java.util.WeakHashMap Maven / Gradle / Ivy
/*
This is not an official specification document, and usage is restricted.
NOTICE
(c) 2005-2007 Sun Microsystems, Inc. All Rights Reserved.
Neither this file nor any files generated from it describe a complete
specification, and they may only be used as described below. For
example, no permission is given for you to incorporate this file, in
whole or in part, in an implementation of a Java specification.
Sun Microsystems Inc. owns the copyright in this file and it is provided
to you for informative, as opposed to normative, use. The file and any
files generated from it may be used to generate other informative
documentation, such as a unified set of documents of API signatures for
a platform that includes technologies expressed as Java APIs. The file
may also be used to produce "compilation stubs," which allow
applications to be compiled and validated for such platforms.
Any work generated from this file, such as unified javadocs or compiled
stub files, must be accompanied by this notice in its entirety.
This work corresponds to the API signatures of JSR 219: Foundation
Profile 1.1. In the event of a discrepency between this work and the
JSR 219 specification, which is available at
http://www.jcp.org/en/jsr/detail?id=219, the latter takes precedence.
*/
package java.util;
import java.lang.ref.WeakReference;
import java.lang.ref.ReferenceQueue;
/**
* 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 1.17, 03/12/05
* @author Doug Lea
* @author Josh Bloch
* @author Mark Reinhold
* @since 1.2
* @see java.util.HashMap
* @see java.lang.ref.WeakReference
*/
public class WeakHashMap extends AbstractMap implements Map
{
/**
* Constructs a new, empty WeakHashMap with the given initial
* capacity and the given load factor.
*
* @param initialCapacity The initial capacity of the WeakHashMap
* @param loadFactor The load factor of the WeakHashMap
* @throws IllegalArgumentException If the initial capacity is negative,
* or if the load factor is nonpositive.
*/
public WeakHashMap(int initialCapacity, float loadFactor) { }
/**
* Constructs a new, empty WeakHashMap with the given initial
* capacity and the default load factor, which is 0.75.
*
* @param initialCapacity The initial capacity of the WeakHashMap
* @throws IllegalArgumentException If the initial capacity is negative.
*/
public WeakHashMap(int initialCapacity) { }
/**
* Constructs a new, empty WeakHashMap with the default initial
* capacity (16) and the default load factor (0.75).
*/
public WeakHashMap() { }
/**
* 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.
*
* @param t the map whose mappings are to be placed in this map.
* @throws NullPointerException if the specified map is null.
* @since 1.3
*/
public WeakHashMap(Map t) { }
/**
* 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.
*/
public int size() {
return 0;
}
/**
* 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.
*/
public boolean isEmpty() {
return false;
}
/**
* 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.
*
* @param key the key whose associated value is to be returned.
* @return the value to which this map maps the specified key, or
* null if the map contains no mapping for this key.
* @see #put(Object, Object)
*/
public Object get(Object key) {
return null;
}
/**
* Returns true if this map contains a mapping for the
* specified key.
*
* @param key The key whose presence in this map is to be tested
* @return true if there is a mapping for key;
* false otherwise
*/
public boolean containsKey(Object key) {
return false;
}
/**
* 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.
*
* @param key key with which the specified value is to be associated.
* @param value value to be associated with the specified key.
* @return previous value associated with specified key, or null
* if there was no mapping for key. A null return can
* also indicate that the HashMap previously associated
* null with the specified key.
*/
public Object put(Object key, Object value) {
return null;
}
/**
* 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.
*
* @param m mappings to be stored in this map.
* @throws NullPointerException if the specified map is null.
*/
public void putAll(Map m) { }
/**
* Removes the mapping for this key from this map if present.
*
* @param key key whose mapping is to be removed from the map.
* @return previous value associated with specified key, or null
* if there was no mapping for key. A null return can
* also indicate that the map previously associated null
* with the specified key.
*/
public Object remove(Object key) {
return null;
}
/**
* Removes all mappings from this map.
*/
public void clear() { }
/**
* Returns true if this map maps one or more keys to the
* specified value.
*
* @param value value whose presence in this map is to be tested.
* @return true if this map maps one or more keys to the
* specified value.
*/
public boolean containsValue(Object value) {
return false;
}
/**
* 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.
*
* @return a set view of the keys contained in this map.
*/
public Set keySet() {
return null;
}
/**
* 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.
*
* @return a collection view of the values contained in this map.
*/
public Collection values() {
return null;
}
/**
* 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.
*
* @return a collection view of the mappings contained in this map.
* @see Map.Entry
*/
public Set entrySet() {
return null;
}
static class Entry implements Map.Entry {
public Object getKey() {
return null;
}
public Object getValue() {
return null;
}
public Object setValue(Object newValue) {
return null;
}
public boolean equals(Object o) {
return false;
}
public int hashCode() {
return 0;
}
public String toString() {
return null;
}
}
}