/*

 * Copyright (c) 1998, 2010, 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 java.util;

import java.lang.ref.WeakReference;

import java.lang.ref.ReferenceQueue;

/**

 * Hash table based implementation of the <tt>Map</tt> interface, with

 * <em>weak keys</em>.

 * An entry in a <tt>WeakHashMap</tt> 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 <tt>Map</tt>

 * implementations.

 *

 * <p> Both null values and the null key are supported. This class has

 * performance characteristics similar to those of the <tt>HashMap</tt>

 * class, and has the same efficiency parameters of <em>initial capacity</em>

 * and <em>load factor</em>.

 *

 * <p> Like most collection classes, this class is not synchronized.

 * A synchronized <tt>WeakHashMap</tt> may be constructed using the

 * {@link Collections#synchronizedMap Collections.synchronizedMap}

 * method.

 *

 * <p> This class is intended primarily for use with key objects whose

 * <tt>equals</tt> methods test for object identity using the

 * <tt>==</tt> 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

 * <tt>WeakHashMap</tt> at some later time and be surprised that its entry

 * has been removed.  This class will work perfectly well with key objects

 * whose <tt>equals</tt> methods are not based upon object identity, such

 * as <tt>String</tt> instances.  With such recreatable key objects,

 * however, the automatic removal of <tt>WeakHashMap</tt> entries whose

 * keys have been discarded may prove to be confusing.

 *

 * <p> The behavior of the <tt>WeakHashMap</tt> class depends in part upon

 * the actions of the garbage collector, so several familiar (though not

 * required) <tt>Map</tt> invariants do not hold for this class.  Because

 * the garbage collector may discard keys at any time, a

 * <tt>WeakHashMap</tt> may behave as though an unknown thread is silently

 * removing entries.  In particular, even if you synchronize on a

 * <tt>WeakHashMap</tt> instance and invoke none of its mutator methods, it

 * is possible for the <tt>size</tt> method to return smaller values over

 * time, for the <tt>isEmpty</tt> method to return <tt>false</tt> and

 * then <tt>true</tt>, for the <tt>containsKey</tt> method to return

 * <tt>true</tt> and later <tt>false</tt> for a given key, for the

 * <tt>get</tt> method to return a value for a given key but later return

 * <tt>null</tt>, for the <tt>put</tt> method to return

 * <tt>null</tt> and the <tt>remove</tt> method to return

 * <tt>false</tt> 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.

 *

 * <p> Each key object in a <tt>WeakHashMap</tt> 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.

 *

 * <p> <strong>Implementation note:</strong> The value objects in a

 * <tt>WeakHashMap</tt> 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 <tt>WeakHashMap</tt> 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

 * <tt>WeakReferences</tt> before

 * inserting, as in: <tt>m.put(key, new WeakReference(value))</tt>,

 * and then unwrapping upon each <tt>get</tt>.

 *

 * <p>The iterators returned by the <tt>iterator</tt> method of the collections

 * returned by all of this class's "collection view methods" are

 * <i>fail-fast</i>: if the map is structurally modified at any time after the

 * iterator is created, in any way except through the iterator's own

 * <tt>remove</tt> 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.

 *

 * <p>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 <tt>ConcurrentModificationException</tt> on a best-effort basis.

 * Therefore, it would be wrong to write a program that depended on this

 * exception for its correctness:  <i>the fail-fast behavior of iterators

 * should be used only to detect bugs.</i>

 *

 * <p>This class is a member of the

 * <a href="{@docRoot}/../technotes/guides/collections/index.html">

 * Java Collections Framework</a>.

 *

 * @param <K> the type of keys maintained by this map

 * @param <V> the type of mapped values

 *

 * @author      Doug Lea

 * @author      Josh Bloch

 * @author      Mark Reinhold

 * @since       1.2

 * @see         java.util.HashMap

 * @see         java.lang.ref.WeakReference

 */

public class WeakHashMap<K,V>

    extends AbstractMap<K,V>

    implements Map<K,V> {

    /**

     * The default initial capacity -- MUST be a power of two.

     */

    private static final int DEFAULT_INITIAL_CAPACITY = 16;

    /**

     * The maximum capacity, used if a higher value is implicitly specified

     * by either of the constructors with arguments.

     * MUST be a power of two <= 1<<30.

     */

    private static final int MAXIMUM_CAPACITY = 1 << 30;

    /**

     * The load factor used when none specified in constructor.

