java.util.concurrent
Class ConcurrentHashMap<K,V>

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

The allowed concurrency among update operations is guided by the optional concurrencyLevel constructor argument (default 16), which is used as a hint for internal sizing. The table is internally partitioned to try to permit the indicated number of concurrent updates without contention. Because placement in hash tables is essentially random, the actual concurrency will vary. Ideally, you should choose a value to accommodate as many threads as will ever concurrently modify the table. Using a significantly higher value than you need can waste space and time, and a significantly lower value can lead to thread contention. But overestimates and underestimates within an order of magnitude do not usually have much noticeable impact. A value of one is appropriate when it is known that only one thread will modify and all others will only read. Also, resizing this or any other kind of hash table is a relatively slow operation, so, when possible, it is a good idea to provide estimates of expected table sizes in constructors.

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.

This class is a member of the Java Collections Framework.

Since:

1.5

See Also:

Serialized Form

Nested Class Summary

Nested classes/interfaces inherited from interface java.util.Map

Map.Entry<K,V>

Constructor Summary

ConcurrentHashMap()
Creates a new, empty map with a default initial capacity, load factor, and concurrencyLevel.

ConcurrentHashMap(int initialCapacity)
Creates a new, empty map with the specified initial capacity, and with default load factor and concurrencyLevel.

ConcurrentHashMap(int initialCapacity, float loadFactor, int concurrencyLevel)
Creates a new, empty map with the specified initial capacity, load factor, and concurrency level.

ConcurrentHashMap(Map<? extends K,? extends V> t)
Creates a new map with the same mappings as the given map.

Method Summary

void	clear() Removes all 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<Map.Entry<K,V>>	entrySet() Returns a collection view of the mappings contained in this map.
V	get(Object key) Returns the value to which the specified key is mapped in this table.
boolean	isEmpty() Returns true if this map contains no key-value mappings.
Enumeration<K>	keys() Returns an enumeration of the keys in this table.
Set<K>	keySet() Returns a set view of the keys contained in this map.
V	put(K key, V value) Maps the specified key to the specified value in this table.
void	putAll(Map<? extends K,? extends V> t) 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.
V	remove(Object key) Removes the key (and its corresponding value) from this table.
boolean	remove(Object key, Object value) Remove entry for key only if currently mapped to given value.
V	replace(K key, V value) Replace entry for key only if currently mapped to some value.
boolean	replace(K key, V oldValue, V newValue) Replace entry for key only if currently mapped to given value.
int	size() Returns the number of key-value mappings in this map.
Collection<V>	values() Returns a collection view of the values contained in this map.

Methods inherited from class java.util.AbstractMap

clone, equals, hashCode, toString

Methods inherited from class java.lang.Object

finalize, getClass, notify, notifyAll, wait, wait, wait

Methods inherited from interface java.util.Map

equals, hashCode

Constructor Detail

ConcurrentHashMap

public ConcurrentHashMap(int initialCapacity,
                         float loadFactor,
                         int concurrencyLevel)

Creates a new, empty map with the specified initial capacity, load factor, and concurrency level.

Parameters:

initialCapacity - the initial capacity. The implementation performs internal sizing to accommodate this many elements.

loadFactor - the load factor threshold, used to control resizing. Resizing may be performed when the average number of elements per bin exceeds this threshold.

concurrencyLevel - the estimated number of concurrently updating threads. The implementation performs internal sizing to try to accommodate this many threads.

Throws:

IllegalArgumentException - if the initial capacity is negative or the load factor or concurrencyLevel are nonpositive.

ConcurrentHashMap

public ConcurrentHashMap(int initialCapacity)

Creates a new, empty map with the specified initial capacity, and with default load factor and concurrencyLevel.

Parameters:

initialCapacity - the initial capacity. The implementation performs internal sizing to accommodate this many elements.

Throws:

IllegalArgumentException - if the initial capacity of elements is negative.

ConcurrentHashMap

public ConcurrentHashMap()

Creates a new, empty map with a default initial capacity, load factor, and concurrencyLevel.

