- java.lang.Object
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- java.util.AbstractMap<K,V>
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- java.util.concurrent.ConcurrentHashMap<K,V>
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- Type Parameters:
K- the type of keys maintained by this mapV- the type of mapped values
- All Implemented Interfaces:
Serializable,ConcurrentMap<K,V>,Map<K,V>
public class ConcurrentHashMap<K,V> extends AbstractMap<K,V> implements ConcurrentMap<K,V>, Serializable
A hash table supporting full concurrency of retrievals and high expected concurrency for updates. This class obeys the same functional specification asHashtable, and includes versions of methods corresponding to each method ofHashtable. 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 withHashtablein 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 (includingputandremove). 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 asputAllandclear, 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 throwConcurrentModificationException. However, iterators are designed to be used by only one thread at a time. Bear in mind that the results of aggregate status methods includingsize,isEmpty, andcontainsValueare 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
initialCapacityconstructor argument. An additional optionalloadFactorconstructor 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 expectedconcurrencyLevelas an additional hint for internal sizing. Note that using many keys with exactly the samehashCode()is a sure way to slow down performance of any hash table. To ameliorate impact, when keys areComparable, this class may use comparison order among keys to help break ties.A
Setprojection of a ConcurrentHashMap may be created (usingnewKeySet()ornewKeySet(int)), or viewed (usingkeySet(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
LongAddervalues and initializing viacomputeIfAbsent. For example, to add a count to aConcurrentHashMap<String,LongAdder> freqs, you can usefreqs.computeIfAbsent(key, k -> new LongAdder()).increment();This class and its views and iterators implement all of the optional methods of the
MapandIteratorinterfaces.Like
Hashtablebut unlikeHashMap, this class does not allownullto be used as a key or value.ConcurrentHashMaps support a set of sequential and parallel bulk operations that, unlike most
Streammethods, 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 onMap.Entryobjects do not support methodsetValue.- 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
parallelismThresholdargument. Methods proceed sequentially if the current map size is estimated to be less than the given threshold. Using a value ofLong.MAX_VALUEsuppresses all parallelism. Using a value of1results in maximal parallelism by partitioning into enough subtasks to fully utilize theForkJoinPool.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
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Nested Class Summary
Nested Classes Modifier and Type Class Description static classConcurrentHashMap.KeySetView<K,V>A 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.AbstractMap
AbstractMap.SimpleEntry<K,V>, AbstractMap.SimpleImmutableEntry<K,V>
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Constructor Summary
Constructors Constructor Description ConcurrentHashMap()Creates a new, empty map with the default initial table size (16).ConcurrentHashMap(int initialCapacity)Creates a new, empty map with an initial table size accommodating the specified number of elements without the need to dynamically resize.ConcurrentHashMap(int initialCapacity, float loadFactor)Creates a new, empty map with an initial table size based on the given number of elements (initialCapacity) and initial table density (loadFactor).ConcurrentHashMap(int initialCapacity, float loadFactor, int concurrencyLevel)Creates a new, empty map with an initial table size based on the given number of elements (initialCapacity), initial table density (loadFactor), and number of concurrently updating threads (concurrencyLevel).ConcurrentHashMap(Map<? extends K,? extends V> m)Creates a new map with the same mappings as the given map.
