ConcurrentHashMap JDK 1.6)

Description

public class ConcurrentHashMap

A hash table supporting full concurrency of retrievals and adjustable expected concurrency for updates.

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.

See also:

Methods

Hide/Show inherited methods

public void clear () [Overrides AbstractMap]

Removes all of the mappings from this map.

protected Object clone () throws CloneNotSupportedException [Inherited From AbstractMap]

Returns a shallow copy of this AbstractMap instance: the keys and values themselves are not cloned.

public boolean contains (Object value)

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

public boolean containsKey (Object key) [Overrides AbstractMap]

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

public boolean containsValue (Object value) [Overrides AbstractMap]

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

public Enumeration< V> elements ()

Returns an enumeration of the values in this table.

public Set<Entry< K, V>> entrySet () [Specified in AbstractMap]

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

public boolean equals (Object o) [Inherited From AbstractMap]

Compares the specified object with this map for equality.

public V get (Object key) [Overrides AbstractMap]

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.)
Throws:
- NullPointerException - if the specified key is null

public int hashCode () [Inherited From AbstractMap]

Returns the hash code value for this map.

public boolean isEmpty () [Overrides AbstractMap]

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

public Enumeration< K> keys ()

Returns an enumeration of the keys in this table.

public Set< K> keySet () [Overrides AbstractMap]

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

public V put ( K key, V value) [Overrides AbstractMap]

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

public void putAll (Map<Object, Object> m) [Overrides AbstractMap]

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

public V putIfAbsent ( K key, V value) [Specified in ConcurrentMap]

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

public V remove (Object key) [Overrides AbstractMap]

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

public boolean remove (Object key, Object value) [Specified in ConcurrentMap]

Removes the entry for a key only if currently mapped to a given 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.
Throws:
- NullPointerException - if the specified key is null

public V replace ( K key, V value) [Specified in ConcurrentMap]

Replaces the entry for a key only if currently mapped to some 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;

public boolean replace ( K key, V oldValue, V newValue) [Specified in ConcurrentMap]

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

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.
Throws:
- NullPointerException - if any of the arguments are null

pack-privatefinal Segment< K, V> segmentFor (int hash)

Returns the segment that should be used for key with given hash

public int size () [Overrides AbstractMap]

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

public String toString () [Inherited From AbstractMap]

Returns a string representation of this map.

public Collection< V> values () [Overrides AbstractMap]

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