| java.lang.Object
 java.util.AbstractMap
java.util.WeakHashMap
| WeakHashMap | | public class WeakHashMap extends AbstractMap implements Map(Code) | | A hashtable-based Map implementation 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 than 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 set, 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 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 or add methods, 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.
version:
1.19, 10/10/06
author:
Doug Lea
author:
Josh Bloch
author:
Mark Reinhold
since:
1.2
See Also: java.util.HashMap
See Also: java.lang.ref.WeakReference |
| Constructor Summary | |
| public | WeakHashMap(int initialCapacity, float loadFactor)
Constructs a new, empty WeakHashMap with the given initial
capacity and the given load factor. | | public | WeakHashMap(int initialCapacity)
Constructs a new, empty WeakHashMap with the given initial
capacity and the default load factor, which is 0.75. | | public | WeakHashMap()
Constructs a new, empty WeakHashMap with the default initial
capacity (16) and the default load factor (0.75). | | public | WeakHashMap(Map t)
Constructs a new WeakHashMap with the same mappings as the
specified Map. |
| Method Summary | |
| public void | clear()
Removes all mappings from this map. | | public boolean | containsKey(Object key)
Returns true if this map contains a mapping for the
specified key. | | public boolean | containsValue(Object value)
Returns true if this map maps one or more keys to the
specified value.
Parameters:
value - value whose presence in this map is to be tested. | | public Set | entrySet()
Returns a collection view of the mappings contained in this map. | | static boolean | eq(Object x, Object y)
Check for equality of non-null reference x and possibly-null y. | | public Object | get(Object key)
Returns the value to which the specified key is mapped in this weak
hash map, or null if the map contains no mapping for
this key. | | Entry | getEntry(Object key)
Returns the entry associated with the specified key in the HashMap. | | static int | indexFor(int h, int length)
Return index for hash code h. | | public boolean | isEmpty()
Returns true if this map contains no key-value mappings. | | public Set | keySet()
Returns a set view of the keys contained in this map. | | public Object | put(Object key, Object 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.
Parameters:
key - key with which the specified value is to be associated.
Parameters:
value - value to be associated with the specified key. | | public void | putAll(Map m)
| | public Object | remove(Object key)
Removes the mapping for this key from this map if present.
Parameters:
key - key whose mapping is to be removed from the map. | | Entry | removeMapping(Object o)
| | void | resize(int newCapacity)
Rehashes the contents of this map into a new array with a
larger capacity. | | public int | size()
Returns the number of key-value mappings in this map. | | public Collection | values()
Returns a collection view of the values contained in this map. |
| WeakHashMap | | public WeakHashMap(int initialCapacity, float loadFactor)(Code) | | Constructs a new, empty WeakHashMap with the given initial
capacity and the given load factor.
Parameters:
initialCapacity - The initial capacity of the WeakHashMap
Parameters:
loadFactor - The load factor of the WeakHashMap
throws:
IllegalArgumentException - If the initial capacity is negative,or if the load factor is nonpositive. |
| WeakHashMap | | public WeakHashMap(int initialCapacity)(Code) | | Constructs a new, empty WeakHashMap with the given initial
capacity and the default load factor, which is 0.75.
Parameters:
initialCapacity - The initial capacity of the WeakHashMap
throws:
IllegalArgumentException - If the initial capacity is negative. |
| WeakHashMap | | public WeakHashMap()(Code) | | Constructs a new, empty WeakHashMap with the default initial
capacity (16) and the default load factor (0.75).
|
| WeakHashMap | | public WeakHashMap(Map t)(Code) | | Constructs a new WeakHashMap with the same mappings as the
specified Map. The WeakHashMap is created with
default load factor, which is 0.75 and an initial capacity
sufficient to hold the mappings in the specified Map.
