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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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package java.util;

import java.lang.ref.WeakReference;
import java.lang.ref.ReferenceQueue;
import java.util.concurrent.ThreadLocalRandom;
import java.util.function.BiConsumer;
import java.util.function.BiFunction;
import java.util.function.Consumer;


Hash table based implementation of the Map interface, 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 from 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 collection, 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. If the values in the map do not rely on the map holding strong references to them, 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 the iterator method of the collections 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 method, 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.
Author:
     Doug Lea,      Josh Bloch,      Mark Reinhold
Type parameters:
<K> – the type of keys maintained by this map
<V> – the type of mapped values
See Also:
HashMap
WeakReference
Since:
      1.2/**
 * Hash table based implementation of the {@code Map} interface, with
 * <em>weak keys</em>.
 * An entry in a {@code 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 from other {@code Map}
 * implementations.
 *
 * <p> Both null values and the null key are supported. This class has
 * performance characteristics similar to those of the {@code HashMap}
 * 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 {@code WeakHashMap} may be constructed using the
 * {@link Collections#synchronizedMap Collections.synchronizedMap}
 * method.
 *
 * <p> This class is intended primarily for use with key objects whose
 * {@code equals} methods test for object identity using the
 * {@code ==} 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
 * {@code WeakHashMap} at some later time and be surprised that its entry
 * has been removed.  This class will work perfectly well with key objects
 * whose {@code equals} methods are not based upon object identity, such
 * as {@code String} instances.  With such recreatable key objects,
 * however, the automatic removal of {@code WeakHashMap} entries whose
 * keys have been discarded may prove to be confusing.
 *
 * <p> The behavior of the {@code WeakHashMap} class depends in part upon
 * the actions of the garbage collector, so several familiar (though not
 * required) {@code Map} invariants do not hold for this class.  Because
 * the garbage collector may discard keys at any time, a
 * {@code WeakHashMap} may behave as though an unknown thread is silently
 * removing entries.  In particular, even if you synchronize on a
 * {@code WeakHashMap} instance and invoke none of its mutator methods, it
 * is possible for the {@code size} method to return smaller values over
 * time, for the {@code isEmpty} method to return {@code false} and
 * then {@code true}, for the {@code containsKey} method to return
 * {@code true} and later {@code false} for a given key, for the
 * {@code get} method to return a value for a given key but later return
 * {@code null}, for the {@code put} method to return
 * {@code null} and the {@code remove} method to return
 * {@code false} 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 {@code 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.
 *
 * <p> <strong>Implementation note:</strong> The value objects in a
 * {@code 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 {@code 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.  If the values
 * in the map do not rely on the map holding strong references to them, one way
 * to deal with this is to wrap values themselves within
 * {@code WeakReferences} before
 * inserting, as in: {@code m.put(key, new WeakReference(value))},
 * and then unwrapping upon each {@code get}.
 *
 * <p>The iterators returned by the {@code iterator} 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
 * {@code remove} 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 {@code ConcurrentModificationException} 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}/java.base/java/util/package-summary.html#CollectionsFramework">
 * 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.
/**
     * 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.
/**
     * 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.
/**
     * 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.
/**
     * 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.
/**
     * 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).
/**
     * The next size value at which to resize (capacity * load factor).
     */
    private int threshold;

    
The load factor for the hash table.
/**
     * The load factor for the hash table.
     */
    private final float loadFactor;

    
Reference queue for cleared WeakEntries
/**
     * 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 Also:
ConcurrentModificationException/**
     * 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 WeakHashMap with the given initial capacity and the given load factor. 
Params:
initialCapacity – The initial capacity of the WeakHashMap
loadFactor –  The load factor of the WeakHashMap
Throws:
IllegalArgumentException – if the initial capacity is negative,
        or if the load factor is nonpositive./**
     * Constructs a new, empty {@code WeakHashMap} with the given initial
     * capacity and the given load factor.
     *
     * @param  initialCapacity The initial capacity of the {@code WeakHashMap}
     * @param  loadFactor      The load factor of the {@code WeakHashMap}
     * @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 WeakHashMap with the given initial capacity and the default load factor (0.75). 
Params:
initialCapacity – The initial capacity of the WeakHashMap
Throws:
IllegalArgumentException – if the initial capacity is negative/**
     * Constructs a new, empty {@code WeakHashMap} with the given initial
     * capacity and the default load factor (0.75).
     *
     * @param  initialCapacity The initial capacity of the {@code WeakHashMap}
     * @throws IllegalArgumentException if the initial capacity is negative
     */
    public WeakHashMap(int initialCapacity) {
        this(initialCapacity, DEFAULT_LOAD_FACTOR);
    }

