| java.util.concurrent.ConcurrentHashMap
| ConcurrentHashMap | | public class ConcurrentHashMap extends AbstractMap implements ConcurrentMap<K, V>,Serializable(Code) | | A hash table supporting full concurrency of retrievals and
adjustable expected concurrency for updates. This class obeys the
same functional specification as
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
author:
Doug Lea<
Parameters:
K - > the type of keys maintained by this map<
Parameters:
V - > the type of mapped values |
| Inner Class :final static class HashEntry | |
| Inner Class :abstract class HashIterator | |
| Inner Class :final class KeyIterator extends HashIterator implements Iterator<K>,Enumeration<K> | |
| Inner Class :final class ValueIterator extends HashIterator implements Iterator<V>,Enumeration<V> | |
| Inner Class :final class EntryIterator extends HashIterator implements Iterator<Entry<K, V>> | |
| Inner Class :final class KeySet extends AbstractSet | |
| Inner Class :final class Values extends AbstractCollection | |
| Inner Class :final class EntrySet extends AbstractSet> | |
| Field Summary | |
| final static int | DEFAULT_CONCURRENCY_LEVEL
The default concurrency level for this table, used when not
otherwise specified in a constructor. | | final static int | DEFAULT_INITIAL_CAPACITY
The default initial capacity for this table,
used when not otherwise specified in a constructor. | | final static float | DEFAULT_LOAD_FACTOR
The default load factor for this table, used when not
otherwise specified in a constructor. | | final static int | MAXIMUM_CAPACITY
The maximum capacity, used if a higher value is implicitly
specified by either of the constructors with arguments. | | final static int | MAX_SEGMENTS
The maximum number of segments to allow; used to bound
constructor arguments. | | final static int | RETRIES_BEFORE_LOCK
Number of unsynchronized retries in size and containsValue
methods before resorting to locking. | | transient Set<Map.Entry<K, V>> | entrySet
| | transient Set<K> | keySet
| | final int | segmentMask
Mask value for indexing into segments. | | final int | segmentShift
Shift value for indexing within segments. | | final Segment<K, V>[] | segments
| | transient Collection<V> | values
|
| Constructor Summary | |
| 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. | | public | ConcurrentHashMap(int initialCapacity, float loadFactor)
Creates a new, empty map with the specified initial capacity
and load factor and with the default concurrencyLevel (16). | | public | ConcurrentHashMap(int initialCapacity)
Creates a new, empty map with the specified initial capacity,
and with default load factor (0.75) and concurrencyLevel (16).
Parameters:
initialCapacity - the initial capacity. | | public | ConcurrentHashMap()
Creates a new, empty map with a default initial capacity (16),
load factor (0.75) and concurrencyLevel (16). | | public | ConcurrentHashMap(Map<? extends K, ? extends V> m)
Creates a new map with the same mappings as the given map. |
| Method Summary | |
| public void | clear()
Removes all of the mappings from this map. | | public boolean | contains(Object value)
Legacy method testing if some key maps into the specified value
in this table. | | public boolean | containsKey(Object key)
Tests if the specified object is a key in this table. | | public boolean | containsValue(Object value)
Returns true if this map maps one or more keys to the
specified value. | | public Enumeration<V> | elements()
Returns an enumeration of the values in this table. | | public Set<Map.Entry<K, V>> | 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. | | public 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.
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 . | | public boolean | isEmpty()
Returns true if this map contains no key-value mappings. | | 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. | | public Enumeration<K> | keys()
Returns an enumeration of the keys in this table. | | public V | put(K key, V value)
Maps the specified key to the specified value in this table. | | public void | putAll(Map<? extends K, ? extends V> m)
Copies all of the mappings from the specified map to this one. | | public V | putIfAbsent(K key, V value)
| | public V | remove(Object key)
Removes the key (and its corresponding value) from this map. | | public boolean | remove(Object key, Object value)
| | public boolean | replace(K key, V oldValue, V newValue)
| | public V | replace(K key, V value)
| | final Segment<K, V> | segmentFor(int hash)
| | public int | size()
Returns the number of key-value mappings in this map. | | 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. |
| DEFAULT_CONCURRENCY_LEVEL | | final static int DEFAULT_CONCURRENCY_LEVEL(Code) | | The default concurrency level for this table, used when not
otherwise specified in a constructor.
|
| DEFAULT_INITIAL_CAPACITY | | final static int DEFAULT_INITIAL_CAPACITY(Code) | | The default initial capacity for this table,
used when not otherwise specified in a constructor.
|
| DEFAULT_LOAD_FACTOR | | final static float DEFAULT_LOAD_FACTOR(Code) | | The default load factor for this table, used when not
otherwise specified in a constructor.
|
| MAXIMUM_CAPACITY | | final static int MAXIMUM_CAPACITY(Code) | | The maximum capacity, used if a higher value is implicitly
specified by either of the constructors with arguments. MUST
be a power of two <= 1<<30 to ensure that entries are indexable
using ints.
|
| MAX_SEGMENTS | | final static int MAX_SEGMENTS(Code) | | The maximum number of segments to allow; used to bound
constructor arguments.
|
| RETRIES_BEFORE_LOCK | | final static int RETRIES_BEFORE_LOCK(Code) | | Number of unsynchronized retries in size and containsValue
methods before resorting to locking. This is used to avoid
unbounded retries if tables undergo continuous modification
which would make it impossible to obtain an accurate result.
|
| segmentMask | | final int segmentMask(Code) | | Mask value for indexing into segments. The upper bits of a
key's hash code are used to choose the segment.
|
| segmentShift | | final int segmentShift(Code) | | Shift value for indexing within segments.
|
| segments | | final Segment<K, V>[] segments(Code) | | The segments, each of which is a specialized hash table
|
| ConcurrentHashMap | | public ConcurrentHashMap(int initialCapacity, float loadFactor, int concurrencyLevel)(Code) | | Creates a new, empty map with the specified initial
capacity, load factor and concurrency level.
