters:
        parallelismThreshold - the (estimated) number of elements needed for this operation to be executed in 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

      • reduceKeysToInt

        public int reduceKeysToInt​(long parallelismThreshold,
                           ToIntFunction<? super K> transformer,
                           int basis,
                           IntBinaryOperator reducer)
        Returns the result of accumulating the given transformation of all keys using the given reducer to combine values, and the given basis as an identity value.
        Parameters:
        parallelismThreshold - the (estimated) number of elements needed for this operation to be executed in 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

      • forEachValue

        public void forEachValue​(long parallelismThreshold,
                         Consumer<? super V> action)
        Performs 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

      • forEachValue

        public <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

      • searchValues

        public <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

      • reduceValues

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

      • reduceValues

        public <U> U reduceValues​(long parallelismThreshold,
                          Function<? super V,? extends U> transformer,
                          BiFunction<? super U,? super U,? extends U> reducer)
        Returns the result of accumulating the given transformation of all values using the given reducer to combine values, or null if none.
        Type Parameters:
        U - the return type of the transformer
        Parameters:
        parallelismThreshold - the (estimated) number of elements needed for this operation to be executed in 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

      • reduceValuesToDouble

        public double reduceValuesToDouble​(long parallelismThreshold,
                                   ToDoubleFunction<? super V> transformer,
                                   double basis,
                                   DoubleBinaryOperator reducer)
        Returns the result of accumulating the given transformation of all values using the given reducer to combine values, and the given basis as an identity value.
        Parameters:
        parallelismThreshold - the (estimated) number of elements needed for this operation to be executed in 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

      • reduceValuesToLong

        public long reduceValuesToLong​(long parallelismThreshold,
                               ToLongFunction<? super V> transformer,
                               long basis,
                               LongBinaryOperator reducer)
        Returns the result of accumulating the given transformation of all values using the given reducer to combine values, and the given basis as an identity value.
        Parameters:
        parallelismThreshold - the (estimated) number of elements needed for this operation to be executed in 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

      • reduceValuesToInt

        public int reduceValuesToInt​(long parallelismThreshold,
                             ToIntFunction<? super V> transformer,
                             int basis,
                             IntBinaryOperator reducer)
        Returns the result of accumulating the given transformation of all values using the given reducer to combine values, and the given basis as an identity value.
        Parameters:
        parallelismThreshold - the (estimated) number of elements needed for this operation to be executed in 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

      • forEachEntry

        public void forEachEntry​(long parallelismThreshold,
                         Consumer<? super Map.Entry<K,V>> action)
        Performs 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

      • forEachEntry

        public <U> void forEachEntry​(long parallelismThreshold,
                             Function<Map.Entry<K,V>,? extends U> transformer,
                             Consumer<? super U> action)
        Performs the given action for each 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

      • searchEntries

        public <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

      • reduceEntries

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

      • reduceEntries

        public <U> U reduceEntries​(long parallelismThreshold,
                           Function<Map.Entry<K,V>,? extends U> transformer,
                           BiFunction<? super U,? super U,? extends U> reducer)
        Returns the result of accumulating the given transformation of all entries using the given reducer to combine values, or null if none.
        Type Parameters:
        U - the return type of the transformer
        Parameters:
        parallelismThreshold - the (estimated) number of elements needed for this operation to be executed in 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

      • reduceEntriesToDouble

        public double reduceEntriesToDouble​(long parallelismThreshold,
                                    ToDoubleFunction<Map.Entry<K,V>> transformer,
                                    double basis,
                                    DoubleBinaryOperator reducer)
        Returns the result of accumulating the given transformation of all entries using the given reducer to combine values, and the given basis as an identity value.
        Parameters:
        parallelismThreshold - the (estimated) number of elements needed for this operation to be executed in 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

      • reduceEntriesToLong

        public long reduceEntriesToLong​(long parallelismThreshold,
                                ToLongFunction<Map.Entry<K,V>> transformer,
                                long basis,
                                LongBinaryOperator reducer)
        Returns the result of accumulating the given transformation of all entries using the given reducer to combine values, and the given basis as an identity value.
        Parameters:
        parallelismThreshold - the (estimated) number of elements needed for this operation to be executed in 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

      • reduceEntriesToInt

        public int reduceEntriesToInt​(long parallelismThreshold,
                              ToIntFunction<Map.Entry<K,V>> transformer,
                              int basis,
                              IntBinaryOperator reducer)
        Returns the result of accumulating the given transformation of all entries using the given reducer to combine values, and the given basis as an identity value.
        Parameters:
        parallelismThreshold - the (estimated) number of elements needed for this operation to be executed in 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