     */

    private static final float DEFAULT_LOAD_FACTOR = 0.75f;

    /**

     * The table, resized as necessary. Length MUST Always be a power of two.

     */

    Entry<K,V>[] table;

    /**

     * The number of key-value mappings contained in this weak hash map.

     */

    private int size;

    /**

     * The next size value at which to resize (capacity * load factor).

     */

    private int threshold;

    /**

     * The load factor for the hash table.

     */

    private final float loadFactor;

    /**

     * Reference queue for cleared WeakEntries

     */

    private final ReferenceQueue<Object> queue = new ReferenceQueue<>();

    /**

     * The number of times this WeakHashMap has been structurally modified.

     * Structural modifications are those that change the number of

     * mappings in the map or otherwise modify its internal structure

     * (e.g., rehash).  This field is used to make iterators on

     * Collection-views of the map fail-fast.

     *

     * @see ConcurrentModificationException

     */

    int modCount;

    @SuppressWarnings("unchecked")

    private Entry<K,V>[] newTable(int n) {

        return (Entry<K,V>[]) new Entry[n];

    }

    /**

     * Constructs a new, empty <tt>WeakHashMap</tt> with the given initial

     * capacity and the given load factor.

     *

     * @param  initialCapacity The initial capacity of the <tt>WeakHashMap</tt>

     * @param  loadFactor      The load factor of the <tt>WeakHashMap</tt>

     * @throws IllegalArgumentException if the initial capacity is negative,

     *         or if the load factor is nonpositive.

     */

    public WeakHashMap(int initialCapacity, float loadFactor) {

        if (initialCapacity < 0)

            throw new IllegalArgumentException("Illegal Initial Capacity: "+

                                               initialCapacity);

        if (initialCapacity > MAXIMUM_CAPACITY)

            initialCapacity = MAXIMUM_CAPACITY;

        if (loadFactor <= 0 || Float.isNaN(loadFactor))

            throw new IllegalArgumentException("Illegal Load factor: "+

                                               loadFactor);

        int capacity = 1;

        while (capacity < initialCapacity)

            capacity <<= 1;

        table = newTable(capacity);

        this.loadFactor = loadFactor;

        threshold = (int)(capacity * loadFactor);

    }

    /**

     * Constructs a new, empty <tt>WeakHashMap</tt> with the given initial

     * capacity and the default load factor (0.75).

     *

     * @param  initialCapacity The initial capacity of the <tt>WeakHashMap</tt>

     * @throws IllegalArgumentException if the initial capacity is negative

     */

    public WeakHashMap(int initialCapacity) {

        this(initialCapacity, DEFAULT_LOAD_FACTOR);

    }

    /**

     * Constructs a new, empty <tt>WeakHashMap</tt> with the default initial

     * capacity (16) and load factor (0.75).

     */

    public WeakHashMap() {

        this.loadFactor = DEFAULT_LOAD_FACTOR;

        threshold = DEFAULT_INITIAL_CAPACITY;

        table = newTable(DEFAULT_INITIAL_CAPACITY);

    }

    /**

     * Constructs a new <tt>WeakHashMap</tt> with the same mappings as the

     * specified map.  The <tt>WeakHashMap</tt> is created with the default

     * load factor (0.75) and an initial capacity sufficient to hold the

     * mappings in the specified map.

     *

     * @param   m 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<? extends K, ? extends V> m) {

        this(Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1, 16),

             DEFAULT_LOAD_FACTOR);

        putAll(m);

    }

    // internal utilities

    /**

     * Value representing null keys inside tables.

     */

    private static final Object NULL_KEY = new Object();

    /**

     * Use NULL_KEY for key if it is null.

     */

    private static Object maskNull(Object key) {

        return (key == null) ? NULL_KEY : key;

    }

    /**

     * Returns internal representation of null key back to caller as null.

     */

    static Object unmaskNull(Object key) {

        return (key == NULL_KEY) ? null : key;

    }

    /**

     * Checks for equality of non-null reference x and possibly-null y.  By

     * default uses Object.equals.

     */

    private static boolean eq(Object x, Object y) {

        return x == y || x.equals(y);

    }

    /**

     * Returns index for hash code h.

     */

    private static int indexFor(int h, int length) {

        return h & (length-1);

    }

    /**

     * Expunges stale entries from the table.

