java.util
class WeakHashMap

java.lang.Object java.util.AbstractMap java.util.WeakHashMap

All Implemented Interfaces:

Map

Most common way to construct:

WeakHashMap map = new WeakHashMap();

Based on 43 examples

public class WeakHashMap
extends AbstractMap
implements Map

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 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 {@link Collections#synchronizedMap 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. 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 {@link 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.

WeakHashMap

public WeakHashMap()

Constructs a new, empty WeakHashMap with the default initial capacity (16) and load factor (0.75).

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 the WeakHashMap

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 the WeakHashMap

loadFactor - The load factor of the WeakHashMap

WeakHashMap

public WeakHashMap(Map 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

clear

public void clear()

Removes all of the mappings from this map. The map will be empty after this call returns.

Overrides:

clear in class AbstractMap

containsKey

public boolean containsKey(Object key)

Returns true if this map contains a mapping for the specified key.

Overrides:

containsKey in class AbstractMap

Parameters:

key - The key whose presence in this map is to be tested

Returns:

true if there is a mapping for key; false otherwise

containsValue

public boolean containsValue(Object value)

Returns true if this map maps one or more keys to the specified value.

Overrides:

containsValue in class AbstractMap

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

entrySet

public Set entrySet()

Returns a {@link 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.

Overrides:

entrySet in class AbstractMap

get

public Object get(Object key)

Returns the value to which the specified key is mapped, or {@code null} if this map contains no mapping for the key.

More formally, if this map contains a mapping from a key {@code k} to a value {@code v} such that {@code (key==null ? k==null : key.equals(k))}, then this method returns {@code v}; otherwise it returns {@code null}. (There can be at most one such mapping.)

A return value of {@code 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 {@code null}. The {@link #containsKey containsKey} operation may be used to distinguish these two cases.

Overrides:

get in class AbstractMap

Parameters:

key

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.

Overrides:

isEmpty in class AbstractMap

keySet

public Set keySet()

Returns a {@link 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.

Overrides:

keySet in class AbstractMap

put

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.

Overrides:

put in class AbstractMap

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, or null if there was no mapping for key. (A null return can also indicate that the map previously associated null with key.)

putAll

public void putAll(Map 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.

Overrides:

putAll in class AbstractMap

Parameters:

m - mappings to be stored in this map.

remove

public Object 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.

Overrides:

remove in class AbstractMap

Parameters:

key - key whose mapping is to be removed from the map

Returns:

the previous value associated with key, or null if there was no mapping for key

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.

Overrides:

size in class AbstractMap

values

public Collection values()

Returns a {@link 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.

Overrides:

values in class AbstractMap