    
Constructs a new, empty WeakHashMap with the default initial capacity (16) and load factor (0.75). /**
     * Constructs a new, empty {@code WeakHashMap} with the default initial
     * capacity (16) and load factor (0.75).
     */
    public WeakHashMap() {
        this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR);
    }

    
Constructs a new WeakHashMap with the same mappings as the specified map. The WeakHashMap is created with the default load factor (0.75) and an initial capacity sufficient to hold the mappings in the specified map. 
Params:
m – the map whose mappings are to be placed in this map
Throws:
NullPointerException – if the specified map is null
Since:
  1.3/**
     * Constructs a new {@code WeakHashMap} with the same mappings as the
     * specified map.  The {@code WeakHashMap} 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,
                DEFAULT_INITIAL_CAPACITY),
             DEFAULT_LOAD_FACTOR);
        putAll(m);
    }

    // internal utilities

    
Value representing null keys inside tables.
/**
     * Value representing null keys inside tables.
     */
    private static final Object NULL_KEY = new Object();

    
Use NULL_KEY for key if it is null.
/**
     * 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.
/**
     * 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.
/**
     * 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);
    }

    
Retrieve object hash code and applies a supplemental hash function to the
result hash, which defends against poor quality hash functions.  This is
critical because HashMap uses power-of-two length hash tables, that
otherwise encounter collisions for hashCodes that do not differ
in lower bits.
/**
     * Retrieve object hash code and applies a supplemental hash function to the
     * result hash, which defends against poor quality hash functions.  This is
     * critical because HashMap uses power-of-two length hash tables, that
     * otherwise encounter collisions for hashCodes that do not differ
     * in lower bits.
     */
    final int hash(Object k) {
        int h = k.hashCode();

        // This function ensures that hashCodes that differ only by
        // constant multiples at each bit position have a bounded
        // number of collisions (approximately 8 at default load factor).
        h ^= (h >>> 20) ^ (h >>> 12);
        return h ^ (h >>> 7) ^ (h >>> 4);
    }

    
Returns index for hash code h.
/**
     * Returns index for hash code h.
     */
    private static int indexFor(int h, int length) {
        return h & (length-1);
    }

    
Expunges stale entries from the table.
/**
     * 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.
/**
     * 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.
/**
     * 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 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. /**
     * Returns {@code 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 size() == 0;
    }

    
Returns the value to which the specified key is mapped, or null if this map contains no mapping for the key. 
More formally, if this map contains a mapping from a key k to a value v such that Objects.equals(key, k), then this method returns v; otherwise it returns null. (There can be at most one such mapping.) 
A return value of null does not necessarily indicate that the map contains no mapping for the key; it's also possible that the map explicitly maps the key to null. The containsKey operation may be used to distinguish these two cases. 
See Also:
put(Object, Object)/**
     * 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 Objects.equals(key, 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 = hash(k);
        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 true if this map contains a mapping for the specified key. 
Params:
key –   The key whose presence in this map is to be tested
Returns:
true if there is a mapping for key; false otherwise/**
     * Returns {@code 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 {@code true} if there is a mapping for {@code key};
     *         {@code false} 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.
/**
     * 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 = hash(k);
        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.
Params:
key – key with which the specified value is to be associated.
value – value to be associated with the specified key.
Returns:
the previous value associated with key, or null if there was no mapping for key. (A null return can also indicate that the map previously associated null with key.)/**
     * 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 {@code key}, or
     *         {@code null} if there was no mapping for {@code key}.
     *         (A {@code null} return can also indicate that the map
     *         previously associated {@code null} with {@code key}.)
     */
    public V put(K key, V value) {
        Object k = maskNull(key);
        int h = hash(k);
        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.
Params:
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)./**
     * 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 /** 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.
Params:
m – mappings to be stored in this map.
Throws:
NullPointerException – if the specified map is 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<? 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 k to value v such that (key==null ?  k==null :
key.equals(k)), that mapping is removed.  (The map can contain
at most one such mapping.)
Returns the value to which this map previously associated the key, or null if the map contained no mapping for the key. A return value of null does not necessarily indicate that the map contained no mapping for the key; it's also possible that the map explicitly mapped the key to null. 
The map will not contain a mapping for the specified key once the
call returns.
Params:
key – key whose mapping is to be removed from the map
Returns:
the previous value associated with key, or null if there was no mapping for key/**
     * 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 {@code k} to
     * value {@code v} 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 {@code null} if the map contained no mapping for the key.  A
     * return value of {@code null} 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 {@code null}.
     *
     * <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 {@code key}, or
     *         {@code null} if there was no mapping for {@code key}
     */
    public V remove(Object key) {
        Object k = maskNull(key);
        int h = hash(k);
        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 /** 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 = hash(k);
        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.
/**
     * 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 true if this map maps one or more keys to the specified value. 
Params:
value – value whose presence in this map is to be tested
Returns:
true if this map maps one or more keys to the specified value/**
     * Returns {@code 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 {@code true} 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
/**
     * 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.
/**
     * 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.
/**
         * 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 Objects.hashCode(k) ^ Objects.hashCode(v);
        }