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Method Summary
All Methods Static Methods Instance Methods Concrete Methods Modifier and Type Method Description voidclear()Removes all of the mappings from this map.Vcompute(K key, BiFunction<? super K,? super V,? extends V> remappingFunction)Attempts to compute a mapping for the specified key and its current mapped value (ornullif there is no current mapping).VcomputeIfAbsent(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.VcomputeIfPresent(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.booleancontains(Object value)Tests 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.Enumeration<V>elements()Returns an enumeration of the values in this table.Set<Map.Entry<K,V>>entrySet()Returns aSetview of the mappings contained in this map.booleanequals(Object o)Compares 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.Vget(Object key)Returns the value to which the specified key is mapped, ornullif this map contains no mapping for the key.VgetOrDefault(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().Enumeration<K>keys()Returns an enumeration of the keys in this table.ConcurrentHashMap.KeySetView<K,V>keySet()Returns aSetview of the keys contained in this map.ConcurrentHashMap.KeySetView<K,V>keySet(V mappedValue)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)).longmappingCount()Returns the number of mappings.Vmerge(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 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.Vput(K key, V value)Maps the specified key to the specified value in this table.voidputAll(Map<? extends K,? extends V> m)Copies all of the mappings from the specified map to this one.VputIfAbsent(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.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> 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.KreduceKeys(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.VreduceValues(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.Vremove(Object key)Removes the key (and its corresponding value) from this map.booleanremove(Object key, Object value)Removes the entry for a key only if currently mapped to a given value.Vreplace(K key, V value)Replaces the entry for a key only if currently mapped to some value.booleanreplace(K key, V oldValue, V newValue)Replaces 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.StringtoString()Returns a string representation of this map.Collection<V>values()Returns aCollectionview of the values contained in this map.-
Methods declared in class java.util.AbstractMap
clone, isEmpty, size
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Methods declared in interface java.util.concurrent.ConcurrentMap
forEach, replaceAll
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Constructor Detail
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ConcurrentHashMap
public ConcurrentHashMap()
Creates a new, empty map with the default initial table size (16).
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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
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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
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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
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ConcurrentHashMap
public ConcurrentHashMap(int initialCapacity, float loadFactor, int concurrencyLevel)Creates a new, empty map with an initial table size based on the given number of elements (initialCapacity), initial table density (loadFactor), and number of concurrently updating threads (concurrencyLevel).- Parameters:
initialCapacity- the initial capacity. The implementation performs internal sizing to accommodate this many elements, given the specified load factor.loadFactor- the load factor (table density) for establishing the initial table sizeconcurrencyLevel- the estimated number of concurrently updating threads. The implementation may use this value as a sizing hint.- Throws:
IllegalArgumentException- if the initial capacity is negative or the load factor or concurrencyLevel are nonpositive
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Method Detail
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get
public V get(Object key)
Returns 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 interfaceMap<K,V>- Overrides:
getin classAbstractMap<K,V>- Parameters:
key- the key whose associated value is to be returned- Returns:
- the value to which the specified key is mapped, or
nullif this map contains no mapping for the key - Throws:
NullPointerException- if the specified key is null
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containsKey
public boolean containsKey(Object key)
Tests if the specified object is a key in this table.- Specified by:
containsKeyin interfaceMap<K,V>- Overrides:
containsKeyin classAbstractMap<K,V>- Parameters:
key- possible key- Returns:
trueif and only if the specified object is a key in this table, as determined by theequalsmethod;falseotherwise- Throws:
NullPointerException- if the specified key is null
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containsValue
public boolean containsValue(Object value)
Returnstrueif 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 interfaceMap<K,V>- Overrides:
containsValuein classAbstractMap<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
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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
getmethod with a key that is equal to the original key.- Specified by:
putin interfaceMap<K,V>- Overrides:
putin classAbstractMap<K,V>- Parameters:
key- key with which the specified value is to be associatedvalue- value to be associated with the specified key- Returns:
- the previous value associated with
key, ornullif there was no mapping forkey - Throws:
NullPointerException- if the specified key or value is null
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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.
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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:
removein interfaceMap<K,V>- Overrides:
removein classAbstractMap<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
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clear
public void clear()
Removes all of the mappings from this map.
