Class WeakHashMap<K,V> 
- Type Parameters:
- K- the type of keys maintained by this map
- V- 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 SummaryNested classes/interfaces declared in class java.util.AbstractMapAbstractMap.SimpleEntry<K,V>, AbstractMap.SimpleImmutableEntry<K, V> 
- 
Constructor SummaryConstructorsConstructorDescriptionConstructs 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 SummaryModifier 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.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.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.AbstractMapclone, equals, hashCode, toStringMethods declared in interface java.util.Mapcompute, computeIfAbsent, computeIfPresent, equals, forEach, getOrDefault, hashCode, merge, putIfAbsent, remove, replace, replace, replaceAll
- 
Constructor Details- 
WeakHashMappublic WeakHashMap(int initialCapacity, float loadFactor) Constructs a new, emptyWeakHashMapwith the given initial capacity and the given load factor.- API Note:
- To create a WeakHashMapwith an initial capacity that accommodates an expected number of mappings, usenewWeakHashMap.
- Parameters:
- initialCapacity- The initial capacity of the- WeakHashMap
- loadFactor- The load factor of the- WeakHashMap
- Throws:
- IllegalArgumentException- if the initial capacity is negative, or if the load factor is nonpositive.
 
- 
WeakHashMappublic WeakHashMap(int initialCapacity) Constructs a new, emptyWeakHashMapwith the given initial capacity and the default load factor (0.75).- API Note:
- To create a WeakHashMapwith an initial capacity that accommodates an expected number of mappings, usenewWeakHashMap.
- Parameters:
- initialCapacity- The initial capacity of the- WeakHashMap
- Throws:
- IllegalArgumentException- if the initial capacity is negative
 
- 
WeakHashMappublic WeakHashMap()Constructs a new, emptyWeakHashMapwith the default initial capacity (16) and load factor (0.75).
- 
WeakHashMapConstructs 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
 
 
- 
- 
Method Details- 
sizepublic 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.
- 
isEmptypublic 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.
- 
getReturns 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.
- 
containsKeyReturnstrueif this map contains a mapping for the specified key.- Specified by:
- containsKeyin interface- Map<K,- V> 
- Overrides:
- containsKeyin class- AbstractMap<K,- V> 
- Parameters:
- key- The key whose presence in this map is to be tested
- Returns:
- trueif there is a mapping for- key;- falseotherwise
 
- 
putAssociates 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 interface- Map<K,- V> 
- Overrides:
- putin class- AbstractMap<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.)
 
- 
putAllCopies 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 interface- Map<K,- V> 
- Overrides:
- putAllin class- AbstractMap<K,- V> 
- Parameters:
- m- mappings to be stored in this map.
- Throws:
- NullPointerException- if the specified map is null.
 
- 
removeRemoves 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. 
- 
clear
- 
containsValueReturnstrueif this map maps one or more keys to the specified value.- Specified by:
- containsValuein interface- Map<K,- V> 
- Overrides:
- containsValuein class- AbstractMap<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
 
- 
keySetReturns 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.
- 
valuesReturns 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.
- 
entrySetReturns 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.
- 
newWeakHashMapCreates 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 map
- V- 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
 
 
-