public class

ConcurrentHashMap

extends AbstractMap<K, V>
implements Serializable ConcurrentMap<K, V>
java.lang.Object
   ↳ java.util.AbstractMap<K, V>
     ↳ java.util.concurrent.ConcurrentHashMap<K, V>

Class Overview

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.

This class implements 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(int initialCapacity, float loadFactor, int concurrencyLevel)
Creates a new, empty map with the specified initial capacity and the specified load factor.
ConcurrentHashMap(int initialCapacity)
Creates a new, empty map with the specified initial capacity, and with default load factor and concurrencyLevel.
ConcurrentHashMap()
Creates a new, empty map with a default initial capacity, load factor, and concurrencyLevel.
ConcurrentHashMap(Map<? extends K, ? extends V> t)
Creates a new map with the same mappings as the given map.
Public Methods
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<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 whether this map is empty.
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> 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.
boolean remove(Object key, Object value)
Remove entry for key only if currently mapped to given value.
V remove(Object key)
Removes the key (and its corresponding value) from this table.
boolean replace(K key, V oldValue, V newValue)
Replace 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.
int size()
Returns the number of elements in this map.
Collection<V> values()
Returns a collection view of the values contained in this map.
[Expand]
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

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

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

Parameters
initialCapacity the initial capacity. The implementation performs internal sizing to accommodate this many elements.
loadFactor the load factor threshold, used to control resizing.
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.

public ConcurrentHashMap (int initialCapacity)

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

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

public ConcurrentHashMap ()

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

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.

Parameters
t the map

Public Methods

public void clear ()

Removes all mappings from this map.

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 value is null.

public boolean containsKey (Object key)

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

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.

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.

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.

public Enumeration<V> elements ()

Returns an enumeration of the values in this table.

Returns
  • an enumeration of the values in this table.
See Also

public Set<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 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.

Returns
  • a collection view of the mappings contained in this map.

public V get (Object key)

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

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.

public boolean isEmpty ()

Returns whether this map is empty.

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

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 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.

Returns
  • a set view of the keys contained in this map.

public Enumeration<K> keys ()

Returns an enumeration of the keys in this table.

Returns
  • an enumeration of the keys in this table.
See Also

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.

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.

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.

Parameters
t Mappings to be stored in this map.

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.

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. A null return can also indicate that the map previously associated null with the specified key, if the implementation supports null values.
Throws
UnsupportedOperationException if the put operation is not supported by this map.
ClassCastException if the class of the specified key or value prevents it from being stored in this map.
NullPointerException if the specified key or value is null.

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.

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.

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.

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.

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.

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.

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.

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.

public int size ()

Returns the number of elements in this map.

Returns
  • the number of elements in this map.

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 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.

Returns
  • a collection view of the values contained in this map.