     */

    private void expungeStaleEntries() {

        for (Object x; (x = queue.poll()) != null; ) {

            synchronized (queue) {

                @SuppressWarnings("unchecked")

                    Entry<K,V> e = (Entry<K,V>) x;

                int i = indexFor(e.hash, table.length);

                Entry<K,V> prev = table[i];

                Entry<K,V> p = prev;

                while (p != null) {

                    Entry<K,V> next = p.next;

                    if (p == e) {

                        if (prev == e)

                            table[i] = next;

                        else

                            prev.next = next;

                        // Must not null out e.next;

                        // stale entries may be in use by a HashIterator

                        e.value = null; // Help GC

                        size--;

                        break;

                    }

                    prev = p;

                    p = next;

                }

            }

        }

    }

    /**

     * Returns the table after first expunging stale entries.

     */

    private Entry<K,V>[] getTable() {

        expungeStaleEntries();

        return table;

    }

    /**

     * 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() {

        if (size == 0)

            return 0;

        expungeStaleEntries();

        return size;

    }

    /**

     * Returns <tt>true</tt> 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 size() == 0;

    }

    /**

     * Returns the value to which the specified key is mapped,

     * or {@code null} if this map contains no mapping for the key.

     *

     * <p>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.)

     *

     * <p>A return value of {@code null} does not <i>necessarily</i>

     * 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.

     *

     * @see #put(Object, Object)

     */

    public V get(Object key) {

        Object k = maskNull(key);

        int h = HashMap.hash(k.hashCode());

        Entry<K,V>[] tab = getTable();

        int index = indexFor(h, tab.length);

        Entry<K,V> e = tab[index];

        while (e != null) {

            if (e.hash == h && eq(k, e.get()))

                return e.value;

            e = e.next;

        }

        return null;

    }

    /**

     * Returns <tt>true</tt> if this map contains a mapping for the

     * specified key.

     *

     * @param  key   The key whose presence in this map is to be tested

     * @return <tt>true</tt> if there is a mapping for <tt>key</tt>;

     *         <tt>false</tt> otherwise

     */

    public boolean containsKey(Object key) {

        return getEntry(key) != null;

    }

    /**

     * Returns the entry associated with the specified key in this map.

     * Returns null if the map contains no mapping for this key.

     */

    Entry<K,V> getEntry(Object key) {

        Object k = maskNull(key);

        int h = HashMap.hash(k.hashCode());

        Entry<K,V>[] tab = getTable();

        int index = indexFor(h, tab.length);

        Entry<K,V> e = tab[index];

        while (e != null && !(e.hash == h && eq(k, e.get())))

            e = e.next;

        return e;

    }

    /**

     * 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 the previous value associated with <tt>key</tt>, or

     *         <tt>null</tt> if there was no mapping for <tt>key</tt>.

     *         (A <tt>null</tt> return can also indicate that the map

     *         previously associated <tt>null</tt> with <tt>key</tt>.)

     */

    public V put(K key, V value) {

        Object k = maskNull(key);

        int h = HashMap.hash(k.hashCode());

        Entry<K,V>[] tab = getTable();

        int i = indexFor(h, tab.length);

        for (Entry<K,V> e = tab[i]; e != null; e = e.next) {

            if (h == e.hash && eq(k, e.get())) {

                V oldValue = e.value;

                if (value != oldValue)

                    e.value = value;

                return oldValue;

            }

        }

        modCount++;

        Entry<K,V> e = tab[i];

        tab[i] = new Entry<>(k, value, queue, h, e);

        if (++size >= threshold)

            resize(tab.length * 2);

        return null;

    }

    /**

     * Rehashes the contents of this map into a new array with a

     * larger capacity.  This method is called automatically when the

     * number of keys in this map reaches its threshold.

     *

     * If current capacity is MAXIMUM_CAPACITY, this method does not

     * resize the map, but sets threshold to Integer.MAX_VALUE.

     * This has the effect of preventing future calls.

     *

     * @param newCapacity the new capacity, MUST be a power of two;

     *        must be greater than current capacity unless current

     *        capacity is MAXIMUM_CAPACITY (in which case value

     *        is irrelevant).