        public String toString() {
            return getKey() + "=" + getValue();
        }
    }

    private abstract class HashIterator<T> implements Iterator<T> {
        private int index;
        private Entry<K,V> entry;
        private Entry<K,V> lastReturned;
        private int expectedModCount = modCount;

        
Strong reference needed to avoid disappearance of key
between hasNext and next
/**
         * Strong reference needed to avoid disappearance of key
         * between hasNext and next
         */
        private Object nextKey;

        
Strong reference needed to avoid disappearance of key
between nextEntry() and any use of the entry
/**
         * Strong reference needed to avoid disappearance of key
         * between nextEntry() and any use of the entry
         */
        private Object currentKey;

        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 /** 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;

    
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. If the map is modified while an iteration over the set is in progress (except through the iterator's own remove operation), the results of the iteration are undefined. The set supports element removal, which removes the corresponding mapping from the map, via the Iterator.remove, Set.remove, removeAll, retainAll, and clear operations. It does not support the add or addAll operations. /**
     * 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 {@code remove} operation), the results of
     * the iteration are undefined.  The set supports element removal,
     * which removes the corresponding mapping from the map, via the
     * {@code Iterator.remove}, {@code Set.remove},
     * {@code removeAll}, {@code retainAll}, and {@code clear}
     * operations.  It does not support the {@code add} or {@code addAll}
     * operations.
     */
    public Set<K> keySet() {
        Set<K> ks = keySet;
        if (ks == null) {
            ks = new KeySet();
            keySet = ks;
        }
        return ks;
    }

    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();
        }

        public Spliterator<K> spliterator() {
            return new KeySpliterator<>(WeakHashMap.this, 0, -1, 0, 0);
        }
    }

    
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. If the map is modified while an iteration over the collection is in progress (except through the iterator's own remove operation), the results of the iteration are undefined. 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. /**
     * 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 {@code remove} operation),
     * the results of the iteration are undefined.  The collection
     * supports element removal, which removes the corresponding
     * mapping from the map, via the {@code Iterator.remove},
     * {@code Collection.remove}, {@code removeAll},
     * {@code retainAll} and {@code clear} operations.  It does not
     * support the {@code add} or {@code addAll} operations.
     */
    public Collection<V> values() {
        Collection<V> vs = values;
        if (vs == null) {
            vs = new Values();
            values = vs;
        }
        return vs;
    }

    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();
        }

        public Spliterator<V> spliterator() {
            return new ValueSpliterator<>(WeakHashMap.this, 0, -1, 0, 0);
        }
    }

    
Returns a 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 remove operation, or through the setValue 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 Iterator.remove, Set.remove, removeAll, retainAll and clear operations. It does not support the add or addAll operations. /**
     * 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 {@code remove} operation, or through the
     * {@code setValue} 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 {@code Iterator.remove},
     * {@code Set.remove}, {@code removeAll}, {@code retainAll} and
     * {@code clear} operations.  It does not support the
     * {@code add} or {@code addAll} 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);
        }

        public Spliterator<Map.Entry<K,V>> spliterator() {
            return new EntrySpliterator<>(WeakHashMap.this, 0, -1, 0, 0);
        }
    }

    @SuppressWarnings("unchecked")
    @Override
    public void forEach(BiConsumer<? super K, ? super V> action) {
        Objects.requireNonNull(action);
        int expectedModCount = modCount;

        Entry<K, V>[] tab = getTable();
        for (Entry<K, V> entry : tab) {
            while (entry != null) {
                Object key = entry.get();
                if (key != null) {
                    action.accept((K)WeakHashMap.unmaskNull(key), entry.value);
                }
                entry = entry.next;

                if (expectedModCount != modCount) {
                    throw new ConcurrentModificationException();
                }
            }
        }
    }

