ConcurrentHashMap<K, V>
Java SE 6
| public class java.util.concurrent ConcurrentHashMap<K, V>
|
Java SE 6 |
java.util.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 |
| K | the type of keys maintained by this map |
| V | the type of mapped values |
| Constructors | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| public | ConcurrentHashMap(int initialCapacity, float loadFactor, int concurrencyLevel) Details
Creates a new, empty map with the specified initial
capacity, load factor and concurrency level.
| ||||||||
| public | ConcurrentHashMap(int initialCapacity, float loadFactor) Details
Creates a new, empty map with the specified initial capacity
and load factor and with the default concurrencyLevel (16).
| ||||||||
| public | ConcurrentHashMap(int initialCapacity) Details
Creates a new, empty map with the specified initial capacity,
and with default load factor (0.75) and concurrencyLevel (16).
| ||||||||
| public | ConcurrentHashMap() Creates a new, empty map with a default initial capacity (16), load factor (0.75) and concurrencyLevel (16). | ||||||||
| public | ConcurrentHashMap(Map m) Details
Creates a new map with the same mappings as the given map.
The map is created with a capacity of 1.5 times the number
of mappings in the given map or 16 (whichever is greater),
and a default load factor (0.75) and concurrencyLevel (16).
| ||||||||
| Methods | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| public void | clear() Removes all of the mappings from this map. | ||||||||
| public boolean | contains(Object value) Details
Legacy method testing if some key maps into the specified value
in this table. This method is identical in functionality to
#containsValue, and exists solely to ensure
full compatibility with class java.util.Hashtable,
which supported this method prior to introduction of the
Java Collections framework.
| ||||||||
| public boolean | containsKey(Object key) Details
Tests if the specified object is a key in this table.
| ||||||||
| public boolean | containsValue(Object value) Details
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.
| ||||||||
| public Enumeration | elements() Details
Returns an enumeration of the values in this table.
| ||||||||
| public Set | entrySet() Returns a 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 | ||||||||
| public Object | get(Object key) Details
Returns the value to which the specified key is mapped,
or null if this map contains no mapping for the key.
More formally, if this map contains a mapping from a key
| ||||||||
| public Enumeration | keys() Details
Returns an enumeration of the keys in this table.
| ||||||||
| public Set | 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 iterator is a "weakly consistent" iterator
that will never throw | ||||||||
| public Object | put(Object key, Object value) Details
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.
| ||||||||
| public void | putAll(Map m) Details
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.
| ||||||||
| public Object | putIfAbsent(Object key, Object value) Details
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.
| ||||||||
| public Object | remove(Object key) Details
Removes the key (and its corresponding value) from this map.
This method does nothing if the key is not in the map.
| ||||||||
| public boolean | remove(Object key, Object value) Details
Removes the entry for a key only if currently mapped to a given value.
This is equivalent to
if (map.containsKey(key) && map.get(key).equals(value)) {
map.remove(key);
return true;
} else return false;
except that the action is performed atomically.
| ||||||||
| public boolean | replace(Object key, Object oldValue, Object newValue) Details
Replaces the entry for a key only if currently mapped to a given value.
This is equivalent to
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.
| ||||||||
| public Object | replace(Object key, Object value) Details
Replaces the entry for a key only if currently mapped to some value.
This is equivalent to
if (map.containsKey(key)) {
return map.put(key, value);
} else return null;
except that the action is performed atomically.
| ||||||||
| public int | size() Details
Returns the number of key-value mappings in this map. If the
map contains more than Integer.MAX_VALUE elements, returns
Integer.MAX_VALUE.
| ||||||||
| public Collection | 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 iterator is a "weakly consistent" iterator
that will never throw | ||||||||
| Properties | |||
|---|---|---|---|
| public boolean | isEmpty() Details
Returns true if this map contains no key-value mappings.
| ||
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