java.util.AbstractMap
java.util.concurrent.ConcurrentHashMap
java.lang.Object
K - the type of keys maintained by this mapV - the type of mapped values
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.
This class implements 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.
| Nested Class Summary |
|---|
| Nested classes inherited from class java.util.Map |
|---|
Map.Entry<K,V> |
| Constructor Summary | |
|---|---|
ConcurrentHashMap()
Constructs a new, empty map with a default initial capacity, load factor, and concurrencyLevel. |
|
ConcurrentHashMap(int initialCapacity)
Constructs a new, empty map with the specified initial capacity, and with default load factor and concurrencyLevel. |
|
ConcurrentHashMap(int initialCapacity,
float loadFactor,
int concurrencyLevel)
Constructs a new, empty map with the specified initial capacity and the specified load factor. |
|
ConcurrentHashMap(Map<A,B> t)
Constructs 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. |
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 |
|---|
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)
initialCapacity - the initial capacity. The implementation
performs internal sizing to accommodate this many elements.loadFactor - the load factor threshold, used to control resizing.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)
initialCapacity - The implementation performs internal
sizing to accommodate this many elements.
IllegalArgumentException - if the initial capacity of
elements is negative.public ConcurrentHashMap()
public ConcurrentHashMap(Map<A,B> t)
| Method Detail |
|---|
public boolean isEmpty()
AbstractMapThis implementation returns size() == 0.
isEmpty in interface MapisEmpty in class AbstractMappublic int size()
AbstractMapThis implementation returns entrySet().size().
size in interface Mapsize in class AbstractMappublic V get(Object key)
get in interface Mapget in class AbstractMapkey - a key in the table.
NullPointerException - if the key is
null.AbstractMap.containsKey(Object)public boolean containsKey(Object key)
containsKey in interface MapcontainsKey in class AbstractMapkey - possible key.
NullPointerException - if the key is
null.public boolean containsValue(Object value)
containsValue in interface MapcontainsValue in class AbstractMapvalue - value whose presence in this map is to be tested.
NullPointerException - if the value is null.public 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 for.
NullPointerException - if the value is null.
public 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 Mapput in class AbstractMapkey - the table key.value - the value.
NullPointerException - if the 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 that the action is performed atomically.
putIfAbsent in interface ConcurrentMapkey - key with which the specified value is to be associated.value - value to be associated with the specified key.
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.public void putAll(Map<? extends K,? extends V> t)
putAll in interface MapputAll in class AbstractMapt - Mappings to be stored in this map.public V remove(Object key)
remove in interface Mapremove in class AbstractMapkey - the key that needs to be removed.
NullPointerException - if the key is
null.
public boolean remove(Object key,
Object value)
if (map.get(key).equals(value)) {
map.remove(key);
return true;
} else return false;
except that the action is performed atomically.
remove in interface ConcurrentMapkey - key with which the specified value is associated.value - value associated with the specified key.
NullPointerException - if the specified key is
null.public void clear()
clear in interface Mapclear in class AbstractMappublic Object clone()
clone in class AbstractMapCloneablepublic Set<K> keySet()
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.
keySet in interface MapkeySet in class AbstractMappublic Collection<V> values()
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.
values in interface Mapvalues in class AbstractMappublic Set<Map.Entry<K,V>> entrySet()
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.
entrySet in interface MapentrySet in class AbstractMappublic Enumeration<K> keys()
keySet()public Enumeration<V> elements()
values()