jsr166e
Class ConcurrentHashMapV8<K,V>

java.lang.Object
  jsr166e.ConcurrentHashMapV8<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 ConcurrentHashMapV8<K,V>
extends Object
implements ConcurrentMap<K,V>, Serializable

A hash table supporting full concurrency of retrievals and high 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. Bear in mind that the results of aggregate status methods including size, isEmpty, and containsValue are typically useful only when a map is not undergoing concurrent updates in other threads. Otherwise the results of these methods reflect transient states that may be adequate for monitoring or estimation purposes, but not for program control.

The table is dynamically expanded when there are too many collisions (i.e., keys that have distinct hash codes but fall into the same slot modulo the table size), with the expected average effect of maintaining roughly two bins per mapping (corresponding to a 0.75 load factor threshold for resizing). There may be much variance around this average as mappings are added and removed, but overall, this maintains a commonly accepted time/space tradeoff for hash tables. However, resizing this or any other kind of hash table may be a relatively slow operation. When possible, it is a good idea to provide a size estimate as an optional initialCapacity constructor argument. An additional optional loadFactor constructor argument provides a further means of customizing initial table capacity by specifying the table density to be used in calculating the amount of space to allocate for the given number of elements. Also, for compatibility with previous versions of this class, constructors may optionally specify an expected concurrencyLevel as an additional hint for internal sizing. Note that using many keys with exactly the same hashCode() is a sure way to slow down performance of any hash table.

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.

jsr166e note: This class is a candidate replacement for java.util.concurrent.ConcurrentHashMap.

Since:

1.5

Author:

Doug Lea

See Also:

Serialized Form

Nested Class Summary

static interface	ConcurrentHashMapV8.MappingFunction<K,V> A function computing a mapping from the given key to a value.
static interface	ConcurrentHashMapV8.RemappingFunction<K,V> A function computing a new mapping given a key and its current mapped value (or null if there is no current mapping).

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

Map.Entry<K,V>

Constructor Summary

ConcurrentHashMapV8()
Creates a new, empty map with the default initial table size (16),

ConcurrentHashMapV8(int initialCapacity)
Creates a new, empty map with an initial table size accommodating the specified number of elements without the need to dynamically resize.

ConcurrentHashMapV8(int initialCapacity, float loadFactor)
Creates a new, empty map with an initial table size based on the given number of elements (initialCapacity) and initial table density (loadFactor).

ConcurrentHashMapV8(int initialCapacity, float loadFactor, int concurrencyLevel)
Creates a new, empty map with an initial table size based on the given number of elements (initialCapacity), table density (loadFactor), and number of concurrently updating threads (concurrencyLevel).

ConcurrentHashMapV8(Map<? extends K,? extends V> m)
Creates a new map with the same mappings as the given map.

Method Summary

void	clear() Removes all of the mappings from this map.
V	compute(K key, ConcurrentHashMapV8.RemappingFunction<? super K,V> remappingFunction) Computes and enters a new mapping value given a key and its current mapped value (or null if there is no current mapping).
V	computeIfAbsent(K key, ConcurrentHashMapV8.MappingFunction<? super K,? extends V> mappingFunction) If the specified key is not already associated with a value, computes its value using the given mappingFunction and enters it into the 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 Set view of the mappings contained in this map.
boolean	equals(Object o) Compares the specified object with this map for equality.
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.
int	hashCode() Returns the hash code value for this Map, i.e., the sum of, for each key-value pair in the map, key.hashCode() ^ value.hashCode().
boolean	isEmpty()
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> 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()
String	toString() Returns a string representation of this map.
Collection<V>	values() Returns a Collection view of the values contained in this map.

Methods inherited from class java.lang.Object

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

Constructor Detail

ConcurrentHashMapV8

public ConcurrentHashMapV8()

Creates a new, empty map with the default initial table size (16),

ConcurrentHashMapV8

public ConcurrentHashMapV8(int initialCapacity)

Creates a new, empty map with an initial table size accommodating the specified number of elements without the need to dynamically resize.

Parameters:

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

Throws:

IllegalArgumentException - if the initial capacity of elements is negative

ConcurrentHashMapV8

public ConcurrentHashMapV8(Map<? extends K,? extends V> m)

Creates a new map with the same mappings as the given map.

Parameters:

m - the map

ConcurrentHashMapV8

public ConcurrentHashMapV8(int initialCapacity,
                           float loadFactor)

Creates a new, empty map with an initial table size based on the given number of elements (initialCapacity) and initial table density (loadFactor).

Parameters:

initialCapacity - the initial capacity. The implementation performs internal sizing to accommodate this many elements, given the specified load factor.

loadFactor - the load factor (table density) for establishing the initial table size

Throws:

IllegalArgumentException - if the initial capacity of elements is negative or the load factor is nonpositive

Since:

1.6

ConcurrentHashMapV8

public ConcurrentHashMapV8(int initialCapacity,
                           float loadFactor,
                           int concurrencyLevel)

Creates a new, empty map with an initial table size based on the given number of elements (initialCapacity), table density (loadFactor), and number of concurrently updating threads (concurrencyLevel).

