This documentation differs from the official API. Jadeite adds extra features to the API including: variable font sizes, constructions examples, placeholders for classes and methods, and auto-generated “See Also” links. Additionally it is missing some items found in standard Javadoc documentation, including: generics type information, “Deprecated” tags and comments, “See Also” links, along with other minor differences. Please send any questions or feedback to bam@cs.cmu.edu.


java.util.concurrent
class ConcurrentHashMap

java.lang.Object extended by java.util.AbstractMap extended by java.util.concurrent.ConcurrentHashMap
All Implemented Interfaces:
Serializable, Map, ConcurrentMap

Most common ways to construct:

ConcurrentHashMap c = new ConcurrentHashMap(5);

Based on 68 examples

 

ConcurrentHashMap empty = new ConcurrentHashMap();

Based on 50 examples


public class ConcurrentHashMap
extends AbstractMap
implements ConcurrentMap, Serializable

A hash table supporting full concurrency of retrievals and adjustable expected concurrency for updates. This class obeys the same functional specification as {@link java.util.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 {@link 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 {@link Map} and {@link Iterator} interfaces.

Like {@link Hashtable} but unlike {@link 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.


Nested Class Summary
 
Nested classes/interfaces inherited from class java.util.AbstractMap
AbstractMap.SimpleEntry, AbstractMap.SimpleImmutableEntry
   
Constructor Summary

          Creates a new, empty map with a default initial capacity (16), load factor (0.75) and concurrencyLevel (16).
ConcurrentHashMap(int initialCapacity)

          Creates a new, empty map with the specified initial capacity, and with default load factor (0.75) and concurrencyLevel (16).
ConcurrentHashMap(int initialCapacity, float loadFactor)

          Creates a new, empty map with the specified initial capacity and load factor and with the default concurrencyLevel (16).
ConcurrentHashMap(int initialCapacity, float loadFactor, int concurrencyLevel)

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

          Creates a new map with the same mappings as the given map.
 
Method Summary
 void

          Removes all of the mappings from this map.
 boolean

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

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

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

          Returns an enumeration of the values in this table.
 Set

          Returns a java.util.Set view of the mappings contained in this map.
 Object
get(Object key)

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

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

          Returns an enumeration of the keys in this table.
 Set

          Returns a java.util.Set view of the keys contained in this map.
 Object
put(Object key, Object value)

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

          Copies all of the mappings from the specified map to this one.
 Object
putIfAbsent(Object key, Object value)

          
 Object

          Removes the key (and its corresponding value) from this map.
 boolean
remove(Object key, Object value)

          
 Object
replace(Object key, Object value)

          
 boolean
replace(Object key, Object oldValue, Object newValue)

          
 int

          Returns the number of key-value mappings in this map.
 Collection

          Returns a java.util.Collection view of the values contained in this map.
 
Methods inherited from class java.util.AbstractMap
clear, clone, containsKey, containsValue, entrySet, equals, get, hashCode, isEmpty, keySet, put, putAll, remove, size, toString, values
 
Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait
 

Constructor Detail

ConcurrentHashMap

public ConcurrentHashMap()
Creates a new, empty map with a default initial capacity (16), load factor (0.75) and concurrencyLevel (16).


ConcurrentHashMap

public ConcurrentHashMap(int initialCapacity)
Creates a new, empty map with the specified initial capacity, and with default load factor (0.75) and concurrencyLevel (16).

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

ConcurrentHashMap

public ConcurrentHashMap(int initialCapacity,
                         float loadFactor)
Creates a new, empty map with the specified initial capacity and load factor and with the default concurrencyLevel (16).

Parameters:
initialCapacity - 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.

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.

ConcurrentHashMap

public ConcurrentHashMap(Map m)
Creates a new map with the same mappings as the given map. The map is created with a capacity of 1.5 times the number of mappings in the given map or 16 (whichever is greater), and a default load factor (0.75) and concurrencyLevel (16).

Parameters:
m - the map
Method Detail

clear

public void clear()
Removes all of the mappings from this map.

Overrides:
clear in class AbstractMap

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 {@link #containsValue}, and exists solely to ensure full compatibility with class {@link java.util.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

containsKey

public boolean containsKey(Object key)
Tests if the specified object is a key in this table.

Overrides:
containsKey in class AbstractMap
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.

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.

Overrides:
containsValue in class AbstractMap
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

elements

public Enumeration elements()
Returns an enumeration of the values in this table.

Returns:
an enumeration of the values in this table

entrySet

public Set entrySet()
Returns a {@link 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. It does not support the add or addAll operations.

The view's iterator is a "weakly consistent" iterator that will never throw {@link 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.

Overrides:
entrySet in class AbstractMap

get

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

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

Overrides:
get in class AbstractMap
Parameters:
key

isEmpty

public boolean isEmpty()
Returns true if this map contains no key-value mappings.

Overrides:
isEmpty in class AbstractMap
Returns:
true if this map contains no key-value mappings

keys

public Enumeration keys()
Returns an enumeration of the keys in this table.

Returns:
an enumeration of the keys in this table

keySet

public Set keySet()
Returns a {@link 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 {@link 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.

Overrides:
keySet in class AbstractMap

put

public Object put(Object key,
                  Object 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.

Overrides:
put in class AbstractMap
Parameters:
key - key with which the specified value is to be associated
value - value to be associated with the specified key
Returns:
the previous value associated with key, or null if there was no mapping for key

putAll

public void putAll(Map 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.

Overrides:
putAll in class AbstractMap
Parameters:
m - mappings to be stored in this map

putIfAbsent

public Object putIfAbsent(Object key,
                          Object value)
{@inheritDoc}

Parameters:
key
value
Returns:
the previous value associated with the specified key, or null if there was no mapping for the key

remove

public Object 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.

Overrides:
remove in class AbstractMap
Parameters:
key - the key that needs to be removed
Returns:
the previous value associated with key, or null if there was no mapping for key

remove

public boolean remove(Object key,
                      Object value)
{@inheritDoc}

Parameters:
key
value

replace

public Object replace(Object key,
                      Object value)
{@inheritDoc}

Parameters:
key
value
Returns:
the previous value associated with the specified key, or null if there was no mapping for the key

replace

public boolean replace(Object key,
                       Object oldValue,
                       Object newValue)
{@inheritDoc}

Parameters:
key
oldValue
newValue

size

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

Overrides:
size in class AbstractMap
Returns:
the number of key-value mappings in this map

values

public Collection values()
Returns a {@link 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 {@link 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.

Overrides:
values in class AbstractMap


This documentation differs from the official API. Jadeite adds extra features to the API including: variable font sizes, constructions examples, placeholders for classes and methods, and auto-generated “See Also” links. Additionally it is missing some items found in standard Javadoc documentation, including: generics type information, “Deprecated” tags and comments, “See Also” links, along with other minor differences. Please send any questions or feedback to bam@cs.cmu.edu.
This page displays the Jadeite version of the documention, which is derived from the offical documentation that contains this copyright notice:
Copyright 2008 Sun Microsystems, Inc. All rights reserved. Use is subject to license terms. Also see the documentation redistribution policy.
The official Sun™ documentation can be found here at http://java.sun.com/javase/6/docs/api/.