java.lang.Object
   ?     java.util.AbstractMap<K, V>
         ?     java.util.WeakHashMap<K, V>
public class WeakHashMap<K,V>
    extends AbstractMap<K,V>
    implements Map<K,V> {}

package java.util;
import java.lang.ref.WeakReference;
import java.lang.ref.ReferenceQueue;
public class WeakHashMap<K,V>
    extends AbstractMap<K,V>
    implements Map<K,V> {
    // 默認(rèn)的初始容量是16，必須是2的冪。
    private static final int DEFAULT_INITIAL_CAPACITY = 16;
    // 最大容量（必須是2的冪且小于2的30次方，傳入容量過(guò)大將被這個(gè)值替換）
    private static final int MAXIMUM_CAPACITY = 1 << 30;
    // 默認(rèn)加載因子
    private static final float DEFAULT_LOAD_FACTOR = 0.75f;
    // 存儲(chǔ)數(shù)據(jù)的Entry數(shù)組，長(zhǎng)度是2的冪。
    // WeakHashMap是采用拉鏈法實(shí)現(xiàn)的，每一個(gè)Entry本質(zhì)上是一個(gè)單向鏈表
    Entry<K,V>[] table;
    // WeakHashMap的大小，它是WeakHashMap保存的鍵值對(duì)的數(shù)量
    private int size;
    // WeakHashMap的閾值，用于判斷是否需要調(diào)整WeakHashMap的容量（threshold = 容量*加載因子）
    private int threshold;
    // 加載因子實(shí)際大小
    private final float loadFactor;
    // queue保存的是“已被GC清除”的“弱引用的鍵”。
    // 弱引用和ReferenceQueue 是聯(lián)合使用的：如果弱引用所引用的對(duì)象被垃圾回收，Java虛擬機(jī)就會(huì)把這個(gè)弱引用加入到與之關(guān)聯(lián)的引用隊(duì)列中
    private final ReferenceQueue<Object> queue = new ReferenceQueue<>();
    // WeakHashMap被改變的次數(shù)
    int modCount;
    // 指定“容量大小”和“加載因子”的構(gòu)造函數(shù)
    public WeakHashMap(int initialCapacity, float loadFactor) {
        if (initialCapacity < 0)
            throw new IllegalArgumentException("Illegal Initial Capacity: "+
                                               initialCapacity);
        // WeakHashMap的最大容量只能是MAXIMUM_CAPACITY
        if (initialCapacity > MAXIMUM_CAPACITY)
            initialCapacity = MAXIMUM_CAPACITY;
        if (loadFactor <= 0 || Float.isNaN(loadFactor))
            throw new IllegalArgumentException("Illegal Load factor: "+
                                               loadFactor);
        // 找出“大于initialCapacity”的最小的2的冪
        int capacity = 1;
        while (capacity < initialCapacity)
            capacity <<= 1;
        // 創(chuàng)建Entry數(shù)組，用來(lái)保存數(shù)據(jù)
        table = newTable(capacity);
        // 設(shè)置“加載因子”
        this.loadFactor = loadFactor;
        // 設(shè)置“WeakHashMap閾值”，當(dāng)WeakHashMap中存儲(chǔ)數(shù)據(jù)的數(shù)量達(dá)到threshold時(shí)，就需要將WeakHashMap的容量加倍。
        threshold = (int)(capacity * loadFactor);
    }
    // 指定“容量大小”的構(gòu)造函數(shù)
    public WeakHashMap(int initialCapacity) {
        this(initialCapacity, DEFAULT_LOAD_FACTOR);
    }
    // 默認(rèn)構(gòu)造函數(shù)。
    public WeakHashMap() {
        this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR);
    }
    // 包含“子Map”的構(gòu)造函數(shù)
    public WeakHashMap(Map<? extends K, ? extends V> m) {
        this(Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1,
                DEFAULT_INITIAL_CAPACITY),
             DEFAULT_LOAD_FACTOR);
        // 將m中的全部元素逐個(gè)添加到WeakHashMap中
        putAll(m);
    }
    // 鍵為null的mask值。
    // 因?yàn)閃eakReference中允許“null的key”，若直接插入“null的key”，將其當(dāng)作弱引用時(shí)，會(huì)被刪除。
    // 因此，這里對(duì)于“key為null”的清空，都統(tǒng)一替換為“key為NULL_KEY”，“NULL_KEY”是“靜態(tài)的final常量”。
    private static final Object NULL_KEY = new Object();
    // 對(duì)“null的key”進(jìn)行特殊處理
    private static Object maskNull(Object key) {
        return (key == null) ? NULL_KEY : key;
    }
    // 還原對(duì)“null的key”的特殊處理
    static Object unmaskNull(Object key) {
        return (key == NULL_KEY) ? null : key;
    }
    // 判斷“x”和“y”是否相等
    private static boolean eq(Object x, Object y) {
        return x == y || x.equals(y);
    }
    // 計(jì)算key的哈希值
    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);
    }
    // 返回哈希值對(duì)應(yīng)的index
    // h & (length-1)保證返回值的小于length
    private static int indexFor(int h, int length) {
        return h & (length-1);
    }
    // 清空table中無(wú)用鍵值對(duì)。原理如下：
    // (01) 當(dāng)WeakHashMap中某個(gè)“弱引用的key”由于沒(méi)有再被引用而被GC收回時(shí)，
    //   被回收的“該弱引用key”也被會(huì)被添加到"ReferenceQueue(queue)"中。
    // (02) 當(dāng)我們執(zhí)行expungeStaleEntries時(shí)，
    //   就遍歷"ReferenceQueue(queue)"中的所有key
