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  * 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

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  * 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);

         }

     }

 }