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.
|
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. |
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. |
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> m)
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 map.
|
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.
|
Collection<V> |
values()
Returns a
Collection view of the values contained in this map. |
clone, equals, hashCode, toStringpublic 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 boolean isEmpty()
public int size()
public 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>get in class AbstractMap<K,V>key - the key whose associated value is to be returnednull if this map contains no mapping for the keyNullPointerException - if the specified key is nullpublic boolean containsKey(Object key)
containsKey in interface Map<K,V>containsKey in class AbstractMap<K,V>key - possible keyNullPointerException - if the specified key is nullpublic boolean containsValue(Object value)
containsValue in interface Map<K,V>containsValue in class AbstractMap<K,V>value - value whose presence in this map is to be testedNullPointerException - 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 forNullPointerException - 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>put in class AbstractMap<K,V>key - key with which the specified value is to be associatedvalue - value to be associated with the specified keyNullPointerException - if the specified key or value is nullpublic V putIfAbsent(K key, V value)
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 keyNullPointerException - if the specified key or value is nullpublic void putAll(Map<? extends K,? extends V> m)
public V remove(Object key)
remove in interface Map<K,V>remove in class AbstractMap<K,V>key - the key that needs to be removedNullPointerException - if the specified key is nullpublic boolean remove(Object key, Object value)
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 keyNullPointerException - if the specified key is nullpublic boolean replace(K key, V oldValue, V newValue)
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 keyNullPointerException - if any of the arguments are nullpublic V replace(K key, V value)
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 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() Submit a bug or feature
For further API reference and developer documentation, see Java SE Documentation. That documentation contains more detailed, developer-targeted descriptions, with conceptual overviews, definitions of terms, workarounds, and working code examples.
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