    //   然后就在“WeakReference的table”中刪除與“ReferenceQueue(queue)中key”對(duì)應(yīng)的鍵值對(duì)
    private void expungeStaleEntries() {
        // 遍歷隊(duì)列
        for (Object x; (x = queue.poll()) != null; ) {
            synchronized (queue) {
                @SuppressWarnings("unchecked")
                    // 由此可以看出隊(duì)列中的元素是Entry
                    Entry<K,V> e = (Entry<K,V>) x;
                // 獲取entry對(duì)應(yīng)桶的index
                int i = indexFor(e.hash, table.length);
                // 根據(jù)index獲取table中對(duì)應(yīng)的桶
                Entry<K,V> prev = table[i];
                Entry<K,V> p = prev;
                // 如果桶不為null，遍歷桶中節(jié)點(diǎn)，找到并刪除與e相等的節(jié)點(diǎn)
                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; // 將e的value置為null
                        size--;// table大小減1
                        break;
                    }
                    // 準(zhǔn)備遍歷下個(gè)節(jié)點(diǎn)
                    prev = p;
                    p = next;
                }
            }
        }
    }
    // 獲取WeakHashMap的table(存放鍵值對(duì)的數(shù)組)
    private Entry<K,V>[] getTable() {
        // 刪除table中“已被GC回收的key對(duì)應(yīng)的鍵值對(duì)”
        expungeStaleEntries();
        return table;
    }
    // 獲取WeakHashMap的實(shí)際大小
    public int size() {
        if (size == 0)
            return 0;
        // 刪除table中“已被GC回收的key對(duì)應(yīng)的鍵值對(duì)”
        expungeStaleEntries();
        return size;
    }
    public boolean isEmpty() {
        return size() == 0;
    }
    // 獲取key對(duì)應(yīng)的value
    public V get(Object key) {
        Object k = maskNull(key);
        // 獲取key的hash值。
        int h = hash(k);
        Entry<K,V>[] tab = getTable();
        int index = indexFor(h, tab.length);
        Entry<K,V> e = tab[index];
        // 在“該hash值對(duì)應(yīng)的鏈表”上查找“鍵值等于key”的元素
        while (e != null) {
            if (e.hash == h && eq(k, e.get()))
                return e.value;
            e = e.next;
        }
        return null;
    }
    // WeakHashMap是否包含key
    public boolean containsKey(Object key) {
        return getEntry(key) != null;
    }
    // 返回“鍵為key”的鍵值對(duì)
    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;
    }
    // 將“key-value”添加到WeakHashMap中
    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) {
            // 若“該key”對(duì)應(yīng)的鍵值對(duì)已經(jīng)存在，則用新的value取代舊的value。然后退出！
            if (h == e.hash && eq(k, e.get())) {
                V oldValue = e.value;
                if (value != oldValue)
                    e.value = value;
                return oldValue;
            }
        }
        // 若“該key”對(duì)應(yīng)的鍵值對(duì)不存在于WeakHashMap中，則將“key-value”添加到table中
        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;
    }
    // 重新調(diào)整WeakHashMap的大小，newCapacity是調(diào)整后的單位
    void resize(int newCapacity) {
        Entry<K,V>[] oldTable = getTable();
        // 記錄table大小
        int oldCapacity = oldTable.length;
        // 如果table大小為MAXIMUM_CAPACITY，就將threshold調(diào)整為Integer.MAX_VALUE，終止執(zhí)行
        if (oldCapacity == MAXIMUM_CAPACITY) {
            threshold = Integer.MAX_VALUE;
            return;
        }
        // 新建一個(gè)newTable，將“舊的table”的全部元素添加到“新的newTable”中，
        // 然后，將“新的newTable”賦值給“舊的table”。
        Entry<K,V>[] newTable = newTable(newCapacity);
        transfer(oldTable, newTable);
        table = newTable;
        // 如果table大小大于等于threshold / 2
        if (size >= threshold / 2) {
            // 重新計(jì)算threshold
            threshold = (int)(newCapacity * loadFactor);
        } else {
            // 刪除table中“已被GC回收的key對(duì)應(yīng)的鍵值對(duì)”
            expungeStaleEntries();
            transfer(newTable, oldTable);
            table = oldTable;
        }
    }
    // 將src的所有鍵值對(duì)復(fù)制到dest中。
    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;
            }
        }
    }
    // 將"m"的全部元素都添加到WeakHashMap中
    public void putAll(Map<? extends K, ? extends V> m) {
        int numKeysToBeAdded = m.size();
        if (numKeysToBeAdded == 0)
            return;
        // 計(jì)算容量是否足夠，
        // 若“當(dāng)前實(shí)際容量 < 需要的容量”，則將容量x2。