     */

    void resize(int newCapacity) {

        Entry<K,V>[] oldTable = getTable();

        int oldCapacity = oldTable.length;

        if (oldCapacity == MAXIMUM_CAPACITY) {

            threshold = Integer.MAX_VALUE;

            return;

        }

        Entry<K,V>[] newTable = newTable(newCapacity);

        transfer(oldTable, newTable);

        table = newTable;

        /*

         * If ignoring null elements and processing ref queue caused massive

         * shrinkage, then restore old table.  This should be rare, but avoids

         * unbounded expansion of garbage-filled tables.

         */

        if (size >= threshold / 2) {

            threshold = (int)(newCapacity * loadFactor);

        } else {

            expungeStaleEntries();

            transfer(newTable, oldTable);

            table = oldTable;

        }

    }

    /** Transfers all entries from src to dest tables */

    private void transfer(Entry<K,V>[] src, Entry<K,V>[] dest) {

        for (int j = 0; j < src.length; ++j) {

            Entry<K,V> e = src[j];

            src[j] = null;

            while (e != null) {

                Entry<K,V> next = e.next;

                Object key = e.get();

                if (key == null) {

                    e.next = null;  // Help GC

                    e.value = null; //  "   "

                    size--;

                } else {

                    int i = indexFor(e.hash, dest.length);

                    e.next = dest[i];

                    dest[i] = e;

                }

                e = next;

            }

        }

    }

    /**

     * 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<? extends K, ? extends V> m) {

        int numKeysToBeAdded = m.size();

        if (numKeysToBeAdded == 0)

            return;

        /*

         * Expand the map if the map if the number of mappings to be added

         * is greater than or equal to threshold.  This is conservative; the

         * obvious condition is (m.size() + size) >= threshold, but this

         * condition could result in a map with twice the appropriate capacity,

         * if the keys to be added overlap with the keys already in this map.

         * By using the conservative calculation, we subject ourself

         * to at most one extra resize.

         */

        if (numKeysToBeAdded > threshold) {

            int targetCapacity = (int)(numKeysToBeAdded / loadFactor + 1);

            if (targetCapacity > MAXIMUM_CAPACITY)

                targetCapacity = MAXIMUM_CAPACITY;

            int newCapacity = table.length;

            while (newCapacity < targetCapacity)

                newCapacity <<= 1;

            if (newCapacity > table.length)

                resize(newCapacity);

        }

        for (Map.Entry<? extends K, ? extends V> e : m.entrySet())

            put(e.getKey(), e.getValue());

    }

    /**

     * 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 <tt>k</tt> to

     * value <tt>v</tt> such that <code>(key==null ?  k==null :

     * key.equals(k))</code>, that mapping is removed.  (The map can contain

     * at most one such mapping.)

     *

     * <p>Returns the value to which this map previously associated the key,

     * or <tt>null</tt> if the map contained no mapping for the key.  A

     * return value of <tt>null</tt> does not <i>necessarily</i> indicate

     * that the map contained no mapping for the key; it's also possible

     * that the map explicitly mapped the key to <tt>null</tt>.

     *

     * <p>The map will not contain a mapping for the specified key once the

     * call returns.

     *

     * @param key key whose mapping is to be removed from the map

     * @return the previous value associated with <tt>key</tt>, or

     *         <tt>null</tt> if there was no mapping for <tt>key</tt>

     */

    public V remove(Object key) {

        Object k = maskNull(key);

        int h = HashMap.hash(k.hashCode());

        Entry<K,V>[] tab = getTable();

        int i = indexFor(h, tab.length);

        Entry<K,V> prev = tab[i];

        Entry<K,V> e = prev;

        while (e != null) {

            Entry<K,V> next = e.next;

            if (h == e.hash && eq(k, e.get())) {

                modCount++;

                size--;

                if (prev == e)

                    tab[i] = next;

                else

                    prev.next = next;

                return e.value;

            }

            prev = e;

            e = next;

        }

        return null;

    }

    /** Special version of remove needed by Entry set */

    boolean removeMapping(Object o) {

        if (!(o instanceof Map.Entry))

            return false;

        Entry<K,V>[] tab = getTable();

        Map.Entry<?,?> entry = (Map.Entry<?,?>)o;

        Object k = maskNull(entry.getKey());

        int h = HashMap.hash(k.hashCode());

        int i = indexFor(h, tab.length);

        Entry<K,V> prev = tab[i];

        Entry<K,V> e = prev;

        while (e != null) {

            Entry<K,V> next = e.next;

            if (h == e.hash && e.equals(entry)) {

                modCount++;

                size--;

                if (prev == e)

                    tab[i] = next;

                else

                    prev.next = next;

                return true;

            }

            prev = e;

            e = next;

        }

        return false;

    }

    /**

     * Removes all of the mappings from this map.

