- 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:
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Nested Class Summary
Nested classes/interfaces declared in class java.util.AbstractMap
AbstractMap.SimpleEntry<K,V>, AbstractMap.SimpleImmutableEntry<K, V> -
Constructor Summary
ConstructorsConstructorDescriptionConstructs a new, emptyWeakHashMapwith the default initial capacity (16) and load factor (0.75).WeakHashMap(int initialCapacity) Constructs a new, emptyWeakHashMapwith the given initial capacity and the default load factor (0.75).WeakHashMap(int initialCapacity, float loadFactor) Constructs a new, emptyWeakHashMapwith the given initial capacity and the given load factor.WeakHashMap(Map<? extends K, ? extends V> m) Constructs a newWeakHashMapwith the same mappings as the specified map. -
Method Summary
Modifier and TypeMethodDescriptionvoidclear()Removes all of the mappings from this map.booleancontainsKey(Object key) Returnstrueif this map contains a mapping for the specified key.booleancontainsValue(Object value) Returnstrueif this map maps one or more keys to the specified value.entrySet()Returns aSetview of the mappings contained in this map.Returns the value to which the specified key is mapped, ornullif this map contains no mapping for the key.booleanisEmpty()Returnstrueif this map contains no key-value mappings.keySet()Returns aSetview of the keys contained in this map.Associates the specified value with the specified key in this map.voidCopies 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.intsize()Returns the number of key-value mappings in this map.values()Returns aCollectionview of the values contained in this map.Methods declared in class java.util.AbstractMap
clone, equals, hashCode, toStringMethods declared in interface java.util.Map
compute, computeIfAbsent, computeIfPresent, equals, forEach, getOrDefault, hashCode, merge, putIfAbsent, remove, replace, replace, replaceAll
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Constructor Details
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WeakHashMap
public WeakHashMap(int initialCapacity, float loadFactor) Constructs a new, emptyWeakHashMapwith the given initial capacity and the given load factor.- Parameters:
initialCapacity- The initial capacity of theWeakHashMaploadFactor- The load factor of theWeakHashMap- Throws:
IllegalArgumentException- if the initial capacity is negative, or if the load factor is nonpositive.
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WeakHashMap
public WeakHashMap(int initialCapacity) Constructs a new, emptyWeakHashMapwith the given initial capacity and the default load factor (0.75).- Parameters:
initialCapacity- The initial capacity of theWeakHashMap- Throws:
IllegalArgumentException- if the initial capacity is negative
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WeakHashMap
public WeakHashMap()Constructs a new, emptyWeakHashMapwith the default initial capacity (16) and load factor (0.75). -
WeakHashMap
Constructs a newWeakHashMapwith the same mappings as the specified map. TheWeakHashMapis 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
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Method Details
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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()Returnstrueif 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, ornullif this map contains no mapping for the key.More formally, if this map contains a mapping from a key
kto a valuevsuch thatObjects.equals(key, k), then this method returnsv; otherwise it returnsnull. (There can be at most one such mapping.)A return value of
nulldoes not necessarily indicate that the map contains no mapping for the key; it's also possible that the map explicitly maps the key tonull. ThecontainsKeyoperation may be used to distinguish these two cases. -
containsKey
Returnstrueif this map contains a mapping for the specified key.- Specified by:
containsKeyin interfaceMap<K,V> - Overrides:
containsKeyin classAbstractMap<K,V> - Parameters:
key- The key whose presence in this map is to be tested- Returns:
trueif there is a mapping forkey;falseotherwise
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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:
putin interfaceMap<K,V> - Overrides:
putin 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, ornullif there was no mapping forkey. (Anullreturn can also indicate that the map previously associatednullwithkey.)
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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:
putAllin interfaceMap<K,V> - Overrides:
putAllin classAbstractMap<K,V> - Parameters:
m- mappings to be stored in this map.- Throws:
NullPointerException- if the specified map is null.
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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 keykto valuevsuch 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
nullif the map contained no mapping for the key. A return value ofnulldoes 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.
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clear
public void clear()Removes all of the mappings from this map. The map will be empty after this call returns. -
containsValue
Returnstrueif this map maps one or more keys to the specified value.- Specified by:
containsValuein interfaceMap<K,V> - Overrides:
containsValuein classAbstractMap<K,V> - Parameters:
value- value whose presence in this map is to be tested- Returns:
trueif this map maps one or more keys to the specified value
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keySet
Returns aSetview 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 ownremoveoperation), 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, andclearoperations. It does not support theaddoraddAlloperations. -
values
Returns aCollectionview 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 ownremoveoperation), 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,retainAllandclearoperations. It does not support theaddoraddAlloperations. -
entrySet
Returns aSetview 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 ownremoveoperation, or through thesetValueoperation 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,retainAllandclearoperations. It does not support theaddoraddAlloperations.
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