ConcurrentHashMap

public ConcurrentHashMap(Map<? extends K,? extends V> t)

Creates a new map with the same mappings as the given map. The map is created with a capacity of twice the number of mappings in the given map or 11 (whichever is greater), and a default load factor and concurrencyLevel.

Parameters:

t - the map

Method Detail

isEmpty

public boolean isEmpty()

Description copied from class: AbstractMap

Returns true if this map contains no key-value mappings.

This implementation returns size() == 0.

Specified by:

isEmpty in interface Map<K,V>

Overrides:

isEmpty in class AbstractMap<K,V>

Returns:

true if this map contains no key-value mappings.

size

public int size()

Description copied from class: AbstractMap

Returns the number of key-value mappings in this map. If the map contains more than Integer.MAX_VALUE elements, returns Integer.MAX_VALUE.

This implementation returns entrySet().size().

Specified by:

size in interface Map<K,V>

Overrides:

size in class AbstractMap<K,V>

Returns:

the number of key-value mappings in this map.

get

public V get(Object key)

Returns the value to which the specified key is mapped in this table.

Specified by:

get in interface Map<K,V>

Overrides:

get in class AbstractMap<K,V>

Parameters:

key - a key in the table.

Returns:

the value to which the key is mapped in this table; null if the key is not mapped to any value in this table.

Throws:

NullPointerException - if the key is null.

See Also:

AbstractMap.containsKey(Object)

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 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 requires a full internal traversal of the hash table, and so 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 value is null.

contains

public 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(java.lang.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 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 - the table key.

value - the value.

Returns:

the previous value of the specified key in this table, or null if it did not have one.

Throws:

NullPointerException - if the key or value is null.

putIfAbsent

public 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.

Specified by:

putIfAbsent in interface ConcurrentMap<K,V>

Parameters:

key - key with which the specified value is to be associated.

value - value to be associated with the specified key.

Returns:

previous value associated with specified 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> t)

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:

t - Mappings to be stored in this map.

remove

public V remove(Object key)

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

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 value to which the key had been mapped in this table, or null if the key did not have a mapping.

Throws:

NullPointerException - if the key is null.

remove

public boolean remove(Object key,
                      Object value)

Remove entry for key only if currently mapped to given value. Acts as

 
  if (map.get(key).equals(value)) {
     map.remove(key);
     return true;
 } else return false;
 

except that the action is performed atomically.

Specified by:

remove in interface ConcurrentMap<K,V>

Parameters:

key - key with which the specified value is associated.

value - value 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)

Replace entry for key only if currently mapped to given value. Acts as

 
  if (map.get(key).equals(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>

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 the specified key or values are null.

replace

public V replace(K key,
                 V value)

Replace entry for key only if currently mapped to some value. Acts as

 
  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>

Parameters:

key - key with which the specified value is associated.

value - value to be associated with the specified key.

Returns:

previous value associated with specified key, or null if there was no mapping for key.

Throws:

NullPointerException - if the specified key or value is null.

clear

public void clear()

Removes all mappings from this map.

Specified by:

clear in interface Map<K,V>

Overrides:

clear in class AbstractMap<K,V>

keySet

public 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 returned 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.

Specified by:

keySet in interface Map<K,V>

Overrides:

keySet in class AbstractMap<K,V>

Returns:

a set view of the keys contained in this map.

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 returned 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.

Specified by:

values in interface Map<K,V>

Overrides:

values in class AbstractMap<K,V>

Returns:

a collection view of the values contained in this map.

entrySet

public Set<Map.Entry<K,V>> entrySet()

Returns a collection view of the mappings contained in this map. Each element in the returned collection is a Map.Entry. 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 the map, via the Iterator.remove, Collection.remove, removeAll, retainAll, and clear operations. It does not support the add or addAll operations. The view's returned 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.

Specified by:

entrySet in interface Map<K,V>

Specified by:

entrySet in class AbstractMap<K,V>

Returns:

a collection view of the mappings contained in this map.

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