java.lang.Object | ||
↳ | java.util.AbstractMap<K, V> | |
↳ | java.util.concurrent.ConcurrentHashMap<K, V> |
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. 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.
Public Constructors | |||||||||||
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Creates a new, empty map with the specified initial
capacity, load factor and concurrency level.
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Creates a new, empty map with the specified initial capacity
and load factor and with the default concurrencyLevel (16).
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Creates a new, empty map with the specified initial capacity,
and with default load factor (0.75) and concurrencyLevel (16).
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Creates a new, empty map with a default initial capacity (16),
load factor (0.75) and concurrencyLevel (16).
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Creates a new map with the same mappings as the given map.
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Public Methods | |||||||||||
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Removes all of the mappings from this map.
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Legacy method testing if some key maps into the specified value
in this table.
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Tests if the specified object is a key in this table.
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Returns true if this map maps one or more keys to the
specified value.
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Returns an enumeration of the values in this table.
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Returns a
Set view of the mappings contained in this map. | |||||||||||
Returns the value to which the specified key is mapped,
or
null if this map contains no mapping for the key. | |||||||||||
Returns true if this map contains no key-value mappings.
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Returns a
Set view of the keys contained in this map. | |||||||||||
Returns an enumeration of the keys in this table.
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Maps the specified key to the specified value in this table.
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Copies all of the mappings from the specified map to this one.
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If the specified key is not already associated
with a value, associate it with the given value.
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Removes the entry for a key only if currently mapped to a given value.
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Removes the key (and its corresponding value) from this map.
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Replaces the entry for a key only if currently mapped to some value.
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Replaces the entry for a key only if currently mapped to a given value.
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Returns the number of key-value mappings in this map.
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Returns a
Collection view of the values contained in this map. |
[Expand]
Inherited Methods | |||||||||||
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From class java.util.AbstractMap
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From class java.lang.Object
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From interface java.util.Map
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From interface java.util.concurrent.ConcurrentMap
|
Creates a new, empty map with the specified initial capacity, load factor and concurrency level.
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. |
---|
Creates a new, empty map with the specified initial capacity and load factor and with the default concurrencyLevel (16).
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 nonpositive |
---|
Creates a new, empty map with the specified initial capacity, and with default load factor (0.75) and concurrencyLevel (16).
initialCapacity | the initial capacity. The implementation performs internal sizing to accommodate this many elements. |
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IllegalArgumentException | if the initial capacity of elements is negative. |
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Creates a new, empty map with a default initial capacity (16), load factor (0.75) and concurrencyLevel (16).
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).
m | the map |
---|
Legacy method testing if some key maps into the specified value
in this table. This method is identical in functionality to
containsValue(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 |
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NullPointerException | if the specified value is null |
---|
Tests if the specified object is a key in this table.
key | possible key |
---|
NullPointerException | if the specified key is null |
---|
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.
value | value whose presence in this map is to be tested |
---|
NullPointerException | if the specified value is null |
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Returns an enumeration of the values in this table.
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.
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.)
key | the key. |
---|
null
if no mapping for the specified key is found.
NullPointerException | if the specified key is null |
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Returns true if this map contains no key-value mappings.
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.
Returns an enumeration of the keys in this table.
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.
key | key with which the specified value is to be associated |
---|---|
value | value to be associated with the specified key |
NullPointerException | if the specified key or value is null |
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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.
m | mappings to be stored in this map |
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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.
key | key with which the specified value is to be associated |
---|---|
value | value to be associated with the specified key |
NullPointerException | if the specified key or value is null |
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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.
key | key with which the specified value is associated |
---|---|
value | value expected to be associated with the specified key |
NullPointerException | if the specified key is null |
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Removes the key (and its corresponding value) from this map. This method does nothing if the key is not in the map.
key | the key that needs to be removed |
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NullPointerException | if the specified key is null |
---|
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.
key | key with which the specified value is associated |
---|---|
value | value to be associated with the specified key |
NullPointerException | if the specified key or value is null |
---|
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.
key | key with which the specified value is associated |
---|---|
oldValue | value expected to be associated with the specified key |
newValue | value to be associated with the specified key |
NullPointerException | if any of the arguments are null |
---|
Returns the number of key-value mappings in this map. If the map contains more than Integer.MAX_VALUE elements, returns Integer.MAX_VALUE.
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.