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java.util.concurrent
Class ConcurrentHashMap<K,V>
java.lang.Object
java.util.AbstractMap<K,V>
java.util.concurrent.ConcurrentHashMap<K,V>
-
-
Type Parameters:
-
K - the type of keys maintained by this map
-
V - the type of mapped values
-
All Implemented Interfaces:
-
Serializable
,
Cloneable
,
ConcurrentMap
<K,V>,
Map
<K,V>
-
public class ConcurrentHashMap<K,V>
- extends AbstractMap<K,V>
- implements ConcurrentMap<K,V>, Serializable
Cloneable, Serializable
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
.
-
Since:
-
1.5
-
See Also:
-
Serialized Form
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, load factor, and concurrencyLevel. |
ConcurrentHashMap
(int initialCapacity)
Creates a new, empty map with the specified initial capacity, and with default load factor and concurrencyLevel. |
ConcurrentHashMap
(int initialCapacity, float loadFactor, int concurrencyLevel)
Creates a new, empty map with the specified initial
capacity,
capacity and the specified
load
factor, and concurrency level.
factor.
|
ConcurrentHashMap
(
Map
<? extends
K
,? extends
V
> t)
Creates a new map with the same mappings as the given map. |
Method Summary
|
void |
clear
()
Removes all mappings from this map. |
Object
|
clone
()
Returns a shallow copy of this
ConcurrentHashMap
instance: the keys and values themselves are not cloned.
|
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 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
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 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
> 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. |
V
|
remove
(
Object
key)
Removes the key (and its corresponding value) from this table. |
boolean |
remove
(
Object
key,
Object
value)
Remove 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. |
boolean |
replace
(
K
key,
V
oldValue,
V
newValue)
Replace entry for key only if currently mapped to given value. |
int |
size
()
Returns the number of key-value mappings in this map. |
Collection
<
V
> |
values
()
Returns a collection view of the values contained in this map. |
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
|
ConcurrentHashMap
public ConcurrentHashMap(int initialCapacity,
float loadFactor,
int concurrencyLevel)
-
Creates a new, empty map with the specified initial
capacity,
capacity and the specified
load
factor, and concurrency level.
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.
Resizing may be performed when the average number of elements per bin exceeds this threshold.
-
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.
ConcurrentHashMap
public ConcurrentHashMap(int initialCapacity)
-
Creates a new, empty map with the specified initial capacity, and with default load factor and concurrencyLevel.
-
Parameters:
-
initialCapacity -
the initial capacity.
The implementation performs internal sizing to accommodate this many elements.
-
Throws:
-
IllegalArgumentException
- if the initial capacity of elements is negative.
ConcurrentHashMap
public ConcurrentHashMap()
-
Creates a new, empty map with a default initial capacity, load factor, and concurrencyLevel.
ConcurrentHashMap
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 and concurrencyLevel.
factor.
-
Parameters:
-
t - the map
isEmpty
public boolean isEmpty()
-
Description copied from class:
AbstractMap
-
Returns
true
if this map contains no key-value mappings.
This implementation returns
size() == 0
.
-
-
Specified by:
-
isEmpty
in interface
Map
<
K
,
V
>
-
Overrides:
-
isEmpty
in class
AbstractMap
<
K
,
V
>
-
-
Returns:
-
true
if this map contains no key-value mappings.
size
public int size()
-
Description copied from class:
AbstractMap
-
Returns the number of key-value mappings in this map. If the map contains more than
Integer.MAX_VALUE
elements, returns
Integer.MAX_VALUE
.
This implementation returns
entrySet().size()
.
-
-
Specified by:
-
size
in interface
Map
<
K
,
V
>
-
Overrides:
-
size
in class
AbstractMap
<
K
,
V
>
-
-
Returns:
-
the number of key-value mappings in this map.
get
public V get(Object key)
-
Returns the value to which the specified key is mapped in this table.
-
-
Specified by:
-
get
in interface
Map
<
K
,
V
>
-
Overrides:
-
get
in class
AbstractMap
<
K
,
V
>
-
-
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
.
-
See Also:
-
AbstractMap.containsKey(Object)
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
>
-
Overrides:
-
containsKey
in class
AbstractMap
<
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 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 requires a full internal traversal of the hash table, and so is much slower than method
containsKey
.
-
-
Specified by:
-
containsValue
in interface
Map
<
K
,
V
>
-
Overrides:
-
containsValue
in class
AbstractMap
<
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 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 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
>
-
Overrides:
-
put
in class
AbstractMap
<
K
,
V
>
-
-
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
.
putIfAbsent
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.
-
-
Specified by:
-
putIfAbsent
in interface
ConcurrentMap
<
K
,
V
>
-
-
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
.
putAll
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.
-
-
Specified by:
-
putAll
in interface
Map
<
K
,
V
>
-
Overrides:
-
putAll
in class
AbstractMap
<
K
,
V
>
-
-
Parameters:
-
t - Mappings to be stored in this map.
remove
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.
-
-
Specified by:
-
remove
in interface
Map
<
K
,
V
>
-
Overrides:
-
remove
in class
AbstractMap
<
K
,
V
>
-
-
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
.
remove
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.
-
-
Specified by:
-
remove
in interface
ConcurrentMap
<
K
,
V
>
-
-
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
.
replace
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.
-
-
Specified by:
-
replace
in interface
ConcurrentMap
<
K
,
V
>
-
-
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
.
replace
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.
-
-
Specified by:
-
replace
in interface
ConcurrentMap
<
K
,
V
>
-
-
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
.
clear
public void clear()
-
Removes all mappings from this map.
-
-
Specified by:
-
clear
in interface
Map
<
K
,
V
>
-
Overrides:
-
clear
in class
AbstractMap
<
K
,
V
>
-
clone
public Objectclone ()
-
Returns a shallow copy of this
ConcurrentHashMap
instance: the keys and values themselves are not cloned.
-
-
Overrides:
-
clone
in class
AbstractMap
<
K
,
V
>
-
-
Returns:
-
a shallow copy of this map.
-
See Also:
-
Cloneable
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
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.
-
-
Specified by:
-
keySet
in interface
Map
<
K
,
V
>
-
Overrides:
-
keySet
in class
AbstractMap
<
K
,
V
>
-
-
Returns:
-
a set view of the keys contained in this map.
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
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.
-
-
Specified by:
-
values
in interface
Map
<
K
,
V
>
-
Overrides:
-
values
in class
AbstractMap
<
K
,
V
>
-
-
Returns:
-
a collection view of the values contained in this map.
entrySet
public Set<Map.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
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.
-
-
Specified by:
-
entrySet
in interface
Map
<
K
,
V
>
-
Specified by:
-
entrySet
in class
AbstractMap
<
K
,
V
>
-
-
Returns:
-
a collection view of the mappings contained in this map.
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