Parameters:
initialCapacity - the initial capacity. The implementationperforms internal sizing to accommodate this many elements.
Parameters:
loadFactor - the load factor threshold, used to control resizing.Resizing may be performed when the average number of elements perbin exceeds this threshold.
Parameters:
concurrencyLevel - the estimated number of concurrentlyupdating threads. The implementation performs internal sizingto try to accommodate this many threads.
throws:
IllegalArgumentException - if the initial capacity isnegative or the load factor or concurrencyLevel arenonpositive. |
| ConcurrentHashMap | | public ConcurrentHashMap(int initialCapacity, float loadFactor)(Code) | | Creates a new, empty map with the specified initial capacity
and load factor and with the default concurrencyLevel (16).
Parameters:
initialCapacity - The implementation performs internalsizing to accommodate this many elements.
Parameters:
loadFactor - the load factor threshold, used to control resizing.Resizing may be performed when the average number of elements perbin exceeds this threshold.
throws:
IllegalArgumentException - if the initial capacity ofelements is negative or the load factor is nonpositive
since:
1.6 |
| ConcurrentHashMap | | public ConcurrentHashMap(int initialCapacity)(Code) | | Creates a new, empty map with the specified initial capacity,
and with default load factor (0.75) and concurrencyLevel (16).
Parameters:
initialCapacity - the initial capacity. The implementationperforms internal sizing to accommodate this many elements.
throws:
IllegalArgumentException - if the initial capacity ofelements is negative. |
| ConcurrentHashMap | | public ConcurrentHashMap()(Code) | | Creates a new, empty map with a default initial capacity (16),
load factor (0.75) and concurrencyLevel (16).
|
| ConcurrentHashMap | | public ConcurrentHashMap(Map<? extends K, ? extends V> m)(Code) | | 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).
Parameters:
m - the map |
| clear | | public void clear()(Code) | | Removes all of the mappings from this map.
|
| contains | | public boolean contains(Object value)(Code) | | Legacy method testing if some key maps into the specified value
in this table. This method is identical in functionality to
ConcurrentHashMap.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.
Parameters:
value - a value to search for true if and only if some key maps to thevalue argument in this table asdetermined by the equals method;false otherwise
throws:
NullPointerException - if the specified value is null |
| containsKey | | public boolean containsKey(Object key)(Code) | | Tests if the specified object is a key in this table.
Parameters:
key - possible key true if and only if the specified objectis a key in this table, as determined by theequals method; false otherwise.
throws:
NullPointerException - if the specified key is null |
| containsValue | | public boolean containsValue(Object value)(Code) | | 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.
Parameters:
value - value whose presence in this map is to be tested true if this map maps one or more keys to thespecified value
throws:
NullPointerException - if the specified value is null |
| entrySet | | public Set<Map.Entry<K, V>> entrySet()(Code) | | 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
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.
|
| get | | public V get(Object key)(Code) | | 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
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.)
throws:
NullPointerException - if the specified key is null |
| isEmpty | | public boolean isEmpty()(Code) | | Returns true if this map contains no key-value mappings.
true if this map contains no key-value mappings |
| keySet | | public Set<K> keySet()(Code) | | 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
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.
|
| put | | public V put(K key, V value)(Code) | | 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.
Parameters:
key - key with which the specified value is to be associated
Parameters:
value - value to be associated with the specified key the previous value associated with key, ornull 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> m)(Code) | | 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.
Parameters:
m - mappings to be stored in this map |
| putIfAbsent | | public V putIfAbsent(K key, V value)(Code) | | the previous value associated with the specified key,or null if there was no mapping for the key
throws:
NullPointerException - if the specified key or value is null |
| remove | | public V remove(Object key)(Code) | | Removes the key (and its corresponding value) from this map.
This method does nothing if the key is not in the map.
Parameters:
key - the key that needs to be removed the previous value associated with key, ornull if there was no mapping for key
throws:
NullPointerException - if the specified key is null |
| replace | | public V replace(K key, V value)(Code) | | the previous value associated with the specified key,or null if there was no mapping for the key
throws:
NullPointerException - if the specified key or value is null |
| segmentFor | | final Segment<K, V> segmentFor(int hash)(Code) | | Returns the segment that should be used for key with given hash
Parameters:
hash - the hash code for the key the segment |
| size | | public int size()(Code) | | Returns the number of key-value mappings in this map. If the
map contains more than Integer.MAX_VALUE elements, returns
Integer.MAX_VALUE.
the number of key-value mappings in this map |
| values | | public Collection<V> values()(Code) | | 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
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.
|
|
|