K - the type of keys maintained by this mapV - the type of mapped valuespublic class ConcurrentHashMap<K,V> extends AbstractMap<K,V> 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.
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.
AbstractMap.SimpleEntry<K,V>, AbstractMap.SimpleImmutableEntry<K,V>| Constructor and Description |
|---|
ConcurrentHashMap()
Creates a new, empty map with a default initial capacity (16),
load factor (0.75) and 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(int initialCapacity,
float loadFactor)
Creates a new, empty map with the specified initial capacity
and load factor and 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> 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 (optional operation).
|
boolean |
containsKey(Object key)
Returns true if this map contains a mapping for the specified
key.
|
boolean |
containsValue(Object value)
Returns true if this map maps one or more keys to the
specified value.
|
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.
|
boolean |
isEmpty()
Returns true if this map contains no key-value mappings.
|
Set<K> |
keySet()
Returns a
Set view of the keys contained in this map. |
V |
put(K key,
V value)
Associates the specified value with the specified key in this map
(optional operation).
|
void |
putAll(Map<? extends K,? extends V> m)
Copies all of the mappings from the specified map to this map
(optional operation).
|
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 mapping for a key from this map if it is present
(optional operation).
|
boolean |
remove(Object key,
Object value)
Removes the entry for a key only if currently mapped to a given value.
|
V |
replace(K key,
V value)
Replaces the entry for a key only if currently mapped to some value.
|
boolean |
replace(K key,
V oldValue,
V newValue)
Replaces the entry for a key only if currently mapped to a given value.
|
int |
size()
Returns the number of key-value mappings in this map.
|
String |
toString()
Returns a string representation of this map.
|
Collection<V> |
values()
Returns a
Collection view of the values contained in this map. |
clonepublic ConcurrentHashMap(int initialCapacity,
float loadFactor,
int concurrencyLevel)
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.IllegalArgumentException - if the initial capacity is
negative or the load factor or concurrencyLevel are
nonpositive.public ConcurrentHashMap(int initialCapacity,
float loadFactor)
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.IllegalArgumentException - if the initial capacity of
elements is negative or the load factor is nonpositivepublic ConcurrentHashMap(int initialCapacity)
initialCapacity - the initial capacity. The implementation
performs internal sizing to accommodate this many elements.IllegalArgumentException - if the initial capacity of
elements is negative.public ConcurrentHashMap()
public ConcurrentHashMap(Map<? extends K,? extends V> m)
m - the mappublic int size()
AbstractMapThis implementation returns entrySet().size().
public boolean isEmpty()
AbstractMapThis implementation returns size() == 0.
public boolean containsKey(Object key)
AbstractMapThis implementation iterates over entrySet() searching for an entry with the specified key. If such an entry is found, true is returned. If the iteration terminates without finding such an entry, false is returned. Note that this implementation requires linear time in the size of the map; many implementations will override this method.
containsKey in interface Map<K,V>containsKey in class AbstractMap<K,V>key - key whose presence in this map is to be testedpublic boolean containsValue(Object value)
AbstractMapThis implementation iterates over entrySet() searching for an entry with the specified value. If such an entry is found, true is returned. If the iteration terminates without finding such an entry, false is returned. Note that this implementation requires linear time in the size of the map.
containsValue in interface Map<K,V>containsValue in class AbstractMap<K,V>value - value whose presence in this map is to be testedpublic V get(Object key)
AbstractMapnull 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==null ? k==null :
key.equals(k)), then this method returns v; otherwise
it returns null. (There can be at most one such mapping.)
If this map permits null values, then a return value of
null does not necessarily indicate that the map
contains no mapping for the key; it's also possible that the map
explicitly maps the key to null. The containsKey operation may be used to distinguish these two cases.
This implementation iterates over entrySet() searching for an entry with the specified key. If such an entry is found, the entry's value is returned. If the iteration terminates without finding such an entry, null is returned. Note that this implementation requires linear time in the size of the map; many implementations will override this method.
public V put(K key, V value)
AbstractMapm.containsKey(k) would return
true.)
This implementation always throws an UnsupportedOperationException.
put in interface Map<K,V>put in class AbstractMap<K,V>key - key with which the specified value is to be associatedvalue - value to be associated with the specified keypublic V remove(Object key)
AbstractMap(key==null ? k==null : key.equals(k)), that mapping
is removed. (The map can contain at most one such mapping.)
Returns the value to which this map previously associated the key, or null if the map contained no mapping for the key.
If this map permits null values, then a return value of null does not necessarily indicate that the map contained no mapping for the key; it's also possible that the map explicitly mapped the key to null.
The map will not contain a mapping for the specified key once the call returns.