     * The map will be empty after this call returns.

     */

    public void clear() {

        // clear out ref queue. We don't need to expunge entries

        // since table is getting cleared.

        while (queue.poll() != null)

            ;

        modCount++;

        Arrays.fill(table, null);

        size = 0;

        // Allocation of array may have caused GC, which may have caused

        // additional entries to go stale.  Removing these entries from the

        // reference queue will make them eligible for reclamation.

        while (queue.poll() != null)

            ;

    }

    /**

     * Returns <tt>true</tt> 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 <tt>true</tt> if this map maps one or more keys to the

     *         specified value

     */

    public boolean containsValue(Object value) {

        if (value==null)

            return containsNullValue();

        Entry<K,V>[] tab = getTable();

        for (int i = tab.length; i-- > 0;)

            for (Entry<K,V> e = tab[i]; e != null; e = e.next)

                if (value.equals(e.value))

                    return true;

        return false;

    }

    /**

     * Special-case code for containsValue with null argument

     */

    private boolean containsNullValue() {

        Entry<K,V>[] tab = getTable();

        for (int i = tab.length; i-- > 0;)

            for (Entry<K,V> e = tab[i]; e != null; e = e.next)

                if (e.value==null)

                    return true;

        return false;

    }

    /**

     * The entries in this hash table extend WeakReference, using its main ref

     * field as the key.

     */

    private static class Entry<K,V> extends WeakReference<Object> implements Map.Entry<K,V> {

        V value;

        final int hash;

        Entry<K,V> next;

        /**

         * Creates new entry.

         */

        Entry(Object key, V value,

              ReferenceQueue<Object> queue,

              int hash, Entry<K,V> next) {

            super(key, queue);

            this.value = value;

            this.hash  = hash;

            this.next  = next;

        }

        @SuppressWarnings("unchecked")

        public K getKey() {

            return (K) WeakHashMap.unmaskNull(get());

        }

        public V getValue() {

            return value;

        }

        public V setValue(V newValue) {

            V oldValue = value;

            value = newValue;

            return oldValue;

        }

        public boolean equals(Object o) {

            if (!(o instanceof Map.Entry))

                return false;

            Map.Entry<?,?> e = (Map.Entry<?,?>)o;

            K k1 = getKey();

            Object k2 = e.getKey();

            if (k1 == k2 || (k1 != null && k1.equals(k2))) {

                V v1 = getValue();

                Object v2 = e.getValue();

                if (v1 == v2 || (v1 != null && v1.equals(v2)))

                    return true;

            }

            return false;

        }

        public int hashCode() {

            K k = getKey();

            V v = getValue();

            return ((k==null ? 0 : k.hashCode()) ^

                    (v==null ? 0 : v.hashCode()));

        }

        public String toString() {

            return getKey() + "=" + getValue();

        }

    }

    private abstract class HashIterator<T> implements Iterator<T> {

        private int index;

        private Entry<K,V> entry = null;

        private Entry<K,V> lastReturned = null;

        private int expectedModCount = modCount;

        /**

         * Strong reference needed to avoid disappearance of key

         * between hasNext and next

         */

        private Object nextKey = null;

        /**

         * Strong reference needed to avoid disappearance of key

         * between nextEntry() and any use of the entry

         */

        private Object currentKey = null;

        HashIterator() {

            index = isEmpty() ? 0 : table.length;

        }

        public boolean hasNext() {

            Entry<K,V>[] t = table;

            while (nextKey == null) {

                Entry<K,V> e = entry;

                int i = index;

                while (e == null && i > 0)

                    e = t[--i];

                entry = e;

                index = i;

                if (e == null) {

                    currentKey = null;

                    return false;

                }

                nextKey = e.get(); // hold on to key in strong ref

                if (nextKey == null)

                    entry = entry.next;

            }

            return true;

        }

        /** The common parts of next() across different types of iterators */

        protected Entry<K,V> nextEntry() {

            if (modCount != expectedModCount)

                throw new ConcurrentModificationException();

            if (nextKey == null && !hasNext())

                throw new NoSuchElementException();

            lastReturned = entry;

            entry = entry.next;

            currentKey = nextKey;

            nextKey = null;

            return lastReturned;

