Class WeakHashMap<K,V>
- Type Parameters:
K
- the type of keys maintained by this mapV
- the type of mapped values
- All Implemented Interfaces:
Map<K,
V>
Map
interface, with
weak keys.
An entry in a WeakHashMap
will automatically be removed when
its key is no longer in ordinary use. More precisely, the presence of a
mapping for a given key will not prevent the key from being discarded by the
garbage collector, that is, made finalizable, finalized, and then reclaimed.
When a key has been discarded its entry is effectively removed from the map,
so this class behaves somewhat differently from other Map
implementations.
Both null values and the null key are supported. This class has
performance characteristics similar to those of the HashMap
class, and has the same efficiency parameters of initial capacity
and load factor.
Like most collection classes, this class is not synchronized.
A synchronized WeakHashMap
may be constructed using the
Collections.synchronizedMap
method.
This class is intended primarily for use with key objects whose
equals
methods test for object identity using the
==
operator. Once such a key is discarded it can never be
recreated, so it is impossible to do a lookup of that key in a
WeakHashMap
at some later time and be surprised that its entry
has been removed. This class will work perfectly well with key objects
whose equals
methods are not based upon object identity, such
as String
instances. With such recreatable key objects,
however, the automatic removal of WeakHashMap
entries whose
keys have been discarded may prove to be confusing.
The behavior of the WeakHashMap
class depends in part upon
the actions of the garbage collector, so several familiar (though not
required) Map
invariants do not hold for this class. Because
the garbage collector may discard keys at any time, a
WeakHashMap
may behave as though an unknown thread is silently
removing entries. In particular, even if you synchronize on a
WeakHashMap
instance and invoke none of its mutator methods, it
is possible for the size
method to return smaller values over
time, for the isEmpty
method to return false
and
then true
, for the containsKey
method to return
true
and later false
for a given key, for the
get
method to return a value for a given key but later return
null
, for the put
method to return
null
and the remove
method to return
false
for a key that previously appeared to be in the map, and
for successive examinations of the key set, the value collection, and
the entry set to yield successively smaller numbers of elements.
Each key object in a WeakHashMap
is stored indirectly as
the referent of a weak reference. Therefore a key will automatically be
removed only after the weak references to it, both inside and outside of the
map, have been cleared by the garbage collector.
Implementation note: The value objects in a
WeakHashMap
are held by ordinary strong references. Thus care
should be taken to ensure that value objects do not strongly refer to their
own keys, either directly or indirectly, since that will prevent the keys
from being discarded. Note that a value object may refer indirectly to its
key via the WeakHashMap
itself; that is, a value object may
strongly refer to some other key object whose associated value object, in
turn, strongly refers to the key of the first value object. If the values
in the map do not rely on the map holding strong references to them, one way
to deal with this is to wrap values themselves within
WeakReferences
before
inserting, as in: m.put(key, new WeakReference(value))
,
and then unwrapping upon each get
.
The iterators returned by the iterator
method of the collections
returned by all of this class's "collection view methods" are
fail-fast: if the map is structurally modified at any time after the
iterator is created, in any way except through the iterator's own
remove
method, the iterator will throw a ConcurrentModificationException
. Thus, in the face of concurrent
modification, the iterator fails quickly and cleanly, rather than risking
arbitrary, non-deterministic behavior at an undetermined time in the future.
Note that the fail-fast behavior of an iterator cannot be guaranteed
as it is, generally speaking, impossible to make any hard guarantees in the
presence of unsynchronized concurrent modification. Fail-fast iterators
throw ConcurrentModificationException
on a best-effort basis.
Therefore, it would be wrong to write a program that depended on this
exception for its correctness: the fail-fast behavior of iterators
should be used only to detect bugs.
This class is a member of the Java Collections Framework.
- Since:
- 1.2
- See Also:
-
Nested Class Summary
Nested classes/interfaces declared in class java.util.AbstractMap
AbstractMap.SimpleEntry<K,
V>, AbstractMap.SimpleImmutableEntry<K, V> -
Constructor Summary
ConstructorDescriptionConstructs a new, emptyWeakHashMap
with the default initial capacity (16) and load factor (0.75).WeakHashMap
(int initialCapacity) Constructs a new, emptyWeakHashMap
with the given initial capacity and the default load factor (0.75).WeakHashMap
(int initialCapacity, float loadFactor) Constructs a new, emptyWeakHashMap
with the given initial capacity and the given load factor.WeakHashMap
(Map<? extends K, ? extends V> m) Constructs a newWeakHashMap
with the same mappings as the specified map. -
Method Summary
Modifier and TypeMethodDescriptionvoid
clear()
Removes all of the mappings from this map.boolean
containsKey
(Object key) Returnstrue
if this map contains a mapping for the specified key.boolean
containsValue
(Object value) Returnstrue
if this map maps one or more keys to the specified value.entrySet()
Returns aSet
view of the mappings contained in this map.Returns the value to which the specified key is mapped, ornull
if this map contains no mapping for the key.boolean
isEmpty()
Returnstrue
if this map contains no key-value mappings.keySet()
Returns aSet
view of the keys contained in this map.static <K,
V> WeakHashMap <K, V> newWeakHashMap
(int numMappings) Creates a new, empty WeakHashMap suitable for the expected number of mappings.Associates the specified value with the specified key in this map.void
Copies all of the mappings from the specified map to this map.Removes the mapping for a key from this weak hash map if it is present.int
size()
Returns the number of key-value mappings in this map.values()
Returns aCollection
view of the values contained in this map.Methods declared in class java.util.AbstractMap
clone, equals, hashCode, toString
Methods declared in interface java.util.Map
compute, computeIfAbsent, computeIfPresent, equals, forEach, getOrDefault, hashCode, merge, putIfAbsent, remove, replace, replace, replaceAll
-
Constructor Details
-
WeakHashMap
public WeakHashMap(int initialCapacity, float loadFactor) Constructs a new, emptyWeakHashMap
with the given initial capacity and the given load factor.- API Note:
- To create a
WeakHashMap
with an initial capacity that accommodates an expected number of mappings, usenewWeakHashMap
. - Parameters:
initialCapacity
- The initial capacity of theWeakHashMap
loadFactor
- The load factor of theWeakHashMap
- Throws:
IllegalArgumentException
- if the initial capacity is negative, or if the load factor is nonpositive.
