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Added in API level 1

ConcurrentHashMap

public class ConcurrentHashMap
extends AbstractMap<K, V> implements ConcurrentMap<K, V>, Serializable

java.lang.Object
   ↳ java.util.AbstractMap<K, V>
     ↳ java.util.concurrent.ConcurrentHashMap<K, V>


A hash table supporting full concurrency of retrievals and high expected concurrency for updates. This class obeys the same functional specification as Hashtable, and includes versions of methods corresponding to each method of Hashtable. However, even though all operations are 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 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.

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.

Summary

Public constructors

ConcurrentHashMap()

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

ConcurrentHashMap(int initialCapacity)

Creates a new, empty map with an initial table size accommodating the specified number of elements without the need to dynamically resize.

ConcurrentHashMap(Map<? extends K, ? extends V> m)

Creates a new map with the same mappings as the given map.

ConcurrentHashMap(int initialCapacity, float loadFactor)

Creates a new, empty map with an initial table size based on the given number of elements (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).

Public methods

void clear()

Removes all of the mappings from this map.

boolean contains(Object value)

Legacy method testing if some key maps into the specified value in this table.

boolean containsKey(Object key)

Tests if the specified object is a key in this table.

boolean containsValue(Object value)

Returns true if this map maps one or more keys to the specified value.

Enumeration<V> elements()

Returns an enumeration of the values in this table.

Set<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.

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.

int hashCode()

Returns the hash code value for this Map, i.e., the sum of, for each key-value pair in the map, key.hashCode() ^ value.hashCode().

boolean isEmpty()

Returns whether this map is empty.

This implementation compares size() to 0.

Set<K> keySet()

Returns a Set view of the keys contained in this map.

Enumeration<K> keys()

Returns an enumeration of the keys in this table.

V put(K key, V value)

Maps the specified key to the specified value in this table.

void putAll(Map<? extends K, ? extends V> m)

Copies all of the mappings from the specified map to this one.

V putIfAbsent(K key, V value)

If the specified key is not already associated with a value, associate it with the given value.

boolean remove(Object key, Object value)

Removes the entry for a key only if currently mapped to a given value.

V remove(Object key)

Removes the key (and its corresponding value) from this map.

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.

int size()

Returns the number of mappings in this Map.

This implementation returns its entry set's size.

String toString()

Returns a string representation of this map.

Collection<V> values()

Returns a Collection view of the values contained in this map.

Inherited methods

From class java.util.AbstractMap
From class java.lang.Object
From interface java.util.Map
From interface java.util.concurrent.ConcurrentMap

Public constructors

ConcurrentHashMap

Added in API level 1
ConcurrentHashMap ()

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

ConcurrentHashMap

Added in API level 1
ConcurrentHashMap (int initialCapacity)

Creates a new, empty map with an initial table size accommodating the specified number of elements without the need to dynamically resize.

Parameters
initialCapacity int: The implementation performs internal sizing to accommodate this many elements.
Throws
IllegalArgumentException if the initial capacity of elements is negative

ConcurrentHashMap

Added in API level 1
ConcurrentHashMap (Map<? extends K, ? extends V> m)

Creates a new map with the same mappings as the given map.

Parameters
m Map: the map

ConcurrentHashMap

Added in API level 9
ConcurrentHashMap (int initialCapacity, 
                float loadFactor)

Creates a new, empty map with an initial table size based on the given number of elements (initialCapacity) and initial table density (loadFactor).

Parameters
initialCapacity int: the initial capacity. The implementation performs internal sizing to accommodate this many elements, given the specified load factor.
loadFactor float: the load factor (table density) for establishing the initial table size
Throws
IllegalArgumentException if the initial capacity of elements is negative or the load factor is nonpositive

ConcurrentHashMap

Added in API level 1
ConcurrentHashMap (int initialCapacity, 
                float loadFactor, 
                int concurrencyLevel)

Creates a new, empty map with an initial table size based on the given number of elements (initialCapacity), table density (loadFactor), and number of concurrently updating threads (concurrencyLevel).

Parameters
initialCapacity int: the initial capacity. The implementation performs internal sizing to accommodate this many elements, given the specified load factor.
loadFactor float: the load factor (table density) for establishing the initial table size
concurrencyLevel int: the estimated number of concurrently updating threads. The implementation may use this value as a sizing hint.
Throws
IllegalArgumentException if the initial capacity is negative or the load factor or concurrencyLevel are nonpositive

Public methods

clear

Added in API level 1
void clear ()

Removes all of the mappings from this map.

contains

Added in API level 1
boolean contains (Object value)

Legacy method testing if some key maps into the specified value in this table. This method is identical in functionality to containsValue(Object), and exists solely to ensure full compatibility with class Hashtable, which supported this method prior to introduction of the Java Collections framework.

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

containsKey

Added in API level 1
boolean containsKey (Object key)

Tests if the specified object is a key in this table.

Parameters
key Object: possible key
Returns
boolean true if and only if the specified object is a key in this table, as determined by the equals method; false otherwise
Throws
NullPointerException if the specified key is null

containsValue

Added in API level 1
boolean containsValue (Object value)

Returns true if this map maps one or more keys to the specified value. Note: This method may require a full traversal of the map, and is much slower than method containsKey.

Parameters
value Object: value whose presence in this map is to be tested
Returns
boolean true if this map maps one or more keys to the specified value
Throws
NullPointerException if the specified value is null

elements

Added in API level 1
Enumeration<V> elements ()

Returns an enumeration of the values in this table.

