- java.lang.Object
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- java.util.AbstractMap<K,V>
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- java.util.WeakHashMap<K,V>
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- Type Parameters:
K- the type of keys maintained by this mapV- the type of mapped values
- All Implemented Interfaces:
Map<K,V>
public class WeakHashMap<K,V> extends AbstractMap<K,V> implements Map<K,V>
Hash table based implementation of theMapinterface, with weak keys. An entry in aWeakHashMapwill 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 otherMapimplementations.Both null values and the null key are supported. This class has performance characteristics similar to those of the
HashMapclass, and has the same efficiency parameters of initial capacity and load factor.Like most collection classes, this class is not synchronized. A synchronized
WeakHashMapmay be constructed using theCollections.synchronizedMapmethod.This class is intended primarily for use with key objects whose
equalsmethods 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 aWeakHashMapat some later time and be surprised that its entry has been removed. This class will work perfectly well with key objects whoseequalsmethods are not based upon object identity, such asStringinstances. With such recreatable key objects, however, the automatic removal ofWeakHashMapentries whose keys have been discarded may prove to be confusing.The behavior of the
WeakHashMapclass depends in part upon the actions of the garbage collector, so several familiar (though not required)Mapinvariants do not hold for this class. Because the garbage collector may discard keys at any time, aWeakHashMapmay behave as though an unknown thread is silently removing entries. In particular, even if you synchronize on aWeakHashMapinstance and invoke none of its mutator methods, it is possible for thesizemethod to return smaller values over time, for theisEmptymethod to returnfalseand thentrue, for thecontainsKeymethod to returntrueand laterfalsefor a given key, for thegetmethod to return a value for a given key but later returnnull, for theputmethod to returnnulland theremovemethod to returnfalsefor 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
WeakHashMapis 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
WeakHashMapare 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 theWeakHashMapitself; 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 withinWeakReferencesbefore inserting, as in:m.put(key, new WeakReference(value)), and then unwrapping upon eachget.The iterators returned by the
iteratormethod 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 ownremovemethod, the iterator will throw aConcurrentModificationException. 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
ConcurrentModificationExceptionon 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:
HashMap,WeakReference
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Nested Class Summary
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Nested classes/interfaces declared in class java.util.AbstractMap
AbstractMap.SimpleEntry<K,V>, AbstractMap.SimpleImmutableEntry<K,V>
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Constructor Summary
Constructors Constructor Description WeakHashMap()Constructs 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.
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Method Summary
All Methods Instance Methods Concrete Methods Modifier and Type Method Description voidclear()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.Set<Map.Entry<K,V>>entrySet()Returns aSetview of the mappings contained in this map.Vget(Object key)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.Set<K>keySet()Returns aSetview of the keys contained in this map.Vput(K key, V value)Associates the specified value with the specified key in this map.voidputAll(Map<? extends K,? extends V> m)Copies all of the mappings from the specified map to this map.Vremove(Object key)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.Collection<V>values()Returns aCollectionview of the values contained in this map.-
Methods declared in class java.util.AbstractMap
clone, equals, hashCode, toString
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Methods declared in interface java.util.Map
compute, computeIfAbsent, computeIfPresent, equals, forEach, getOrDefault, hashCode, merge, putIfAbsent, remove, replace, replace, replaceAll
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Constructor Detail
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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).
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WeakHashMap
public WeakHashMap(Map<? extends K,? extends V> m)
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 Detail
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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.
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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.
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get
public V get(Object key)
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.- Specified by:
getin interfaceMap<K,V>- Overrides:
getin 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
nullif this map contains no mapping for the key - See Also:
put(Object, Object)
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containsKey
public boolean containsKey(Object key)
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
public V put(K key, V value)
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
public void putAll(Map<? extends K,? extends V> m)
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
public V remove(Object key)
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.
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containsValue
public boolean containsValue(Object value)
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
public Set<K> 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.
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values
public Collection<V> 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.
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entrySet
public Set<Map.Entry<K,V>> 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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