Parameters:
t - the map whose mappings are to be placed in this map.
throws:
NullPointerException - if the specified map is null.
since:
1.3 |
| clear | | public void clear()(Code) | | Removes all mappings from this map.
|
| containsKey | | public boolean containsKey(Object key)(Code) | | Returns true if this map contains a mapping for the
specified key.
Parameters:
key - The key whose presence in this map is to be tested true if there is a mapping for key;false otherwise |
| containsValue | | public boolean containsValue(Object value)(Code) | | Returns true if this map maps one or more keys to the
specified value.
Parameters:
value - value whose presence in this map is to be tested. true if this map maps one or more keys to thespecified value. |
| entrySet | | public Set entrySet()(Code) | | Returns a collection view of the mappings contained in this map. Each
element in the returned collection is a Map.Entry. The
collection is backed by the map, so changes to the map are reflected in
the collection, and vice-versa. 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.
a collection view of the mappings contained in this map.
See Also: Map.Entry |
| eq | | static boolean eq(Object x, Object y)(Code) | | Check for equality of non-null reference x and possibly-null y. By
default uses Object.equals.
|
| get | | public Object get(Object key)(Code) | | Returns the value to which the specified key is mapped in this weak
hash map, or null if the map contains no mapping for
this key. A return value of null does not necessarily
indicate that the map contains no mapping for the key; it is also
possible that the map explicitly maps the key to null. The
containsKey method may be used to distinguish these two
cases.
Parameters:
key - the key whose associated value is to be returned. the value to which this map maps the specified key, ornull if the map contains no mapping for this key.
See Also: WeakHashMap.put(Object,Object) |
| getEntry | | Entry getEntry(Object key)(Code) | | Returns the entry associated with the specified key in the HashMap.
Returns null if the HashMap contains no mapping for this key.
|
| indexFor | | static int indexFor(int h, int length)(Code) | | Return index for hash code h.
|
| isEmpty | | public boolean isEmpty()(Code) | | 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.
|
| keySet | | public Set keySet()(Code) | | 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. The set supports element removal, which removes the
corresponding mapping from this map, via the Iterator.remove,
Set.remove, removeAll, retainAll, and
clear operations. It does not support the add or
addAll operations.
a set view of the keys contained in this map. |
| put | | public Object put(Object key, Object value)(Code) | | 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.
Parameters:
key - key with which the specified value is to be associated.
Parameters:
value - value to be associated with the specified key. previous value associated with specified key, or nullif there was no mapping for key. A null return canalso indicate that the HashMap previously associatednull with the specified key. |
| putAll | | public void putAll(Map m)(Code) | | 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.
Parameters:
m - mappings to be stored in this map.
throws:
NullPointerException - if the specified map is null. |
| remove | | public Object remove(Object key)(Code) | | Removes the mapping for this key from this map if present.
Parameters:
key - key whose mapping is to be removed from the map. previous value associated with specified key, or nullif there was no mapping for key. A null return canalso indicate that the map previously associated nullwith the specified key. |
| removeMapping | | Entry removeMapping(Object o)(Code) | | Special version of remove needed by Entry set
|
| resize | | void resize(int newCapacity)(Code) | | Rehashes the contents of this map into a new array with a
larger capacity. This method is called automatically when the
number of keys in this map reaches its threshold.
If current capacity is MAXIMUM_CAPACITY, this method does not
resize the map, but but sets threshold to Integer.MAX_VALUE.
This has the effect of preventing future calls.
Parameters:
newCapacity - the new capacity, MUST be a power of two;must be greater than current capacity unless currentcapacity is MAXIMUM_CAPACITY (in which case valueis irrelevant). |
| size | | public int size()(Code) | | 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.
|
| values | | public Collection values()(Code) | | 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. The collection supports element
removal, which removes the corresponding mapping from this map, via the
Iterator.remove, Collection.remove,
removeAll, retainAll, and clear operations.
It does not support the add or addAll operations.
a collection view of the values contained in this map. |
|
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