This implementation iterates over entrySet() searching for an entry with the specified key. If such an entry is found, its value is obtained with its getValue operation, the entry is removed from the collection (and the backing map) with the iterator's remove operation, and the saved value is returned. If the iteration terminates without finding such an entry, null is returned. Note that this implementation requires linear time in the size of the map; many implementations will override this method.
Note that this implementation throws an UnsupportedOperationException if the entrySet iterator does not support the remove method and this map contains a mapping for the specified key.
public void putAll(Map<? extends K,? extends V> m)
AbstractMapput(k, v) on this map once
for each mapping from key k to value v in the
specified map. The behavior of this operation is undefined if the
specified map is modified while the operation is in progress.
This implementation iterates over the specified map's entrySet() collection, and calls this map's put operation once for each entry returned by the iteration.
Note that this implementation throws an UnsupportedOperationException if this map does not support the put operation and the specified map is nonempty.
public void clear()
AbstractMapThis implementation calls entrySet().clear().
Note that this implementation throws an UnsupportedOperationException if the entrySet does not support the clear operation.
public Set<K> keySet()
AbstractMapSet 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. If the map is modified
while an iteration over the set is in progress (except through
the iterator's own remove operation), the results of
the iteration are undefined. 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.
This implementation returns a set that subclasses AbstractSet.
The subclass's iterator method returns a "wrapper object" over this
map's entrySet() iterator. The size method
delegates to this map's size method and the
contains method delegates to this map's
containsKey method.
The set is created the first time this method is called, and returned in response to all subsequent calls. No synchronization is performed, so there is a slight chance that multiple calls to this method will not all return the same set.
public Collection<V> values()
AbstractMapCollection 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. If the map is
modified while an iteration over the collection is in progress
(except through the iterator's own remove operation),
the results of the iteration are undefined. 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.
This implementation returns a collection that subclasses AbstractCollection. The subclass's iterator method returns a
"wrapper object" over this map's entrySet() iterator.
The size method delegates to this map's size
method and the contains method delegates to this map's
containsValue method.
The collection is created the first time this method is called, and returned in response to all subsequent calls. No synchronization is performed, so there is a slight chance that multiple calls to this method will not all return the same collection.
public Set<Map.Entry<K,V>> entrySet()
MapSet 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. If the map is modified
while an iteration over the set is in progress (except through
the iterator's own remove operation, or through the
setValue operation on a map entry returned by the
iterator) the results of the iteration are undefined. 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.public boolean equals(Object o)
AbstractMapThis implementation first checks if the specified object is this map; if so it returns true. Then, it checks if the specified object is a map whose size is identical to the size of this map; if not, it returns false. If so, it iterates over this map's entrySet collection, and checks that the specified map contains each mapping that this map contains. If the specified map fails to contain such a mapping, false is returned. If the iteration completes, true is returned.
equals in interface Map<K,V>equals in class AbstractMap<K,V>o - object to be compared for equality with this mapObject.hashCode(),
HashMappublic int hashCode()
AbstractMapObject.hashCode().
This implementation iterates over entrySet(), calling
hashCode() on each element (entry) in the
set, and adding up the results.
hashCode in interface Map<K,V>hashCode in class AbstractMap<K,V>Map.Entry.hashCode(),
Object.equals(Object),
Set.equals(Object)public String toString()
AbstractMapString.valueOf(Object).toString in class AbstractMap<K,V>public V putIfAbsent(K key, V value)
ConcurrentMap
if (!map.containsKey(key))
return map.put(key, value);
else
return map.get(key);
except that the action is performed atomically.putIfAbsent in interface ConcurrentMap<K,V>key - key with which the specified value is to be associatedvalue - value to be associated with the specified keypublic boolean remove(Object key, Object value)
ConcurrentMap
if (map.containsKey(key) && map.get(key).equals(value)) {
map.remove(key);
return true;
} else return false;
except that the action is performed atomically.remove in interface ConcurrentMap<K,V>key - key with which the specified value is associatedvalue - value expected to be associated with the specified keypublic boolean replace(K key, V oldValue, V newValue)
ConcurrentMap
if (map.containsKey(key) && map.get(key).equals(oldValue)) {
map.put(key, newValue);
return true;
} else return false;
except that the action is performed atomically.replace in interface ConcurrentMap<K,V>key - key with which the specified value is associatedoldValue - value expected to be associated with the specified keynewValue - value to be associated with the specified keypublic V replace(K key, V value)
ConcurrentMap
if (map.containsKey(key)) {
return map.put(key, value);
} else return null;
except that the action is performed atomically.replace in interface ConcurrentMap<K,V>key - key with which the specified value is associatedvalue - value to be associated with the specified keyCopyright © 2015 API Design. All Rights Reserved.