java.lang.Object | ||
↳ | java.util.AbstractMap<K, V> | |
↳ | java.util.concurrent.ConcurrentHashMap<K, V> |
A hash table supporting full concurrency of retrievals and
high 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. (More formally, an update operation for a given key bears a
happens-before relation with any (non-null) retrieval for
that key reporting the updated value.) 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. Bear in mind that the
results of aggregate status methods including size
, isEmpty
, and containsValue
are typically useful only when
a map is not undergoing concurrent updates in other threads.
Otherwise the results of these methods reflect transient states
that may be adequate for monitoring or estimation purposes, but not
for program control.
The table is dynamically expanded when there are too many
collisions (i.e., keys that have distinct hash codes but fall into
the same slot modulo the table size), with the expected average
effect of maintaining roughly two bins per mapping (corresponding
to a 0.75 load factor threshold for resizing). There may be much
variance around this average as mappings are added and removed, but
overall, this maintains a commonly accepted time/space tradeoff for
hash tables. However, resizing this or any other kind of hash
table may be a relatively slow operation. When possible, it is a
good idea to provide a size estimate as an optional initialCapacity
constructor argument. An additional optional
loadFactor
constructor argument provides a further means of
customizing initial table capacity by specifying the table density
to be used in calculating the amount of space to allocate for the
given number of elements. Also, for compatibility with previous
versions of this class, constructors may optionally specify an
expected concurrencyLevel
as an additional hint for
internal sizing. Note that using many keys with exactly the same
hashCode()
is a sure way to slow down performance of any
hash table. To ameliorate impact, when keys are Comparable
,
this class may use comparison order among keys to help break ties.
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 | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|
Creates a new, empty map with the default initial table size (16).
| |||||||||||
Creates a new, empty map with an initial table size
accommodating the specified number of elements without the need
to dynamically resize.
| |||||||||||
Creates a new map with the same mappings as the given map.
| |||||||||||
Creates a new, empty map with an initial table size based on
the given number of elements (
initialCapacity ) and
initial table density (loadFactor ). | |||||||||||
Creates a new, empty map with an initial table size based on
the given number of elements (
initialCapacity ), table
density (loadFactor ), and number of concurrently
updating threads (concurrencyLevel ). |
Public Methods | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|
Removes all of the mappings from this map.
| |||||||||||
Legacy method testing if some key maps into the specified value
in this table.
| |||||||||||
Tests if the specified object is a key in this table.
| |||||||||||
Returns
true if this map maps one or more keys to the
specified value. | |||||||||||
Returns an enumeration of the values in this table.
| |||||||||||
Returns a
Set view of the mappings contained in this map. | |||||||||||
Compares the specified object with this map for equality.
| |||||||||||
Returns the value to which the specified key is mapped,
or
null if this map contains no mapping for the key. | |||||||||||
Returns the hash code value for this
Map , i.e.,
the sum of, for each key-value pair in the map,
key.hashCode() ^ value.hashCode() . | |||||||||||
Returns whether this map is empty.
This implementation compares | |||||||||||
Returns a
Set view of the keys contained in this map. | |||||||||||
Returns an enumeration of the keys in this table.
| |||||||||||
Maps the specified key to the specified value in this table.
| |||||||||||
Copies all of the mappings from the specified map to this one.
| |||||||||||
If the specified key is not already associated
with a value, associate it with the given value.
| |||||||||||
Removes the entry for a key only if currently mapped to a given value.
| |||||||||||
Removes the key (and its corresponding value) from this map.
| |||||||||||
Replaces the entry for a key only if currently mapped to some value.
| |||||||||||
Replaces the entry for a key only if currently mapped to a given value.
| |||||||||||
Returns the number of mappings in this
Map .
This implementation returns its entry set's size. | |||||||||||
Returns a string representation of this map.
| |||||||||||
Returns a
Collection view of the values contained in this map. |
[Expand]
Inherited Methods | |||||||||||
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From class
java.util.AbstractMap
| |||||||||||
From class
java.lang.Object
| |||||||||||
From interface
java.util.Map
| |||||||||||
From interface
java.util.concurrent.ConcurrentMap
|
Creates a new, empty map with the default initial table size (16).
Creates a new, empty map with an initial table size accommodating the specified number of elements without the need to dynamically resize.
initialCapacity | The implementation performs internal sizing to accommodate this many elements. |
---|
IllegalArgumentException | if the initial capacity of elements is negative |
---|
Creates a new map with the same mappings as the given map.
m | the map |
---|
Creates a new, empty map with an initial table size based on
the given number of elements (initialCapacity
) and
initial table density (loadFactor
).
initialCapacity | the initial capacity. The implementation performs internal sizing to accommodate this many elements, given the specified load factor. |
---|---|
loadFactor | the load factor (table density) for establishing the initial table size |
IllegalArgumentException | if the initial capacity of elements is negative or the load factor is nonpositive |
---|
Creates a new, empty map with an initial table size based on
the given number of elements (initialCapacity
), table
density (loadFactor
), and number of concurrently
updating threads (concurrencyLevel
).
initialCapacity | the initial capacity. The implementation performs internal sizing to accommodate this many elements, given the specified load factor. |
---|---|
loadFactor | the load factor (table density) for establishing the initial table size |
concurrencyLevel | the estimated number of concurrently updating threads. The implementation may use this value as a sizing hint. |
IllegalArgumentException | if the initial capacity is negative or the load factor or concurrencyLevel are nonpositive |
---|
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
.
value | a value to search for |
---|
true
if and only if some key maps to the
value
argument in this table as
determined by the equals
method;
false
otherwiseNullPointerException | if the specified value is null |
---|
Tests if the specified object is a key in this table.
key | possible key |
---|
true
if and only if the specified object
is a key in this table, as determined by the
equals
method; false
otherwiseNullPointerException | if the specified key is null |
---|
Returns true
if this map maps one or more keys to the
specified value. Note: This method may require a full traversal
of the map, and is much slower than method containsKey
.
value | value whose presence in this map is to be tested |
---|
true
if this map maps one or more keys to the
specified valueNullPointerException | if the specified value is null |
---|
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.
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.
Compares the specified object with this map for equality.
Returns true
if the given object is a map with the same
mappings as this map. This operation may return misleading
results if either map is concurrently modified during execution
of this method.
o | object to be compared for equality with this map |
---|
true
if the specified object is equal to this map
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 |
---|
Returns the hash code value for this Map
, i.e.,
the sum of, for each key-value pair in the map,
key.hashCode() ^ value.hashCode()
.
Returns whether this map is empty.
This implementation compares size()
to 0.
true
if this map has no elements, false
otherwise.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 |
key
, or
null
if there was no mapping for key
NullPointerException | if the specified key or value is null |
---|
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 |
---|
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 |
null
if there was no mapping for the keyNullPointerException | if the specified key or value is null |
---|
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 |
true
if the value was removedNullPointerException | if the specified key is null |
---|
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 |
---|
key
, or
null
if there was no mapping for key
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 |
null
if there was no mapping for the keyNullPointerException | 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 |
true
if the value was replacedNullPointerException | if any of the arguments are null |
---|
Returns the number of mappings in this Map
.
This implementation returns its entry set's size.
Map
.
Returns a string representation of this map. The string
representation consists of a list of key-value mappings (in no
particular order) enclosed in braces ("{
}"). Adjacent
mappings are separated by the characters ", "
(comma
and space). Each key-value mapping is rendered as the key
followed by an equals sign ("=
") followed by the
associated 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.