Class WeakHashMap<K,V>
- java.lang.Object
-
- java.util.AbstractMap<K,V>
-
- java.util.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>
public class WeakHashMap<K,V> extends AbstractMap<K,V> implements Map<K,V>
Hash table based implementation of the 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:
-
HashMap
,WeakReference
Nested Class Summary
Nested classes/interfaces declared in class java.util.AbstractMap
AbstractMap.SimpleEntry<K,V>, AbstractMap.SimpleImmutableEntry<K,V>
Constructor Summary
Constructor | Description |
---|---|
WeakHashMap() | Constructs a new, empty |
WeakHashMap(int initialCapacity) | Constructs a new, empty |
WeakHashMap(int initialCapacity,
float loadFactor) | Constructs a new, empty |
WeakHashMap(Map<? extends K,? extends V> m) | Constructs a new |
Method Summary
Modifier and Type | Method | Description |
---|---|---|
void | clear() | Removes all of the mappings from this map. |
boolean | containsKey(Object key) | Returns |
boolean | containsValue(Object value) | Returns |
Set<Map.Entry<K,V>> | entrySet() | Returns a |
V | get(Object key) | Returns the value to which the specified key is mapped, or |
boolean | isEmpty() | Returns |
Set<K> | keySet() | Returns a |
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 |
Methods declared in class java.util.AbstractMap
clone, equals, hashCode, toString
Methods declared in class java.lang.Object
finalize, getClass, notify, notifyAll, wait, wait, wait
Methods declared in interface java.util.Map
compute, computeIfAbsent, computeIfPresent, equals, forEach, getOrDefault, hashCode, merge, putIfAbsent, remove, replace, replace, replaceAll
Constructor Detail
WeakHashMap
public WeakHashMap(int initialCapacity, float loadFactor)
Constructs a new, empty WeakHashMap
with the given initial capacity and the given load factor.
- Parameters:
-
initialCapacity
- The initial capacity of theWeakHashMap
-
loadFactor
- The load factor of theWeakHashMap
- Throws:
-
IllegalArgumentException
- if the initial capacity is negative, or if the load factor is nonpositive.
WeakHashMap
public WeakHashMap(int initialCapacity)
Constructs a new, empty WeakHashMap
with 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
WeakHashMap
public WeakHashMap()
Constructs a new, empty WeakHashMap
with the default initial capacity (16) and load factor (0.75).
WeakHashMap
public WeakHashMap(Map<? extends K,? extends V> m)
Constructs a new 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.
- 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 Detail
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.
- Specified by:
-
size
in interfaceMap<K,V>
- Overrides:
-
size
in classAbstractMap<K,V>
- Returns:
- the number of key-value mappings in this map
isEmpty
public boolean isEmpty()
Returns 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.
- Specified by:
-
isEmpty
in interfaceMap<K,V>
- Overrides:
-
isEmpty
in classAbstractMap<K,V>
- Returns:
-
true
if this map contains no key-value mappings
get
public 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.
More formally, if this map contains a mapping from a key k
to a value v
such that Objects.equals(key, 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.
- Specified by:
-
get
in interfaceMap<K,V>
- Overrides:
-
get
in 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
null
if this map contains no mapping for the key - See Also:
put(Object, Object)
containsKey
public boolean containsKey(Object key)
Returns true
if this map contains a mapping for the specified key.
- Specified by:
-
containsKey
in interfaceMap<K,V>
- Overrides:
-
containsKey
in classAbstractMap<K,V>
- Parameters:
-
key
- The key whose presence in this map is to be tested - Returns:
-
true
if there is a mapping forkey
;false
otherwise
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:
-
put
in interfaceMap<K,V>
- Overrides:
-
put
in 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
, ornull
if there was no mapping forkey
. (Anull
return can also indicate that the map previously associatednull
withkey
.)
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:
-
putAll
in interfaceMap<K,V>
- Overrides:
-
putAll
in classAbstractMap<K,V>
- Parameters:
-
m
- mappings to be stored in this map. - Throws:
-
NullPointerException
- if the specified map is null.
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 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.
- Specified by:
-
remove
in interfaceMap<K,V>
- Overrides:
-
remove
in classAbstractMap<K,V>
- Parameters:
-
key
- key whose mapping is to be removed from the map - Returns:
- the previous value associated with
key
, ornull
if there was no mapping forkey
clear
public void clear()
Removes all of the mappings from this map. The map will be empty after this call returns.
containsValue
public boolean containsValue(Object value)
Returns true
if this map maps one or more keys to the specified value.
- Specified by:
-
containsValue
in interfaceMap<K,V>
- Overrides:
-
containsValue
in classAbstractMap<K,V>
- Parameters:
-
value
- value whose presence in this map is to be tested - Returns:
-
true
if this map maps one or more keys to the specified value
keySet
public Set<K> keySet()
Returns a 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.
- Specified by:
-
keySet
in interfaceMap<K,V>
- Overrides:
-
keySet
in classAbstractMap<K,V>
- Returns:
- a set view of the keys contained in this map
values
public Collection<V> values()
Returns a 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.
- Specified by:
-
values
in interfaceMap<K,V>
- Overrides:
-
values
in classAbstractMap<K,V>
- Returns:
- a collection view of the values contained in this map
entrySet
public Set<Map.Entry<K,V>> entrySet()
Returns a 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.