    @SuppressWarnings("unchecked")
    @Override
    public void replaceAll(BiFunction<? super K, ? super V, ? extends V> function) {
        Objects.requireNonNull(function);
        int expectedModCount = modCount;

        Entry<K, V>[] tab = getTable();;
        for (Entry<K, V> entry : tab) {
            while (entry != null) {
                Object key = entry.get();
                if (key != null) {
                    entry.value = function.apply((K)WeakHashMap.unmaskNull(key), entry.value);
                }
                entry = entry.next;

                if (expectedModCount != modCount) {
                    throw new ConcurrentModificationException();
                }
            }
        }
    }

    
Similar form as other hash Spliterators, but skips dead
elements.
/**
     * Similar form as other hash Spliterators, but skips dead
     * elements.
     */
    static class WeakHashMapSpliterator<K,V> {
        final WeakHashMap<K,V> map;
        WeakHashMap.Entry<K,V> current; // current node
        int index;             // current index, modified on advance/split
        int fence;             // -1 until first use; then one past last index
        int est;               // size estimate
        int expectedModCount;  // for comodification checks

        WeakHashMapSpliterator(WeakHashMap<K,V> m, int origin,
                               int fence, int est,
                               int expectedModCount) {
            this.map = m;
            this.index = origin;
            this.fence = fence;
            this.est = est;
            this.expectedModCount = expectedModCount;
        }

        final int getFence() { // initialize fence and size on first use
            int hi;
            if ((hi = fence) < 0) {
                WeakHashMap<K,V> m = map;
                est = m.size();
                expectedModCount = m.modCount;
                hi = fence = m.table.length;
            }
            return hi;
        }

        public final long estimateSize() {
            getFence(); // force init
            return (long) est;
        }
    }

    static final class KeySpliterator<K,V>
        extends WeakHashMapSpliterator<K,V>
        implements Spliterator<K> {
        KeySpliterator(WeakHashMap<K,V> m, int origin, int fence, int est,
                       int expectedModCount) {
            super(m, origin, fence, est, expectedModCount);
        }

        public KeySpliterator<K,V> trySplit() {
            int hi = getFence(), lo = index, mid = (lo + hi) >>> 1;
            return (lo >= mid) ? null :
                new KeySpliterator<>(map, lo, index = mid, est >>>= 1,
                                     expectedModCount);
        }

        public void forEachRemaining(Consumer<? super K> action) {
            int i, hi, mc;
            if (action == null)
                throw new NullPointerException();
            WeakHashMap<K,V> m = map;
            WeakHashMap.Entry<K,V>[] tab = m.table;
            if ((hi = fence) < 0) {
                mc = expectedModCount = m.modCount;
                hi = fence = tab.length;
            }
            else
                mc = expectedModCount;
            if (tab.length >= hi && (i = index) >= 0 &&
                (i < (index = hi) || current != null)) {
                WeakHashMap.Entry<K,V> p = current;
                current = null; // exhaust
                do {
                    if (p == null)
                        p = tab[i++];
                    else {
                        Object x = p.get();
                        p = p.next;
                        if (x != null) {
                            @SuppressWarnings("unchecked") K k =
                                (K) WeakHashMap.unmaskNull(x);
                            action.accept(k);
                        }
                    }
                } while (p != null || i < hi);
            }
            if (m.modCount != mc)
                throw new ConcurrentModificationException();
        }

        public boolean tryAdvance(Consumer<? super K> action) {
            int hi;
            if (action == null)
                throw new NullPointerException();
            WeakHashMap.Entry<K,V>[] tab = map.table;
            if (tab.length >= (hi = getFence()) && index >= 0) {
                while (current != null || index < hi) {
                    if (current == null)
                        current = tab[index++];
                    else {
                        Object x = current.get();
                        current = current.next;
                        if (x != null) {
                            @SuppressWarnings("unchecked") K k =
                                (K) WeakHashMap.unmaskNull(x);
                            action.accept(k);
                            if (map.modCount != expectedModCount)
                                throw new ConcurrentModificationException();
                            return true;
                        }
                    }
                }
            }
            return false;
        }

        public int characteristics() {
            return Spliterator.DISTINCT;
        }
    }

    static final class ValueSpliterator<K,V>
        extends WeakHashMapSpliterator<K,V>
        implements Spliterator<V> {
        ValueSpliterator(WeakHashMap<K,V> m, int origin, int fence, int est,
                         int expectedModCount) {
            super(m, origin, fence, est, expectedModCount);
        }

