- java.lang.Object
-
- java.util.AbstractMap<K,V>
-
- java.util.concurrent.ConcurrentHashMap<K,V>
-
- Type Parameters:
K
- the type of keys maintained by this mapV
- the type of mapped values
- All Implemented Interfaces:
- Serializable, ConcurrentMap<K,V>, Map<K,V>
public class ConcurrentHashMap<K,V> extends AbstractMap<K,V> implements ConcurrentMap<K,V>, Serializable
A hash table supporting full concurrency of retrievals and adjustable expected concurrency for updates. This class obeys the same functional specification asHashtable
, 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
andIterator
interfaces.Like
Hashtable
but unlikeHashMap
, 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
- See Also:
- Serialized Form
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Nested Class Summary
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Nested classes/interfaces inherited from class java.util.AbstractMap
AbstractMap.SimpleEntry<K,V>, AbstractMap.SimpleImmutableEntry<K,V>
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Constructor Summary
Constructors 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.
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Method Summary
Methods 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 aSet
view of the mappings contained in this map.V
get(Object key)
Returns the value to which the specified key is mapped, ornull
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 aSet
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 aCollection
view of the values contained in this map.
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Constructor Detail
-
ConcurrentHashMap
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. 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.- Throws:
IllegalArgumentException
- if the initial capacity is negative or the load factor or concurrencyLevel are nonpositive.
-
ConcurrentHashMap
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).- Parameters:
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.- Throws:
IllegalArgumentException
- if the initial capacity of elements is negative or the load factor is nonpositive- Since:
- 1.6
-
ConcurrentHashMap
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. The implementation performs internal sizing to accommodate this many elements.- Throws:
IllegalArgumentException
- if the initial capacity of elements is negative.
-
ConcurrentHashMap
public ConcurrentHashMap()
Creates a new, empty map with a default initial capacity (16), load factor (0.75) and concurrencyLevel (16).
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ConcurrentHashMap
public ConcurrentHashMap(Map<? extends K,? extends V> m)
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
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Method Detail
-
isEmpty
public boolean isEmpty()
Returns true if this map contains no key-value mappings.
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size
public int size()
Returns the number of key-value mappings in this map. If the map contains more than Integer.MAX_VALUE elements, returns Integer.MAX_VALUE.
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get
public V get(Object key)
Returns the value to which the specified key is mapped, ornull
if this map contains no mapping for the key.More formally, if this map contains a mapping from a key
k
to a valuev
such thatkey.equals(k)
, then this method returnsv
; otherwise it returnsnull
. (There can be at most one such mapping.)- Specified by:
get
in interfaceMap<K,V>
- Overrides:
get
in classAbstractMap<K,V>
- Parameters:
key
- the key whose associated value is to be returned- Returns:
- the value to which the specified key is mapped, or
null
if this map contains no mapping for the key - Throws:
NullPointerException
- if the specified key is null
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containsKey
public boolean containsKey(Object key)
Tests if the specified object is a key in this table.- Specified by:
containsKey
in interfaceMap<K,V>
- Overrides:
containsKey
in classAbstractMap<K,V>
- Parameters:
key
- possible key- Returns:
- true if and only if the specified object is a key in this table, as determined by the equals method; false otherwise.
- Throws:
NullPointerException
- if the specified key is null
-
containsValue
public boolean containsValue(Object value)
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.- Specified by:
containsValue
in interfaceMap<K,V>
- Overrides:
containsValue
in classAbstractMap<K,V>
- Parameters:
value
- value whose presence in this map is to be tested- Returns:
- true if this map maps one or more keys to the specified value
- Throws:
NullPointerException
- if the specified value is null
-
contains
public boolean contains(Object value)
Legacy method testing if some key maps into the specified value in this table. This method is identical in functionality tocontainsValue(java.lang.Object)
, and exists solely to ensure full compatibility with classHashtable
, which supported this method prior to introduction of the Java Collections framework.- Parameters:
value
- a value to search for- Returns:
- true if and only if some key maps to the value argument in this table as determined by the equals method; false otherwise
- Throws:
NullPointerException
- if the specified value is null
-
put
public V put(K key, V value)
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.
- Specified by:
put
in interfaceMap<K,V>
- Overrides:
put
in classAbstractMap<K,V>
- Parameters:
key
- key with which the specified value is to be associatedvalue
- value to be associated with the specified key- Returns:
- the previous value associated with key, or null if there was no mapping for key
- Throws:
NullPointerException
- if the specified key or value is null
-
putIfAbsent
public V putIfAbsent(K key, V value)
If the specified key is not already associated with a value, associate it with the given value. This is equivalent toif (!map.containsKey(key)) return map.put(key, value); else return map.get(key);
except that the action is performed atomically.- Specified by:
putIfAbsent
in interfaceConcurrentMap<K,V>
- Parameters:
key
- key with which the specified value is to be associatedvalue
- value to be associated with the specified key- Returns:
- 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
-
putAll
public void putAll(Map<? extends K,? extends V> m)
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.
-
remove
public V remove(Object key)
Removes the key (and its corresponding value) from this map. This method does nothing if the key is not in the map.- Specified by:
remove
in interfaceMap<K,V>
- Overrides:
remove
in classAbstractMap<K,V>
- Parameters:
key
- the key that needs to be removed- Returns:
- the previous value associated with key, or null if there was no mapping for key
- Throws:
NullPointerException
- if the specified key is null
-
remove
public boolean remove(Object key, Object value)
Removes the entry for a key only if currently mapped to a given value. This is equivalent toif (map.containsKey(key) && map.get(key).equals(value)) { map.remove(key); return true; } else return false;
except that the action is performed atomically.- Specified by:
remove
in interfaceConcurrentMap<K,V>
- Parameters:
key
- key with which the specified value is associatedvalue
- value expected to be associated with the specified key- Returns:
- true if the value was removed
- Throws:
NullPointerException
- if the specified key is null
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replace
public boolean replace(K key, V oldValue, V newValue)
Replaces the entry for a key only if currently mapped to a given value. This is equivalent toif (map.containsKey(key) && map.get(key).equals(oldValue)) { map.put(key, newValue); return true; } else return false;
except that the action is performed atomically.- Specified by:
replace
in interfaceConcurrentMap<K,V>
- Parameters:
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 key- Returns:
- true if the value was replaced
- Throws:
NullPointerException
- if any of the arguments are null
-
replace
public V replace(K key, V value)
Replaces the entry for a key only if currently mapped to some value. This is equivalent toif (map.containsKey(key)) { return map.put(key, value); } else return null;
except that the action is performed atomically.- Specified by:
replace
in interfaceConcurrentMap<K,V>
- Parameters:
key
- key with which the specified value is associatedvalue
- value to be associated with the specified key- Returns:
- 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
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clear
public void clear()
Removes all of the mappings from this map.
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keySet
public Set<K> keySet()
Returns aSet
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.
-
values
public Collection<V> values()
Returns aCollection
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.
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entrySet
public Set<Map.Entry<K,V>> entrySet()
Returns aSet
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.
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keys
public Enumeration<K> keys()
Returns an enumeration of the keys in this table.- Returns:
- an enumeration of the keys in this table
- See Also:
keySet()
-
elements
public Enumeration<V> elements()
Returns an enumeration of the values in this table.- Returns:
- an enumeration of the values in this table
- See Also:
values()
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