java.util.AbstractMap<K,V>
java.util.concurrent.ConcurrentHashMap<K,V>
is new.
java.lang.Object
public class ConcurrentHashMap<K,V>
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
),
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
NOT
allow
null
to be used as a key or value.
This class is a member of the Java Collections Framework .
| Nested Class Summary |
|---|
|
Nested classes/interfaces inherited from class java.util.
AbstractMap
|
|---|
|
AbstractMap.SimpleEntry
<
K
,
V
>,
AbstractMap.SimpleImmutableEntry
<
K
,
V
|
| Nested classes/interfaces inherited from interface java.util. Map |
|---|
| Map.Entry < K , V > |
| Constructor Summary | |
|---|---|
|
ConcurrentHashMap
() Creates a new, empty map with a default initial
capacity (16),
factor (0.75)
concurrencyLevel (16).
|
|
|
ConcurrentHashMap
(int initialCapacity) Creates a new, empty map with the specified initial capacity, and with default load factor
(0.75)
and
concurrencyLevel (16).
|
|
|
ConcurrentHashMap
float loadFactor)
Creates a new, empty map with the specified initial
capacity and
factor
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.
|
|
|
|
|
|
ConcurrentHashMap
(
Map
<? extends
K
,? extends
V
Creates a new map with the same mappings as the given map.
|
|
| 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 < V > |
elements
() Returns an enumeration of the values in this table. |
| Set < Map.Entry < K , V |
entrySet
() Returns a
Set
|
| V |
get
(
Object
Returns the value to which the specified key is
mapped, or null if
map contains no mapping for the key.
|
| boolean |
isEmpty
() Returns true if this map contains no key-value mappings. |
| Enumeration < K > |
keys
() Returns an enumeration of the keys in this table. |
| Set < K |
keySet
() Returns a
Set
|
| V |
put
(
K
key,
V
Maps the specified key to the specified value in this table. |
| void |
putAll
(
Map
<? extends
K
,? extends
V
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. |
| V |
remove
(
Object
Removes the key (and its corresponding value) from this
map.
|
| boolean |
remove
(
Object
key,
Object
Removes the
a
key only if currently mapped to
a
given value. |
| V |
replace
(
K
key,
V
Replaces the
a
key only if currently mapped to some value. |
| boolean |
replace
(
K
key,
V
oldValue,
V
Replaces the
a
key only if currently mapped to
a
given value. |
| int |
size
() Returns the number of key-value mappings in this map. |
| Collection < V |
values
() Returns a
Collection
|
| Methods inherited from class java.util. AbstractMap |
|---|
| clone , equals , hashCode , toString |
| Methods inherited from class java.lang. Object |
|---|
| finalize , getClass , notify , notifyAll , wait , wait , wait |
| Methods inherited from interface java.util. Map |
|---|
| equals , hashCode |
| Constructor Detail |
|---|
public ConcurrentHashMap(int initialCapacity,
float loadFactor,
int concurrencyLevel)
factor
ConcurrentHashMap
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
public ConcurrentHashMap(int initialCapacity)
(0.75)
and
concurrencyLevel (16).
public ConcurrentHashMap()
capacity (16),
factor (0.75)
concurrencyLevel (16).
public ConcurrentHashMap(Map<? extends K,? extends V> t)
1.5 times
16
(0.75)
and
concurrencyLevel (16).
m
| Method Detail |
|---|
public boolean isEmpty()
This implementation returns
size() == 0
.
mappings
public int size()
This implementation returns
entrySet().size()
.
map
public V get(Object key)
mapped, or null if
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.)
key - the key whose associated value is to be returned
the value to which the specified key is mapped, or null if this map contains no mapping for the key
specified
key is
null
public boolean containsKey(Object key)
key
specified
key is
null
public boolean containsValue(Object value)
tested
value
specified
value is
null
public boolean contains(Object value)
for
otherwise
specified
value is
null
public V put(K key,
V value)
null.
The value can be retrieved by calling the get method with a key that is equal to the original key.
key - key with which the specified value is to be associated
value - value to be associated with the specified key
the previous value associated with
key
, or
null
if there was no mapping for
key
specified
key or value is
null
public V putIfAbsent(K key,
V value)
if (!map.containsKey(key)) return map.put(key, value); else return map.get(key);
except
associated
key
the
previous value associated with
the
specified key, or
null
if there was no mapping for
the key
public void putAll(Map<? extends K,? extends V> t)
map.
m
mappings
map
public V remove(Object key)
map.
map.
removed
the previous value associated with
key
, or
null
if there was no mapping for
key
specified
key is
null
public boolean remove(Object key,
Object value)
Removes the
a
key only if currently mapped to
a
given value.
This is equivalent to
ifexcept that the action is performed atomically.(map.get(key).equals(value)){ map.remove(key); return true; } else return false;
associated
expected to be
associated with the specified
key
public boolean replace(K key,
V oldValue,
V newValue)
Replaces the
a
key only if currently mapped to
a
given value.
This is equivalent to
ifexcept that the action is performed atomically.(map.get(key).equals(oldValue)){ map.put(key, newValue); return true; } else return false;
associated
key
key
any of
the
arguments
public V replace(K key,
V value)
Replaces the
a
key only if currently mapped to some value.
This is equivalent to
ifexcept that the action is performed atomically.((map.containsKey(key)){ return map.put(key, value); } else return null;
associated
key
the
previous value associated with
the
specified key, or
null
if there was no mapping for
the key
public void clear()
of the
mappings from this map.
public Set<K> keySet()
Set
The view's
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.
map
public Collection<V> values()
Collection
The view's
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.
map
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
Set.remove
The view's
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.
set
map
public Enumeration<K> keys()
table
public Enumeration<V> elements()
table