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keySet
public ConcurrentHashMap.KeySetView<K,V> keySet()
Returns 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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values
public Collection<V> values()
Returns 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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entrySet
public Set<Map.Entry<K,V>> entrySet()
Returns 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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hashCode
public 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().- Specified by:
hashCodein interfaceMap<K,V>- Overrides:
hashCodein classAbstractMap<K,V>- Returns:
- the hash code value for this map
- See Also:
Map.Entry.hashCode(),Object.equals(Object),Set.equals(Object)
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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:
toStringin classAbstractMap<K,V>- Returns:
- a string representation of this map
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equals
public boolean equals(Object o)
Compares 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.- Specified by:
equalsin interfaceMap<K,V>- Overrides:
equalsin classAbstractMap<K,V>- Parameters:
o- object to be compared for equality with this map- Returns:
trueif the specified object is equal to this map- See Also:
Object.hashCode()
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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 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 interfaceConcurrentMap<K,V>- Specified by:
putIfAbsentin interfaceMap<K,V>- Parameters:
key- key with which the specified value is to be associatedvalue- value to be associated with the specified key- Returns:
- the previous value associated with the specified key,
or
nullif there was no mapping for the key - Throws:
NullPointerException- if the specified key or value is null
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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 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 interfaceConcurrentMap<K,V>- Specified by:
removein interfaceMap<K,V>- Parameters:
key- key with which the specified value is associatedvalue- value expected to be associated with the specified key- Returns:
trueif the value was removed- Throws:
NullPointerException- if the specified key is null
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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 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 interfaceConcurrentMap<K,V>- Specified by:
replacein interfaceMap<K,V>- Parameters:
key- key with which the specified value is associatedoldValue- value expected to be associated with the specified keynewValue- value to be associated with the specified key- Returns:
trueif the value was replaced- Throws:
NullPointerException- if any of the arguments are null
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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 thismap:
except that the action is performed atomically.if (map.containsKey(key)) return map.put(key, value); else return null;- Specified by:
replacein interfaceConcurrentMap<K,V>- Specified by:
replacein interfaceMap<K,V>- Parameters:
key- key with which the specified value is associatedvalue- value to be associated with the specified key- Returns:
- the previous value associated with the specified key,
or
nullif there was no mapping for the key - Throws:
NullPointerException- if the specified key or value is null
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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:
getOrDefaultin interfaceConcurrentMap<K,V>- Specified by:
getOrDefaultin interfaceMap<K,V>- Parameters:
key- the key whose associated value is to be returneddefaultValue- the value to return if this map contains no mapping for the given key- Returns:
- the mapping for the key, if present; else the default value
- Throws:
NullPointerException- if the specified key is null
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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 unlessnull. The entire method invocation is performed atomically, so the function is applied at most once per key. Some attempted update operations on this map by other threads may be blocked while computation is in progress, so the computation should be short and simple, and must not attempt to update any other mappings of this map.- Specified by:
computeIfAbsentin interfaceConcurrentMap<K,V>- Specified by:
computeIfAbsentin interfaceMap<K,V>- Parameters:
key- key with which the specified value is to be associatedmappingFunction- the function to compute a value- Returns:
- the current (existing or computed) value associated with the specified key, or null if the computed value is null
- Throws:
NullPointerException- if the specified key or mappingFunction is nullIllegalStateException- if the computation detectably attempts a recursive update to this map that would otherwise never completeRuntimeException- or Error if the mappingFunction does so, in which case the mapping is left unestablished
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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:
computeIfPresentin interfaceConcurrentMap<K,V>- Specified by:
computeIfPresentin interfaceMap<K,V>- Parameters:
key- key with which a value may be associatedremappingFunction- the function to compute a value- Returns:
- the new value associated with the specified key, or null if none
- Throws:
NullPointerException- if the specified key or remappingFunction is nullIllegalStateException- if the computation detectably attempts a recursive update to this map that would otherwise never completeRuntimeException- or Error if the remappingFunction does so, in which case the mapping is unchanged
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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 (ornullif 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:
computein interfaceConcurrentMap<K,V>- Specified by:
computein interfaceMap<K,V>- Parameters:
key- key with which the specified value is to be associatedremappingFunction- the function to compute a value- Returns:
- the new value associated with the specified key, or null if none
- Throws:
NullPointerException- if the specified key or remappingFunction is nullIllegalStateException- if the computation detectably attempts a recursive update to this map that would otherwise never completeRuntimeException- or Error if the remappingFunction does so, in which case the mapping is unchanged
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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 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 interfaceConcurrentMap<K,V>- Specified by:
mergein interfaceMap<K,V>- Parameters:
key- key with which the specified value is to be associatedvalue- the value to use if absentremappingFunction- the function to recompute a value if present- Returns:
- the new value associated with the specified key, or null if none
- Throws:
NullPointerException- if the specified key or the remappingFunction is nullRuntimeException- or Error if the remappingFunction does so, in which case the mapping is unchanged
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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 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 thevalueargument in this table as determined by theequalsmethod;falseotherwise- Throws:
NullPointerException- if the specified value is null
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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()
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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()
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mappingCount
public long mappingCount()
Returns the number of mappings. This method should be used instead ofMap.size()because a ConcurrentHashMap may contain more mappings than can be represented as an int. The value returned is an estimate; the actual count may differ if there are concurrent insertions or removals.- Returns:
- the number of mappings
- Since:
- 1.8
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newKeySet
public static <K> ConcurrentHashMap.KeySetView<K,Boolean> newKeySet()
Creates 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
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newKeySet
public static <K> ConcurrentHashMap.KeySetView<K,Boolean> newKeySet(int initialCapacity)
Creates 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
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keySet
public ConcurrentHashMap.KeySetView<K,V> keySet(V mappedValue)
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)). 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
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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 parallelaction- the action- Since:
- 1.8
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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 paralleltransformer- a function returning the transformation for an element, or null if there is no transformation (in which case the action is not applied)action- the action- Since:
- 1.8
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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 parallelsearchFunction- 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
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reduce
public <U> U reduce(long parallelismThreshold, BiFunction<? super K,? super V,? extends U> transformer, BiFunction<? super U,? super U,? extends U> reducer)Returns the result of accumulating the given transformation of all (key, value) pairs using the given reducer to combine values, or null if none.- Type Parameters:
U- the return type of the transformer- Parameters:
parallelismThreshold- the (estimated) number of elements needed for this operation to be executed in paralleltransformer- a function returning the transformation for an element, or null if there is no transformation (in which case it is not combined)reducer- a commutative associative combining function- Returns:
- the result of accumulating the given transformation of all (key, value) pairs
- Since:
- 1.8
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reduceToDouble
public double reduceToDouble(long parallelismThreshold, ToDoubleBiFunction<? super K,? super V> transformer, double basis, DoubleBinaryOperator reducer)Returns the result of accumulating the given transformation of all (key, value) pairs using the given reducer to combine values, and the given basis as an identity value.- Parameters:
parallelismThreshold- the (estimated) number of elements needed for this operation to be executed in paralleltransformer- a function returning the transformation for an elementbasis- the identity (initial default value) for the reductionreducer- a commutative associative combining function- Returns:
- the result of accumulating the given transformation of all (key, value) pairs
- Since:
- 1.8
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reduceToLong
public long reduceToLong(long parallelismThreshold, ToLongBiFunction<? super K,? super V> transformer, long basis, LongBinaryOperator reducer)Returns the result of accumulating the given transformation of all (key, value) pairs using the given reducer to combine values, and the given basis as an identity value.- Parameters:
parallelismThreshold- the (estimated) number of elements needed for this operation to be executed in paralleltransformer- a function returning the transformation for an elementbasis- the identity (initial default value) for the reductionreducer- a commutative associative combining function- Returns:
- the result of accumulating the given transformation of all (key, value) pairs
- Since:
- 1.8
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reduceToInt
public int reduceToInt(long parallelismThreshold, ToIntBiFunction<? super K,? super V> transformer, int basis, IntBinaryOperator reducer)Returns the result of accumulating the given transformation of all (key, value) pairs using the given reducer to combine values, and the given basis as an identity value.- Parameters:
parallelismThreshold- the (estimated) number of elements needed for this operation to be executed in paralleltransformer- a function returning the transformation for an elementbasis- the identity (initial default value) for the reductionreducer- a commutative associative combining function- Returns:
- the result of accumulating the given transformation of all (key, value) pairs
- Since:
- 1.8
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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 parallelaction- the action- Since:
- 1.8
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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 paralleltransformer- a function returning the transformation for an element, or null if there is no transformation (in which case the action is not applied)action- the action- Since:
- 1.8
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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 parallelsearchFunction- 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
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reduceKeys
public K reduceKeys(long parallelismThreshold, BiFunction<? super K,? super K,? extends K> reducer)
Returns the result of accumulating all keys using the given reducer to combine values, or null if none.- Parameters:
parallelismThreshold- the (estimated) number of elements needed for this operation to be executed in parallelreducer- a commutative associative combining function- Returns:
- the result of accumulating all keys using the given reducer to combine values, or null if none
- Since:
- 1.8
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reduceKeys
public <U> U reduceKeys(long parallelismThreshold, Function<? super K,? extends U> transformer, BiFunction<? super U,? super U,? extends U> reducer)Returns the result of accumulating the given transformation of all keys using the given reducer to combine values, or null if none.- Type Parameters:
U- the return type of the transformer- Parameters:
parallelismThreshold- the (estimated) number of elements needed for this operation to be executed in paralleltransformer- a function returning the transformation for an element, or null if there is no transformation (in which case it is not combined)reducer- a commutative associative combining function- Returns:
- the result of accumulating the given transformation of all keys
- Since:
- 1.8
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reduceKeysToDouble
public double reduceKeysToDouble(long parallelismThreshold, ToDoubleFunction<? super K> transformer, double basis, DoubleBinaryOperator reducer)Returns the result of accumulating the given transformation of all keys using the given reducer to combine values, and the given basis as an identity value.- Parameters:
parallelismThreshold- the (estimated) number of elements needed for this operation to be executed in paralleltransformer- a function returning the transformation for an elementbasis- the identity (initial default value) for the reductionreducer- a commutative associative combining function- Returns:
- the result of accumulating the given transformation of all keys
- Since:
- 1.8
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reduceKeysToLong
public long reduceKeysToLong(long parallelismThreshold, ToLongFunction<? super K> transformer, long basis, LongBinaryOperator reducer)Returns the result of accumulating the given transformation of all keys using the given reducer to combine values, and the given basis as an identity value.- Parameters:
parallelismThreshold- the (estimated) number of elements needed for this operation to be executed in paralleltransformer- a function returning the transformation for an elementbasis- the identity (initial default value) for the reductionreducer- a commutative associative combining function- Returns:
- the result of accumulating the given transformation of all keys
- Since:
- 1.8
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reduceKeysToInt
public int reduceKeysToInt(long parallelismThreshold, ToIntFunction<? super K> transformer, int basis, IntBinaryOperator reducer)Returns the result of accumulating the given transformation of all keys using the given reducer to combine values, and the given basis as an identity value.- Parameters:
parallelismThreshold- the (estimated) number of elements needed for this operation to be executed in paralleltransformer- a function returning the transformation for an elementbasis- the identity (initial default value) for the reductionreducer- a commutative associative combining function- Returns:
- the result of accumulating the given transformation of all keys
- Since:
- 1.8
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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 parallelaction- the action- Since:
- 1.8
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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 paralleltransformer- a function returning the transformation for an element, or null if there is no transformation (in which case the action is not applied)action- the action- Since:
- 1.8
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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 parallelsearchFunction- 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
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reduceValues
public V reduceValues(long parallelismThreshold, BiFunction<? super V,? super V,? extends V> reducer)
Returns the result of accumulating all values using the given reducer to combine values, or null if none.- Parameters:
parallelismThreshold- the (estimated) number of elements needed for this operation to be executed in parallelreducer- a commutative associative combining function- Returns:
- the result of accumulating all values
- Since:
- 1.8
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reduceValues
public <U> U reduceValues(long parallelismThreshold, Function<? super V,? extends U> transformer, BiFunction<? super U,? super U,? extends U> reducer)Returns the result of accumulating the given transformation of all values using the given reducer to combine values, or null if none.- Type Parameters:
U- the return type of the transformer- Parameters:
parallelismThreshold- the (estimated) number of elements needed for this operation to be executed in paralleltransformer- a function returning the transformation for an element, or null if there is no transformation (in which case it is not combined)reducer- a commutative associative combining function- Returns:
- the result of accumulating the given transformation of all values
- Since:
- 1.8
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reduceValuesToDouble
public double reduceValuesToDouble(long parallelismThreshold, ToDoubleFunction<? super V> transformer, double basis, DoubleBinaryOperator reducer)Returns the result of accumulating the given transformation of all values using the given reducer to combine values, and the given basis as an identity value.- Parameters:
