K - the type of keys maintained by this mapV - the type of mapped valuespublic class ConcurrentHashMapV8<K,V> extends Object implements ConcurrentMap<K,V>, Serializable
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.
| Modifier and Type | Class and Description |
|---|---|
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). |
| Constructor and Description |
|---|
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.
|
| Modifier and Type | Method and Description |
|---|---|
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. |
public ConcurrentHashMapV8()
public ConcurrentHashMapV8(int initialCapacity)
initialCapacity - The implementation performs internal
sizing to accommodate this many elements.IllegalArgumentException - if the initial capacity of
elements is negativepublic ConcurrentHashMapV8(Map<? extends K,? extends V> m)
m - the mappublic ConcurrentHashMapV8(int initialCapacity,
float loadFactor)
initialCapacity) and
initial table density (loadFactor).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 sizeIllegalArgumentException - if the initial capacity of
elements is negative or the load factor is nonpositivepublic ConcurrentHashMapV8(int initialCapacity,
float loadFactor,
int concurrencyLevel)
initialCapacity), table
density (loadFactor), and number of concurrently
updating threads (concurrencyLevel).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 sizeconcurrencyLevel - the estimated number of concurrently
updating threads. The implementation may use this value as
a sizing hint.IllegalArgumentException - if the initial capacity is
negative or the load factor or concurrencyLevel are
nonpositivepublic V get(Object key)
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.)
get in interface Map<K,V>NullPointerException - if the specified key is nullpublic boolean containsKey(Object key)
containsKey in interface Map<K,V>key - possible keytrue if and only if the specified object
is a key in this table, as determined by the
equals method; false otherwiseNullPointerException - if the specified key is nullpublic boolean containsValue(Object value)
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.containsValue in interface Map<K,V>value - value whose presence in this map is to be testedtrue if this map maps one or more keys to the
specified valueNullPointerException - if the specified value is nullpublic boolean contains(Object value)
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.value - a value to search fortrue if and only if some key maps to the
value argument in this table as
determined by the equals method;
false otherwiseNullPointerException - if the specified value is nullpublic V put(K key, V value)
The value can be retrieved by calling the get method
with a key that is equal to the original key.
put in interface Map<K,V>key - key with which the specified value is to be associatedvalue - value to be associated with the specified keykey, or
null if there was no mapping for keyNullPointerException - if the specified key or value is nullpublic V putIfAbsent(K key, V value)
putIfAbsent in interface ConcurrentMap<K,V>null if there was no mapping for the keyNullPointerException - if the specified key or value is nullpublic void putAll(Map<? extends K,? extends V> m)
public V computeIfAbsent(K key, ConcurrentHashMapV8.MappingFunction<? super K,? extends V> mappingFunction)
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)); }});key - key with which the specified value is to be associatedmappingFunction - the function to compute a valueNullPointerException - if the specified key, mappingFunction,
or computed value is nullIllegalStateException - if the computation detectably
attempts a recursive update to this map that would
otherwise never completeRuntimeException - or Error if the mappingFunction does so,
in which case the mapping is left unestablishedpublic V compute(K key, ConcurrentHashMapV8.RemappingFunction<? super K,V> remappingFunction)
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;});}key - key with which the specified value is to be associatedremappingFunction - the function to compute a valueNullPointerException - if the specified key or remappingFunction
or computed value is nullIllegalStateException - if the computation detectably
attempts a recursive update to this map that would
otherwise never completeRuntimeException - or Error if the remappingFunction does so,
in which case the mapping is unchangedpublic V remove(Object key)
remove in interface Map<K,V>key - the key that needs to be removedkey, or
null if there was no mapping for keyNullPointerException - if the specified key is nullpublic boolean remove(Object key, Object value)
remove in interface ConcurrentMap<K,V>NullPointerException - if the specified key is nullpublic boolean replace(K key, V oldValue, V newValue)
replace in interface ConcurrentMap<K,V>NullPointerException - if any of the arguments are nullpublic V replace(K key, V value)
replace in interface ConcurrentMap<K,V>null if there was no mapping for the keyNullPointerException - if the specified key or value is nullpublic void clear()
public Set<K> keySet()
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.
public Collection<V> values()
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.
public Set<Map.Entry<K,V>> entrySet()
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.
public Enumeration<K> keys()
keySet()public Enumeration<V> elements()
values()public int hashCode()
Map, i.e.,
the sum of, for each key-value pair in the map,
key.hashCode() ^ value.hashCode().public String toString()
{}"). 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.public boolean equals(Object o)
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.Copyright © 2012 JBoss by Red Hat. All Rights Reserved.