java.util.concurrent

Class ConcurrentHashMap<K,V>

Implemented Interfaces:
ConcurrentMap<K,V>, Map<K,V>, Serializable

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 ../../../../technotes/guides/collections/index.html"> Java Collections Framework.

Since:
1.5
See Also:
Serialized Form

Nested Class Summary

Nested classes/interfaces inherited from class java.util.AbstractMap<K,V>

AbstractMap.SimpleEntry, AbstractMap.SimpleImmutableEntry

Constructor Summary

ConcurrentHashMap()
Creates a new, empty map with a default initial capacity (16), load factor (0.75) and concurrencyLevel (16).
ConcurrentHashMap(extends K, V> m)
Creates a new map with the same mappings as the given map.
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.

Method Summary

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
elements()
Returns an enumeration of the values in this table.
Set>
entrySet()
Returns a Set view of the mappings contained in this map.
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.
boolean
isEmpty()
Returns true if this map contains no key-value mappings.
Set
keySet()
Returns a Set view of the keys contained in this map.
Enumeration
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(extends K, V> m)
Copies all of the mappings from the specified map to this one.
V
putIfAbsent(K key, V value)
V
remove(Object key)
Removes the key (and its corresponding value) from this map.
boolean
remove(Object key, Object value)
V
replace(K key, V value)
boolean
replace(K key, V oldValue, V newValue)
int
size()
Returns the number of key-value mappings in this map.
Collection
values()
Returns a Collection view of the values contained in this map.

Methods inherited from class java.util.AbstractMap<K,V>

V>> entrySet, clear, clone, containsKey, containsValue, equals, get, hashCode, isEmpty, keySet, put, putAll, remove, size, toString, values

Methods inherited from class java.lang.Object

clone, equals, extends Object> getClass, finalize, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Details

ConcurrentHashMap

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

ConcurrentHashMap

public ConcurrentHashMap(extends K,
                         V> 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

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.
Throws:
IllegalArgumentException - if the initial capacity of elements is negative.

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.
Throws:
IllegalArgumentException - if the initial capacity of elements is negative or the load factor is nonpositive
Since:
1.6

ConcurrentHashMap

public ConcurrentHashMap(int initialCapacity,
                         float loadFactor,
                         int concurrencyLevel)
Creates a new, empty map with 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.

Method Details

clear

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

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(Object), and exists solely to ensure full compatibility with class Hashtable, which supported this method prior to introduction of the Java Collections framework.
Parameters:
value - a value to search for
Returns:
true if and only if some key maps to the value argument in this table as determined by the equals method; false otherwise
Throws:
NullPointerException - if the specified value is null

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 interface AbstractMap<K,V>
Parameters:
key - possible key
Returns:
true if and only if the specified object is a key in this table, as determined by the equals method; false otherwise.
Throws:
NullPointerException - if the specified key is null

containsValue

public boolean containsValue(Object value)
Returns true if this map maps one or more keys to the specified value. Note: This method 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 interface AbstractMap<K,V>
Parameters:
value - value whose presence in this map is to be tested
Returns:
true if this map maps one or more keys to the specified value
Throws:
NullPointerException - if the specified value is null

elements

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

entrySet

public Set> 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. 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.


get

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

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

Specified by:
get in interface Map<K,V>
Overrides:
get in interface AbstractMap<K,V>
Throws:
NullPointerException - if the specified key is null

isEmpty

public boolean isEmpty()
Returns true if this map contains no key-value mappings.
Specified by:
isEmpty in interface Map<K,V>
Overrides:
isEmpty in interface AbstractMap<K,V>
Returns:
true if this map contains no key-value mappings

keySet

public Set 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.

Specified by:
keySet in interface Map<K,V>
Overrides:
keySet in interface AbstractMap<K,V>

keys

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

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

putAll

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

putIfAbsent

public V putIfAbsent(K key,
                     V value)
Specified by:
putIfAbsent in interface ConcurrentMap<K,V>
Returns:
the previous value associated with the specified key, or null if there was no mapping for the key
Throws:
NullPointerException - if the specified key or value is null

remove

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

remove

public boolean remove(Object key,
                      Object value)
Specified by:
remove in interface ConcurrentMap<K,V>
Throws:
NullPointerException - if the specified key is null

replace

public V replace(K key,
                 V value)
Specified by:
replace in interface ConcurrentMap<K,V>
Returns:
the previous value associated with the specified key, or null if there was no mapping for the key
Throws:
NullPointerException - if the specified key or value is null

replace

public boolean replace(K key,
                       V oldValue,
                       V newValue)
Specified by:
replace in interface ConcurrentMap<K,V>
Throws:
NullPointerException - if any of the arguments are null

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.
Specified by:
size in interface Map<K,V>
Overrides:
size in interface AbstractMap<K,V>
Returns:
the number of key-value mappings in this map

values

public Collection 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.

Specified by:
values in interface Map<K,V>
Overrides:
values in interface AbstractMap<K,V>