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/*
* Licensed to the Apache Software Foundation (ASF) under one or more
* contributor license agreements. See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* The ASF licenses this file to You under the Apache License, Version 2.0
* (the "License"); you may not use this file except in compliance with
* the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
import java.lang.ref.ReferenceQueue;
import java.lang.ref.WeakReference;
import java.util.HashSet;
import java.util.Iterator;
/**
* A weak HashSet. An element stored in the WeakHashSet might be
* garbage collected, if there is no strong reference to this element.
*/
public class WeakHashSet extends HashSet {
/**
* Helps to detect garbage collected values.
*/
ReferenceQueue queue = new ReferenceQueue();
/**
* Returns an iterator over the elements in this set. The elements
* are returned in no particular order.
*
* @return an Iterator over the elements in this set.
*/
public Iterator iterator() {
// remove garbage collected elements
processQueue();
// get an iterator of the superclass WeakHashSet
final Iterator i = super.iterator();
return new Iterator() {
public boolean hasNext() {
return i.hasNext();
}
public Object next() {
// unwrap the element
return getReferenceObject((WeakReference) i.next());
}
public void remove() {
// remove the element from the HashSet
i.remove();
}
};
}
/**
* Returns <code>true</code> if this set contains the specified element.
*
* @param o element whose presence in this set is to be tested.
* @return <code>true</code> if this set contains the specified element.
*/
public boolean contains(Object o) {
return super.contains(WeakElement.create(o));
}
/**
* Adds the specified element to this set if it is not already
* present.
*
* @param o element to be added to this set.
* @return <code>true</code> if the set did not already contain the specified
* element.
*/
public boolean add(Object o) {
processQueue();
return super.add(WeakElement.create(o, this.queue));
}
/**
* Removes the given element from this set if it is present.
*
* @param o object to be removed from this set, if present.
* @return <code>true</code> if the set contained the specified element.
*/
public boolean remove(Object o) {
boolean ret = super.remove(WeakElement.create(o));
processQueue();
return ret;
}
/**
* A convenience method to return the object held by the
* weak reference or <code>null</code> if it does not exist.
*/
private final Object getReferenceObject(WeakReference ref) {
return (ref == null) ? null : ref.get();
}
/**
* Removes all garbage collected values with their keys from the map.
* Since we don't know how much the ReferenceQueue.poll() operation
* costs, we should call it only in the add() method.
*/
private final void processQueue() {
WeakElement wv = null;
while ((wv = (WeakElement) this.queue.poll()) != null) {
super.remove(wv);
}
}
/**
* A WeakHashSet stores objects of class WeakElement.
* A WeakElement wraps the element that should be stored in the WeakHashSet.
* WeakElement inherits from java.lang.ref.WeakReference.
* It redefines equals and hashCode which delegate to the corresponding methods
* of the wrapped element.
*/
static private class WeakElement extends WeakReference {
private int hash; /* Hashcode of key, stored here since the key
may be tossed by the GC */
private WeakElement(Object o) {
super(o);
hash = o.hashCode();
}
private WeakElement(Object o, ReferenceQueue q) {
super(o, q);
hash = o.hashCode();
}
private static WeakElement create(Object o) {
return (o == null) ? null : new WeakElement(o);
}
private static WeakElement create(Object o, ReferenceQueue q) {
return (o == null) ? null : new WeakElement(o, q);
}
/* A WeakElement is equal to another WeakElement iff they both refer to objects
that are, in turn, equal according to their own equals methods */
public boolean equals(Object o) {
if (this == o)
return true;
if (!(o instanceof WeakElement))
return false;
Object t = this.get();
Object u = ((WeakElement) o).get();
if (t == u)
return true;
if ((t == null) || (u == null))
return false;
return t.equals(u);
}
public int hashCode() {
return hash;
}
}
}
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