Parameters:

initialCapacity - the initial capacity. The implementation performs internal sizing to accommodate this many elements, given the specified load factor.

loadFactor - the load factor (table density) for establishing the initial table size

concurrencyLevel - the estimated number of concurrently updating threads. The implementation may use this value as a sizing hint.

Throws:

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

Method Detail

isEmpty

public boolean isEmpty()

Specified by:

isEmpty in interface Map<K,V>

size

public int size()

Specified by:

size in interface Map<K,V>

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>

Throws:

NullPointerException - if the specified key 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>

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 may require a full traversal of the map, and is much slower than method containsKey.

Specified by:

containsValue in interface Map<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

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 specified 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>

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

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

putAll

public void putAll(Map<? extends K,? extends 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>

Parameters:

m - mappings to be stored in this map

computeIfAbsent

public V computeIfAbsent(K key,
                         ConcurrentHashMapV8.MappingFunction<? super K,? extends V> mappingFunction)

If the specified key is not already associated with a value, computes its value using the given mappingFunction and enters it into the map. This is equivalent to

 if (map.containsKey(key))
   return map.get(key);
 value = mappingFunction.map(key);
 map.put(key, value);
 return value;

except that the action is performed atomically. If the function returns null (in which case a NullPointerException is thrown), or the function itself throws an (unchecked) exception, the exception is rethrown to its caller, and no mapping is recorded. Some attempted update operations on this map by other threads may be blocked while computation is in progress, so the computation should be short and simple, and must not attempt to update any other mappings of this Map. The most appropriate usage is to construct a new object serving as an initial mapped value, or memoized result, as in:

 map.computeIfAbsent(key, new MappingFunction<K, V>() {
   public V map(K k) { return new Value(f(k)); }});

Parameters:

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

mappingFunction - the function to compute a value

Returns:

the current (existing or computed) value associated with the specified key.

Throws:

NullPointerException - if the specified key, mappingFunction, or computed value is null

IllegalStateException - if the computation detectably attempts a recursive update to this map that would otherwise never complete

RuntimeException - or Error if the mappingFunction does so, in which case the mapping is left unestablished

compute

public V compute(K key,
                 ConcurrentHashMapV8.RemappingFunction<? super K,V> remappingFunction)

Computes and enters a new mapping value given a key and its current mapped value (or null if there is no current mapping). This is equivalent to

 map.put(key, remappingFunction.remap(key, map.get(key));
 

except that the action is performed atomically. If the function returns null (in which case a NullPointerException is thrown), or the function itself throws an (unchecked) exception, the exception is rethrown to its caller, and current mapping is left unchanged. Some attempted update operations on this map by other threads may be blocked while computation is in progress, so the computation should be short and simple, and must not attempt to update any other mappings of this Map. For example, to either create or append new messages to a value mapping:

 Map<Key, String> map = ...;
 final String msg = ...;
 map.compute(key, new RemappingFunction<Key, String>() {
   public String remap(Key k, String v) {
    return (v == null) ? msg : v + msg;});}

Parameters:

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

remappingFunction - the function to compute a value

Returns:

the new value associated with the specified key.

Throws:

NullPointerException - if the specified key or remappingFunction or computed value is null

IllegalStateException - if the computation detectably attempts a recursive update to this map that would otherwise never complete

RuntimeException - or Error if the remappingFunction does so, in which case the mapping is unchanged

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>

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 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

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

clear

public void clear()

Removes all of the mappings from this map.

Specified by:

clear in interface Map<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 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>

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 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>

entrySet

public Set<Map.Entry<K,V>> 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.

Specified by:

entrySet in interface Map<K,V>

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()

hashCode

public int hashCode()

Returns the hash code value for this Map, i.e., the sum of, for each key-value pair in the map, key.hashCode() ^ value.hashCode().

Specified by:

hashCode in interface Map<K,V>

Overrides:

hashCode in class Object

Returns:

the hash code value for this map

toString

public String toString()

Returns a string representation of this map. The string representation consists of a list of key-value mappings (in no particular order) enclosed in braces ("{}"). Adjacent mappings are separated by the characters ", " (comma and space). Each key-value mapping is rendered as the key followed by an equals sign ("=") followed by the associated value.

Overrides:

toString in class Object

Returns:

a string representation of this map

equals

public boolean equals(Object o)

Compares the specified object with this map for equality. Returns true if the given object is a map with the same mappings as this map. This operation may return misleading results if either map is concurrently modified during execution of this method.

Specified by:

equals in interface Map<K,V>

Overrides:

equals in class Object

Parameters:

o - object to be compared for equality with this map

Returns:

true if the specified object is equal to this map