public final class WeakHasherMap<K,V>
extends java.util.AbstractMap<K,V>
implements java.util.Map<K,V>
WeakHashMap from JDK 1.2.2, that adds a
constructor that takes a Hasher argument. I also replaced uses of hashCode() and equals() by uses
of Hasher, where appropriate. I've changed the name so that this version is not used
accidentally. It is slightly less efficient, in the absence of a Hasher, and has not been as
extensively tested.
The original documentation follows.
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.
This problem may be fixed in a future release.
HashMap,
WeakReference| Constructor and Description |
|---|
WeakHasherMap()
Constructs a new, empty
WeakHashMap with the default capacity and the default load
factor, which is 0.75. |
WeakHasherMap(Hasher h)
Constructs a new, empty
WeakHashMap with the default capacity and the default load
factor, which is 0.75. |
WeakHasherMap(int initialCapacity)
Constructs a new, empty
WeakHashMap with the given initial capacity and the
default load factor, which is 0.75. |
WeakHasherMap(int initialCapacity,
float loadFactor)
Constructs a new, empty
WeakHashMap with the given initial capacity and the given
load factor. |
| Modifier and Type | Method and Description |
|---|---|
void |
clear()
Removes all mappings from this map.
|
boolean |
containsKey(java.lang.Object key)
Returns
true if this map contains a mapping for the specified key. |
java.util.Set<java.util.Map.Entry<K,V>> |
entrySet()
Returns a
Set view of the mappings in this map. |
V |
get(java.lang.Object key)
Returns the value to which this map maps the specified
key. |
boolean |
isEmpty()
Returns
true if this map contains no key-value mappings. |
V |
put(K key,
V value)
Updates this map so that the given
key maps to the given value. |
V |
remove(java.lang.Object key)
Removes the mapping for the given
key from this map, if present. |
int |
size()
Returns the number of key-value mappings in this map.
|
clone, containsValue, equals, hashCode, keySet, putAll, toString, valuespublic WeakHasherMap(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 WeakHashMapjava.lang.IllegalArgumentException - If the initial capacity is less than zero, or if the load
factor is nonpositivepublic WeakHasherMap(int initialCapacity)
WeakHashMap with the given initial capacity and the
default load factor, which is 0.75.initialCapacity - the initial capacity of the WeakHashMapjava.lang.IllegalArgumentException - If the initial capacity is less than zeropublic WeakHasherMap()
WeakHashMap with the default capacity and the default load
factor, which is 0.75.public WeakHasherMap(Hasher h)
WeakHashMap with the default capacity and the default load
factor, which is 0.75. The WeakHashMap uses the specified hasher for
hashing keys and comparing them for equality.h - the Hasher to use when hashing values for this map@Pure public int size()
Map interface, the time required by this operation is
linear in the size of the map.@Pure public boolean isEmpty()
true if this map contains no key-value mappings.@Pure public boolean containsKey(java.lang.Object key)
true if this map contains a mapping for the specified key.@Pure public V get(java.lang.Object key)
key. If this map does not
contain a value for this key, then return null.public V put(K key, V value)
key maps to the given value. If
the map previously contained a mapping for key then that mapping is replaced and
the previous value is returned.put in interface java.util.Map<K,V>put in class java.util.AbstractMap<K,V>key - the key that is to be mapped to the given valuevalue - the value to which the given key is to be mappednull if if there was
no mapping for the keypublic V remove(java.lang.Object key)
key from this map, if present.public void clear()