        public ValueSpliterator<K,V> trySplit() {
            int hi = getFence(), lo = index, mid = (lo + hi) >>> 1;
            return (lo >= mid) ? null :
                new ValueSpliterator<>(map, lo, index = mid, est >>>= 1,
                                       expectedModCount);
        }

        public void forEachRemaining(Consumer<? super V> action) {
            int i, hi, mc;
            if (action == null)
                throw new NullPointerException();
            WeakHashMap<K,V> m = map;
            WeakHashMap.Entry<K,V>[] tab = m.table;
            if ((hi = fence) < 0) {
                mc = expectedModCount = m.modCount;
                hi = fence = tab.length;
            }
            else
                mc = expectedModCount;
            if (tab.length >= hi && (i = index) >= 0 &&
                (i < (index = hi) || current != null)) {
                WeakHashMap.Entry<K,V> p = current;
                current = null; // exhaust
                do {
                    if (p == null)
                        p = tab[i++];
                    else {
                        Object x = p.get();
                        V v = p.value;
                        p = p.next;
                        if (x != null)
                            action.accept(v);
                    }
                } while (p != null || i < hi);
            }
            if (m.modCount != mc)
                throw new ConcurrentModificationException();
        }

        public boolean tryAdvance(Consumer<? super V> action) {
            int hi;
            if (action == null)
                throw new NullPointerException();
            WeakHashMap.Entry<K,V>[] tab = map.table;
            if (tab.length >= (hi = getFence()) && index >= 0) {
                while (current != null || index < hi) {
                    if (current == null)
                        current = tab[index++];
                    else {
                        Object x = current.get();
                        V v = current.value;
                        current = current.next;
                        if (x != null) {
                            action.accept(v);
                            if (map.modCount != expectedModCount)
                                throw new ConcurrentModificationException();
                            return true;
                        }
                    }
                }
            }
            return false;
        }

        public int characteristics() {
            return 0;
        }
    }

    static final class EntrySpliterator<K,V>
        extends WeakHashMapSpliterator<K,V>
        implements Spliterator<Map.Entry<K,V>> {
        EntrySpliterator(WeakHashMap<K,V> m, int origin, int fence, int est,
                       int expectedModCount) {
            super(m, origin, fence, est, expectedModCount);
        }

        public EntrySpliterator<K,V> trySplit() {
            int hi = getFence(), lo = index, mid = (lo + hi) >>> 1;
            return (lo >= mid) ? null :
                new EntrySpliterator<>(map, lo, index = mid, est >>>= 1,
                                       expectedModCount);
        }


        public void forEachRemaining(Consumer<? super Map.Entry<K, V>> action) {
            int i, hi, mc;
            if (action == null)
                throw new NullPointerException();
            WeakHashMap<K,V> m = map;
            WeakHashMap.Entry<K,V>[] tab = m.table;
            if ((hi = fence) < 0) {
                mc = expectedModCount = m.modCount;
                hi = fence = tab.length;
            }
            else
                mc = expectedModCount;
            if (tab.length >= hi && (i = index) >= 0 &&
                (i < (index = hi) || current != null)) {
                WeakHashMap.Entry<K,V> p = current;
                current = null; // exhaust
                do {
                    if (p == null)
                        p = tab[i++];
                    else {
                        Object x = p.get();
                        V v = p.value;
                        p = p.next;
                        if (x != null) {
                            @SuppressWarnings("unchecked") K k =
                                (K) WeakHashMap.unmaskNull(x);
                            action.accept
                                (new AbstractMap.SimpleImmutableEntry<>(k, v));
                        }
                    }
                } while (p != null || i < hi);
            }
            if (m.modCount != mc)
                throw new ConcurrentModificationException();
        }

        public boolean tryAdvance(Consumer<? super Map.Entry<K,V>> action) {
            int hi;
            if (action == null)
                throw new NullPointerException();
            WeakHashMap.Entry<K,V>[] tab = map.table;
            if (tab.length >= (hi = getFence()) && index >= 0) {
                while (current != null || index < hi) {
                    if (current == null)
                        current = tab[index++];
                    else {
                        Object x = current.get();
                        V v = current.value;
                        current = current.next;
                        if (x != null) {
                            @SuppressWarnings("unchecked") K k =
                                (K) WeakHashMap.unmaskNull(x);
                            action.accept
                                (new AbstractMap.SimpleImmutableEntry<>(k, v));
                            if (map.modCount != expectedModCount)
                                throw new ConcurrentModificationException();
                            return true;
                        }
                    }
                }
            }
            return false;
        }

        public int characteristics() {
            return Spliterator.DISTINCT;
        }
    }

}