JavaTM 2 Platform Std. Ed. v1.5.0
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 mapV - the type of mapped values
- All Implemented Interfaces:
- Serializable, ConcurrentMap<K,V>, Map<K,V>
public class ConcurrentHashMap<K,V> - extends AbstractMap<K,V>
- implements ConcurrentMap<K,V>, 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, load factor, and concurrency level. |
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. |
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. |
ConcurrentHashMap
public ConcurrentHashMap(int initialCapacity,
float loadFactor,
int concurrencyLevel)
- Creates a new, empty map with the specified initial
capacity, load factor, and concurrency level.
- 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.
- 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.
- Throws:
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>
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
Copyright 2003 Sun Microsystems, Inc. All rights reserved
|