-
WeakHashMap
public WeakHashMap(int initialCapacity) Constructs a new, emptyWeakHashMap
with the given initial capacity and the default load factor (0.75).- API Note:
- To create a
WeakHashMap
with an initial capacity that accommodates an expected number of mappings, usenewWeakHashMap
. - Parameters:
initialCapacity
- The initial capacity of theWeakHashMap
- Throws:
IllegalArgumentException
- if the initial capacity is negative
-
WeakHashMap
public WeakHashMap()Constructs a new, emptyWeakHashMap
with the default initial capacity (16) and load factor (0.75). -
WeakHashMap
Constructs a newWeakHashMap
with the same mappings as the specified map. TheWeakHashMap
is created with the default load factor (0.75) and an initial capacity sufficient to hold the mappings in the specified map.- Parameters:
m
- the map whose mappings are to be placed in this map- Throws:
NullPointerException
- if the specified map is null- Since:
- 1.3
-
-
Method Details
-
size
public int size()Returns the number of key-value mappings in this map. This result is a snapshot, and may not reflect unprocessed entries that will be removed before next attempted access because they are no longer referenced. -
isEmpty
public boolean isEmpty()Returnstrue
if this map contains no key-value mappings. This result is a snapshot, and may not reflect unprocessed entries that will be removed before next attempted access because they are no longer referenced. -
get
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 thatObjects.equals(key, k)
, then this method returnsv
; otherwise it returnsnull
. (There can be at most one such mapping.)A return value of
null
does not necessarily indicate that the map contains no mapping for the key; it's also possible that the map explicitly maps the key tonull
. ThecontainsKey
operation may be used to distinguish these two cases. -
containsKey
Returnstrue
if this map contains a mapping for the specified key.- Specified by:
containsKey
in interfaceMap<K,
V> - Overrides:
containsKey
in classAbstractMap<K,
V> - Parameters:
key
- The key whose presence in this map is to be tested- Returns:
true
if there is a mapping forkey
;false
otherwise
-
put
Associates the specified value with the specified key in this map. If the map previously contained a mapping for this key, the old value is replaced.- Specified by:
put
in interfaceMap<K,
V> - Overrides:
put
in classAbstractMap<K,
V> - Parameters:
key
- key with which the specified value is to be associated.value
- value to be associated with the specified key.- Returns:
- the previous value associated with
key
, ornull
if there was no mapping forkey
. (Anull
return can also indicate that the map previously associatednull
withkey
.)
-
putAll
Copies all of the mappings from the specified map to this map. These mappings will replace any mappings that this map had for any of the keys currently in the specified map.- Specified by:
putAll
in interfaceMap<K,
V> - Overrides:
putAll
in classAbstractMap<K,
V> - Parameters:
m
- mappings to be stored in this map.- Throws:
NullPointerException
- if the specified map is null.
-
remove
Removes the mapping for a key from this weak hash map if it is present. More formally, if this map contains a mapping from keyk
to valuev
such that(key==null ? k==null : key.equals(k))
, that mapping is removed. (The map can contain at most one such mapping.)Returns the value to which this map previously associated the key, or
null
if the map contained no mapping for the key. A return value ofnull
does not necessarily indicate that the map contained no mapping for the key; it's also possible that the map explicitly mapped the key tonull
.The map will not contain a mapping for the specified key once the call returns.
-
clear
-
containsValue
Returnstrue
if this map maps one or more keys to the specified value.- 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
-
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. If the map is modified while an iteration over the set is in progress (except through the iterator's ownremove
operation), the results of the iteration are undefined. The set supports element removal, which removes the corresponding mapping from the map, via theIterator.remove
,Set.remove
,removeAll
,retainAll
, andclear
operations. It does not support theadd
oraddAll
operations. -
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. If the map is modified while an iteration over the collection is in progress (except through the iterator's ownremove
operation), the results of the iteration are undefined. The collection supports element removal, which removes the corresponding mapping from the map, via theIterator.remove
,Collection.remove
,removeAll
,retainAll
andclear
operations. It does not support theadd
oraddAll
operations. -
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. If the map is modified while an iteration over the set is in progress (except through the iterator's ownremove
operation, or through thesetValue
operation on a map entry returned by the iterator) the results of the iteration are undefined. The set supports element removal, which removes the corresponding mapping from the map, via theIterator.remove
,Set.remove
,removeAll
,retainAll
andclear
operations. It does not support theadd
oraddAll
operations. -
newWeakHashMap
Creates a new, empty WeakHashMap suitable for the expected number of mappings. The returned map uses the default load factor of 0.75, and its initial capacity is generally large enough so that the expected number of mappings can be added without resizing the map.- Type Parameters:
K
- the type of keys maintained by the new mapV
- the type of mapped values- Parameters:
numMappings
- the expected number of mappings- Returns:
- the newly created map
- Throws:
IllegalArgumentException
- if numMappings is negative- Since:
- 19
-