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>
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.
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()
WeakHashMap with the default initial capacity (16) and load
factor (0.75).public WeakHashMap(int initialCapacity)
WeakHashMap with the given initial capacity and the default load
factor (0.75).initialCapacity - The initial capacity of the WeakHashMapIllegalArgumentException - if the initial capacity is negativepublic WeakHashMap(int initialCapacity,
float loadFactor)
WeakHashMap with the given initial capacity and the given load
factor.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)
WeakHashMap with the same mappings as the specified map. The
WeakHashMap is created with the default load factor (0.75) and an initial capacity
sufficient to hold the mappings in the specified map.m - the map whose mappings are to be placed in this mapNullPointerException - if the specified map is nullpublic void clear()
public boolean containsKey(@Nullable Object key)
true if this map contains a mapping for the specified key.containsKey in interface Map<K,V>containsKey in class AbstractMap<K,V>key - The key whose presence in this map is to be testedtrue if there is a mapping for key; false otherwisepublic boolean containsValue(@Nullable Object value)
true if this map maps one or more keys to the specified value.containsValue in interface Map<K,V>containsValue in class AbstractMap<K,V>value - value whose presence in this map is to be testedtrue if this map maps one or more keys to the specified valuepublic 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.@Nullable public V get(@Nullable 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()
true 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.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.@Nullable public V put(@Nullable K key, @Nullable 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.key, or null if there was no mapping
for key. (A null return can also indicate that the map previously
associated null with 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.@Nullable public V remove(@Nullable Object key)
k to value v 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 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.