        }

        public void remove() {

            if (lastReturned == null)

                throw new IllegalStateException();

            if (modCount != expectedModCount)

                throw new ConcurrentModificationException();

            WeakHashMap.this.remove(currentKey);

            expectedModCount = modCount;

            lastReturned = null;

            currentKey = null;

        }

    }

    private class ValueIterator extends HashIterator<V> {

        public V next() {

            return nextEntry().value;

        }

    }

    private class KeyIterator extends HashIterator<K> {

        public K next() {

            return nextEntry().getKey();

        }

    }

    private class EntryIterator extends HashIterator<Map.Entry<K,V>> {

        public Map.Entry<K,V> next() {

            return nextEntry();

        }

    }

    // Views

    private transient Set<Map.Entry<K,V>> entrySet = null;

    /**

     * 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 <tt>remove</tt> operation), the results of

     * the iteration are undefined.  The set supports element removal,

     * which removes the corresponding mapping from the map, via the

     * <tt>Iterator.remove</tt>, <tt>Set.remove</tt>,

     * <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt>

     * operations.  It does not support the <tt>add</tt> or <tt>addAll</tt>

     * operations.

     */

    public Set<K> keySet() {

        Set<K> ks = keySet;

        return (ks != null ? ks : (keySet = new KeySet()));

    }

    private class KeySet extends AbstractSet<K> {

        public Iterator<K> iterator() {

            return new KeyIterator();

        }

        public int size() {

            return WeakHashMap.this.size();

        }

        public boolean contains(Object o) {

            return containsKey(o);

        }

        public boolean remove(Object o) {

            if (containsKey(o)) {

                WeakHashMap.this.remove(o);

                return true;

            }

            else

                return false;

        }

        public void clear() {

            WeakHashMap.this.clear();

        }

    }

    /**

     * 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 <tt>remove</tt> operation),

     * the results of the iteration are undefined.  The collection

     * supports element removal, which removes the corresponding

     * mapping from the map, via the <tt>Iterator.remove</tt>,

     * <tt>Collection.remove</tt>, <tt>removeAll</tt>,

     * <tt>retainAll</tt> and <tt>clear</tt> operations.  It does not

     * support the <tt>add</tt> or <tt>addAll</tt> operations.

     */

    public Collection<V> values() {

        Collection<V> vs = values;

        return (vs != null) ? vs : (values = new Values());

    }

    private class Values extends AbstractCollection<V> {

        public Iterator<V> iterator() {

            return new ValueIterator();

        }

        public int size() {

            return WeakHashMap.this.size();

        }

        public boolean contains(Object o) {

            return containsValue(o);

        }

        public void clear() {

            WeakHashMap.this.clear();

        }

    }

    /**

     * 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 <tt>remove</tt> operation, or through the

     * <tt>setValue</tt> 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 <tt>Iterator.remove</tt>,

     * <tt>Set.remove</tt>, <tt>removeAll</tt>, <tt>retainAll</tt> and

     * <tt>clear</tt> operations.  It does not support the

     * <tt>add</tt> or <tt>addAll</tt> operations.

     */

    public Set<Map.Entry<K,V>> entrySet() {

        Set<Map.Entry<K,V>> es = entrySet;

        return es != null ? es : (entrySet = new EntrySet());

    }

    private class EntrySet extends AbstractSet<Map.Entry<K,V>> {

        public Iterator<Map.Entry<K,V>> iterator() {

            return new EntryIterator();

        }

        public boolean contains(Object o) {

            if (!(o instanceof Map.Entry))

                return false;

            Map.Entry<?,?> e = (Map.Entry<?,?>)o;

            Entry<K,V> candidate = getEntry(e.getKey());

            return candidate != null && candidate.equals(e);

        }

        public boolean remove(Object o) {

            return removeMapping(o);

        }

        public int size() {

            return WeakHashMap.this.size();

        }

        public void clear() {

            WeakHashMap.this.clear();

        }

        private List<Map.Entry<K,V>> deepCopy() {

            List<Map.Entry<K,V>> list = new ArrayList<>(size());

            for (Map.Entry<K,V> e : this)

                list.add(new AbstractMap.SimpleEntry<>(e));

            return list;

        }

        public Object[] toArray() {

            return deepCopy().toArray();

        }

        public <T> T[] toArray(T[] a) {

            return deepCopy().toArray(a);

        }

    }

}