K - the type of keys maintained by this mapV - the type of mapped valuespublic class WeakHashMap<K,V> extends AbstractMap<K,V> implements Map<K,V>
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.
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. 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
HashMap,
WeakReferenceAbstractMap.SimpleEntry<K,V>, AbstractMap.SimpleImmutableEntry<K,V>| Constructor and Description |
|---|
WeakHashMap()
Constructs a new, empty WeakHashMap with the default initial capacity (16) and load
factor (0.75).
|
WeakHashMap(int initialCapacity)
Constructs a new, empty WeakHashMap with the given initial capacity and the default
load factor (0.75).
|
WeakHashMap(int initialCapacity,
float loadFactor)
Constructs a new, empty WeakHashMap with the given initial capacity and the given
load factor.
|
WeakHashMap(Map<? extends K,? extends V> m)
Constructs a new WeakHashMap with the same mappings as the specified map.
|
| Modifier and Type | Method and Description |
|---|---|
void |
clear()
Removes all of the mappings from this map.
|
boolean |
containsKey(Object key)
Returns true if this map contains a mapping for the specified key.
|
boolean |
containsValue(Object value)
Returns true if this map maps one or more keys to the specified value.
|
Set<Map.Entry<K,V>> |
entrySet()
Returns a
Set view of the mappings contained in this map. |
V |
get(Object key)
Returns the value to which the specified key is mapped, or
null if this map contains
no mapping for the key. |
boolean |
isEmpty()
Returns true if this map contains no key-value mappings.
|
Set<K> |
keySet()
Returns a
Set view of the keys contained in this map. |
V |
put(K key,
V value)
Associates the specified value with the specified key in this map.
|
void |
putAll(Map<? extends K,? extends V> m)
Copies all of the mappings from the specified map to this map.
|
V |
remove(Object key)
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.
|
Collection<V> |
values()
Returns a
Collection view of the values contained in this map. |
clone, equals, hashCode, toStringpublic WeakHashMap()
public WeakHashMap(int initialCapacity)
initialCapacity - The initial capacity of the WeakHashMapIllegalArgumentException - if the initial capacity is negativepublic WeakHashMap(int initialCapacity,
float loadFactor)
initialCapacity - The initial capacity of the WeakHashMaploadFactor - The load factor of the WeakHashMapIllegalArgumentException - if the initial capacity is negative, or if the load factor is nonpositive.public WeakHashMap(Map<? extends K,? extends V> m)
m - the map whose mappings are to be placed in this mapNullPointerException - if the specified map is nullpublic void clear()
public boolean containsKey(Object key)
containsKey in interface Map<K,V>containsKey in class AbstractMap<K,V>key - The key whose presence in this map is to be testedpublic boolean containsValue(Object value)
containsValue in interface Map<K,V>containsValue in class AbstractMap<K,V>value - value whose presence in this map is to be testedpublic Set<Map.Entry<K,V>> entrySet()
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. If the map is modified
while an iteration over the set is in progress (except through the iterator's own
remove operation, or through the setValue 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 the
Iterator.remove, Set.remove, removeAll, retainAll and
clear operations. It does not support the add or addAll
operations.public V get(Object key)
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==null ? k==null :
key.equals(k)), then this method returns v; otherwise it returns null.
(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 to null.
The containsKey operation may be used to distinguish these two cases.
get in interface Map<K,V>get in class AbstractMap<K,V>key - the key whose associated value is to be returnednull if this map contains
no mapping for the keyput(Object, Object)public boolean isEmpty()
public Set<K> keySet()
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. If the map is modified while
an iteration over the set is in progress (except through the iterator's own remove
operation), the results of the iteration are undefined. The set supports element removal,
which removes the corresponding mapping from the map, via the Iterator.remove,
Set.remove, removeAll, retainAll, and clear operations.
It does not support the add or addAll operations.public V put(K key, V value)
put in interface Map<K,V>put in class AbstractMap<K,V>key - key with which the specified value is to be associated.value - value to be associated with the specified key.public void putAll(Map<? extends K,? extends V> m)
putAll in interface Map<K,V>putAll in class AbstractMap<K,V>m - mappings to be stored in this map.NullPointerException - if the specified map is null.public V remove(Object key)
(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 of null does not necessarily indicate that the map contained no mapping for the key; it's also possible that the map explicitly mapped the key to null.
The map will not contain a mapping for the specified key once the call returns.
public int size()
public Collection<V> values()
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. If
the map is modified while an iteration over the collection is in progress (except through the
iterator's own remove operation), the results of the iteration are undefined. The
collection supports element removal, which removes the corresponding mapping from the map,
via the Iterator.remove, Collection.remove, removeAll,
retainAll and clear operations. It does not support the add or
addAll operations.