Returns
Enumeration<V> an enumeration of the values in this table

See also:

entrySet

Added in API level 1
Set<Entry<K, V>> entrySet ()

Returns a Set view of the mappings contained in this map. The set is backed by the map, so changes to the map are reflected in the set, and 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 iterator is a "weakly consistent" iterator that will never throw ConcurrentModificationException, and guarantees to traverse elements as they existed upon construction of the iterator, and may (but is not guaranteed to) reflect any modifications subsequent to construction.

Returns
Set<Entry<K, V>> the set view

equals

Added in API level 1
boolean equals (Object o)

Compares the specified object with this map for equality. Returns true if the given object is a map with the same mappings as this map. This operation may return misleading results if either map is concurrently modified during execution of this method.

Parameters
o Object: object to be compared for equality with this map
Returns
boolean true if the specified object is equal to this map

get

Added in API level 1
V get (Object key)

Returns the value to which the specified key is mapped, or null if this map contains no mapping for the key.

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

Parameters
key Object: the key.
Returns
V the value of the mapping with the specified key, or null if no mapping for the specified key is found.
Throws
NullPointerException if the specified key is null

hashCode

Added in API level 1
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().

Returns
int the hash code value for this map

isEmpty

Added in API level 1
boolean isEmpty ()

Returns whether this map is empty.

This implementation compares size() to 0.

Returns
boolean true if this map has no elements, false otherwise.

keySet

Added in API level 1
Set<K> keySet ()

Returns a Set view of the keys contained in this map. The set is backed by the map, so changes to the map are reflected in the set, and vice-versa. The set supports element removal, which removes the corresponding mapping from this map, via the Iterator.remove, Set.remove, removeAll, retainAll, and clear operations. It does not support the add or addAll operations.

The view's iterator is a "weakly consistent" iterator that will never throw ConcurrentModificationException, and guarantees to traverse elements as they existed upon construction of the iterator, and may (but is not guaranteed to) reflect any modifications subsequent to construction.

Returns
Set<K> the set view

keys

Added in API level 1
Enumeration<K> keys ()

Returns an enumeration of the keys in this table.

Returns
Enumeration<K> an enumeration of the keys in this table

See also:

put

Added in API level 1
V put (K key, 
                V value)

Maps the specified key to the specified value in this table. Neither the key nor the value can be null.

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

Parameters
key K: key with which the specified value is to be associated
value V: value to be associated with the specified key
Returns
V the previous value associated with key, or null if there was no mapping for key
Throws
NullPointerException if the specified key or value is null

putAll

Added in API level 1
void putAll (Map<? extends K, ? extends V> m)

Copies all of the mappings from the specified map to this one. These mappings replace any mappings that this map had for any of the keys currently in the specified map.

Parameters
m Map: mappings to be stored in this map

putIfAbsent

Added in API level 1
V putIfAbsent (K key, 
                V value)

If the specified key is not already associated with a value, associate it with the given value. This is equivalent to

 if (!map.containsKey(key))
   return map.put(key, value);
 else
   return map.get(key);
except that the action is performed atomically.

Parameters
key K: key with which the specified value is to be associated
value V: value to be associated with the specified key
Returns
V the previous value associated with the specified key, or null if there was no mapping for the key
Throws
NullPointerException if the specified key or value is null

remove

Added in API level 1
boolean remove (Object key, 
                Object value)

Removes the entry for a key only if currently mapped to a given value. This is equivalent to

 if (map.containsKey(key) && map.get(key).equals(value)) {
   map.remove(key);
   return true;
  else
   return false;}
except that the action is performed atomically.

Parameters
key Object: key with which the specified value is associated
value Object: value expected to be associated with the specified key
Returns
boolean true if the value was removed
Throws
NullPointerException if the specified key is null

remove

Added in API level 1
V remove (Object key)

Removes the key (and its corresponding value) from this map. This method does nothing if the key is not in the map.

Parameters
key Object: the key that needs to be removed
Returns
V the previous value associated with key, or null if there was no mapping for key
Throws
NullPointerException if the specified key is null

replace

Added in API level 1
V replace (K key, 
                V value)

Replaces the entry for a key only if currently mapped to some value. This is equivalent to

 if (map.containsKey(key)) {
   return map.put(key, value);
  else
   return null;}
except that the action is performed atomically.

Parameters
key K: key with which the specified value is associated
value V: value to be associated with the specified key
Returns
V the previous value associated with the specified key, or null if there was no mapping for the key
Throws
NullPointerException if the specified key or value is null

replace

Added in API level 1
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

 if (map.containsKey(key) && map.get(key).equals(oldValue)) {
   map.put(key, newValue);
   return true;
  else
   return false;}
except that the action is performed atomically.

Parameters
key K: key with which the specified value is associated
oldValue V: value expected to be associated with the specified key
newValue V: value to be associated with the specified key
Returns
boolean true if the value was replaced
Throws
NullPointerException if any of the arguments are null

size

Added in API level 1
int size ()

Returns the number of mappings in this Map.

This implementation returns its entry set's size.

Returns
int the number of mappings in this Map.

toString

Added in API level 1
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.

Returns
String a string representation of this map

values

Added in API level 1
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 iterator is a "weakly consistent" iterator that will never throw ConcurrentModificationException, and guarantees to traverse elements as they existed upon construction of the iterator, and may (but is not guaranteed to) reflect any modifications subsequent to construction.

Returns
Collection<V> the collection view
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