parallelismThreshold- the (estimated) number of elements needed for this operation to be executed in paralleltransformer- a function returning the transformation for an elementbasis- the identity (initial default value) for the reductionreducer- a commutative associative combining function- Returns:
- the result of accumulating the given transformation of all values
- Since:
- 1.8
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reduceValuesToLong
public long reduceValuesToLong(long parallelismThreshold, ToLongFunction<? super V> transformer, long basis, LongBinaryOperator reducer)Returns the result of accumulating the given transformation of all values using the given reducer to combine values, and the given basis as an identity value.- Parameters:
parallelismThreshold- the (estimated) number of elements needed for this operation to be executed in paralleltransformer- a function returning the transformation for an elementbasis- the identity (initial default value) for the reductionreducer- a commutative associative combining function- Returns:
- the result of accumulating the given transformation of all values
- Since:
- 1.8
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reduceValuesToInt
public int reduceValuesToInt(long parallelismThreshold, ToIntFunction<? super V> transformer, int basis, IntBinaryOperator reducer)Returns the result of accumulating the given transformation of all values using the given reducer to combine values, and the given basis as an identity value.- Parameters:
parallelismThreshold- the (estimated) number of elements needed for this operation to be executed in paralleltransformer- a function returning the transformation for an elementbasis- the identity (initial default value) for the reductionreducer- a commutative associative combining function- Returns:
- the result of accumulating the given transformation of all values
- Since:
- 1.8
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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 parallelaction- the action- Since:
- 1.8
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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 paralleltransformer- a function returning the transformation for an element, or null if there is no transformation (in which case the action is not applied)action- the action- Since:
- 1.8
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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 parallelsearchFunction- 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
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reduceEntries
public Map.Entry<K,V> reduceEntries(long parallelismThreshold, BiFunction<Map.Entry<K,V>,Map.Entry<K,V>,? extends Map.Entry<K,V>> reducer)
Returns the result of accumulating all entries using the given reducer to combine values, or null if none.- Parameters:
parallelismThreshold- the (estimated) number of elements needed for this operation to be executed in parallelreducer- a commutative associative combining function- Returns:
- the result of accumulating all entries
- Since:
- 1.8
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reduceEntries
public <U> U reduceEntries(long parallelismThreshold, Function<Map.Entry<K,V>,? extends U> transformer, BiFunction<? super U,? super U,? extends U> reducer)Returns the result of accumulating the given transformation of all entries using the given reducer to combine values, or null if none.- Type Parameters:
U- the return type of the transformer- Parameters:
parallelismThreshold- the (estimated) number of elements needed for this operation to be executed in paralleltransformer- a function returning the transformation for an element, or null if there is no transformation (in which case it is not combined)reducer- a commutative associative combining function- Returns:
- the result of accumulating the given transformation of all entries
- Since:
- 1.8
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reduceEntriesToDouble
public double reduceEntriesToDouble(long parallelismThreshold, ToDoubleFunction<Map.Entry<K,V>> transformer, double basis, DoubleBinaryOperator reducer)Returns the result of accumulating the given transformation of all entries using the given reducer to combine values, and the given basis as an identity value.- Parameters:
parallelismThreshold- the (estimated) number of elements needed for this operation to be executed in paralleltransformer- a function returning the transformation for an elementbasis- the identity (initial default value) for the reductionreducer- a commutative associative combining function- Returns:
- the result of accumulating the given transformation of all entries
- Since:
- 1.8
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reduceEntriesToLong
public long reduceEntriesToLong(long parallelismThreshold, ToLongFunction<Map.Entry<K,V>> transformer, long basis, LongBinaryOperator reducer)Returns the result of accumulating the given transformation of all entries using the given reducer to combine values, and the given basis as an identity value.- Parameters:
parallelismThreshold- the (estimated) number of elements needed for this operation to be executed in paralleltransformer- a function returning the transformation for an elementbasis- the identity (initial default value) for the reductionreducer- a commutative associative combining function- Returns:
- the result of accumulating the given transformation of all entries
- Since:
- 1.8
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reduceEntriesToInt
public int reduceEntriesToInt(long parallelismThreshold, ToIntFunction<Map.Entry<K,V>> transformer, int basis, IntBinaryOperator reducer)Returns the result of accumulating the given transformation of all entries using the given reducer to combine values, and the given basis as an identity value.- Parameters:
parallelismThreshold- the (estimated) number of elements needed for this operation to be executed in paralleltransformer- a function returning the transformation for an elementbasis- the identity (initial default value) for the reductionreducer- a commutative associative combining function- Returns:
- the result of accumulating the given transformation of all entries
- Since:
- 1.8
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