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 * Copyright (c) 2011, 2015, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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package com.sun.javafx.util;

import java.lang.ref.ReferenceQueue;
import java.lang.ref.WeakReference;
import java.util.Iterator;

This is a helper class for handling weak references across all devices.
We tried to use WeakHashMap, but it isn't available on mobile. We tried to
add it to mobile, but it requires ReferenceQueue and it appears that
ReferenceQueue requires support from the VM which we don't know that we
have on mobile. So this class attempts to lesson the likelyhood of
memory leaks.
As we abandoned mobile, we considered removal of this class. But replacement
by WeakHashMap is not always possible as we use mutable elements. At least
it was now possible to optimize this class using the ReferenceQueue.
/**
 * This is a helper class for handling weak references across all devices.
 * We tried to use WeakHashMap, but it isn't available on mobile. We tried to
 * add it to mobile, but it requires ReferenceQueue and it appears that
 * ReferenceQueue requires support from the VM which we don't know that we
 * have on mobile. So this class attempts to lesson the likelyhood of
 * memory leaks.
 *
 * As we abandoned mobile, we considered removal of this class. But replacement
 * by WeakHashMap is not always possible as we use mutable elements. At least
 * it was now possible to optimize this class using the ReferenceQueue.
 */
public class WeakReferenceQueue<E> {
    
Reference queue for cleared weak references
/**
     * Reference queue for cleared weak references
     */
    private final ReferenceQueue garbage = new ReferenceQueue();

    
Strongly referenced list head
/**
     * Strongly referenced list head
     */
    private Object strongRef = new Object();
    private ListEntry head = new ListEntry(strongRef, garbage);

    
Size of the queue
/**
     * Size of the queue
     */
    int size = 0;

    @SuppressWarnings("unchecked")
    public void add(E obj) {
        cleanup();
        size++;
        new ListEntry(obj, garbage).insert(head.prev);
    }

    public void remove(E obj) {
        cleanup();

        ListEntry entry = head.next;
        while (entry != head) {
            Object other = entry.get();
            if (other == obj) {
                size--;
                entry.remove();
                return;
            }
            entry = entry.next;
        }
    }

    public void cleanup() {
        ListEntry entry;
        while ((entry = (ListEntry) garbage.poll()) != null) {
            size--;
            entry.remove();
        }
    }

    public Iterator<? super E> iterator() {
        return new Iterator() {
            private ListEntry index = head;
            private Object next = null;

            public boolean hasNext() {
                next = null;
                while (next == null) {
                    ListEntry nextIndex = index.prev;
                    if (nextIndex == head) {
                        break;
                    }
                    next = nextIndex.get();
                    if (next == null) {
                        size--;
                        nextIndex.remove();
                    }
                }

                return next != null;
            }

            public Object next() {
                hasNext(); // forces us to clear out crap up to the next
                           // valid spot
                index = index.prev;
                return next;
            }

            public void remove() {
                if (index != head) {
                    ListEntry nextIndex = index.next;
                    size--;
                    index.remove();
                    index = nextIndex;
                }
            }
        };
    }

    private static class ListEntry extends WeakReference {
        ListEntry prev, next;

        public ListEntry(Object o, ReferenceQueue queue) {
            super(o, queue);
            prev = this;
            next = this;
        }

        public void insert(ListEntry where) {
            prev = where;
            next = where.next;
            where.next = this;
            next.prev = this;
        }

        public void remove() {
            prev.next = next;
            next.prev = prev;
            next = this;
            prev = this;
        }
    }

}