        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);
        }
        // 將“m”中的元素逐個(gè)添加到WeakHashMap中。
        for (Map.Entry<? extends K, ? extends V> e : m.entrySet())
            put(e.getKey(), e.getValue());
    }
    // 刪除“鍵為key”元素
    public V remove(Object key) {
        Object k = maskNull(key);
        // 獲取哈希值。
        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;
        // 刪除鏈表中“鍵為key”的元素
        // 本質(zhì)是“刪除單向鏈表中的節(jié)點(diǎn)”
        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;
    }
    // 刪除“鍵值對(duì)”
    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;
        // 刪除鏈表中的“鍵值對(duì)e”
        // 本質(zhì)是“刪除單向鏈表中的節(jié)點(diǎn)”
        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;
    }
    // 清空WeakHashMap，將所有的元素設(shè)為null
    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)
            ;
    }
    // 是否包含“值為value”的元素
    public boolean containsValue(Object value) {
        // 若“value為null”，則調(diào)用containsNullValue()查找
        if (value==null)
            return containsNullValue();
        // 若“value不為null”，則查找WeakHashMap中是否有值為value的節(jié)點(diǎn)。
        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;
    }
    // 是否包含null值
    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;
    }
    // Entry是單向鏈表。
    // 它是 “WeakHashMap鏈?zhǔn)酱鎯?chǔ)法”對(duì)應(yīng)的鏈表。
    // 它實(shí)現(xiàn)了Map.Entry 接口，即實(shí)現(xiàn)getKey(), getValue(), setValue(V value), equals(Object o), hashCode()這些函數(shù)
    private static class Entry<K,V> extends WeakReference<Object> implements Map.Entry<K,V> {
        V value;
        final int hash;
        // 指向下一個(gè)節(jié)點(diǎn)
        Entry<K,V> next;
        // 構(gòu)造函數(shù)。
        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;
        }
        // 判斷兩個(gè)Entry是否相等
        // 若兩個(gè)Entry的“key”和“value”都相等，則返回true。
        // 否則，返回false
        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;
        }
        // 實(shí)現(xiàn)hashCode()
        public int hashCode() {
            K k = getKey();
            V v = getValue();
            return Objects.hashCode(k) ^ Objects.hashCode(v);
        }
        public String toString() {
            return getKey() + "=" + getValue();
        }
    }
    // HashIterator是WeakHashMap迭代器的抽象出來(lái)的父類，實(shí)現(xiàn)了公共了函數(shù)。
    // 它包含“key迭代器(KeyIterator)”、“Value迭代器(ValueIterator)”和“Entry迭代器(EntryIterator)”3個(gè)子類。
    private abstract class HashIterator<T> implements Iterator<T> {
        // 當(dāng)前索引
        private int index;
        // 當(dāng)前元素
        private Entry<K,V> entry;
        // 上一次返回元素
        private Entry<K,V> lastReturned;
        // expectedModCount用于實(shí)現(xiàn)fast-fail機(jī)制。
        private int expectedModCount = modCount;
        // 下一個(gè)鍵(強(qiáng)引用)
        private Object nextKey;
        // 當(dāng)前鍵(強(qiáng)引用)
        private Object currentKey;
        // 構(gòu)造函數(shù)
        HashIterator() {
            index = isEmpty() ? 0 : table.length;
        }
        // 是否存在下一個(gè)元素
        public boolean hasNext() {
            Entry<K,V>[] t = table;
            // 一個(gè)Entry就是一個(gè)單向鏈表
            // 若該Entry的下一個(gè)節(jié)點(diǎn)不為空，就將next指向下一個(gè)節(jié)點(diǎn);
            // 否則，將next指向下一個(gè)鏈表(也是下一個(gè)Entry)的不為null的節(jié)點(diǎn)。
            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;
        }
        // 獲取下一個(gè)元素
        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;
        }
        // 刪除當(dāng)前元素
        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;
        }
    }
    // value的迭代器
    private class ValueIterator extends HashIterator<V> {
        public V next() {
            return nextEntry().value;
        }
    }
    // key的迭代器
    private class KeyIterator extends HashIterator<K> {
        public K next() {
            return nextEntry().getKey();
        }
    }
    // Entry的迭代器
    private class EntryIterator extends HashIterator<Map.Entry<K,V>> {
        public Map.Entry<K,V> next() {
            return nextEntry();
        }
    }
    // WeakHashMap的Entry對(duì)應(yīng)的集合
    private transient Set<Map.Entry<K,V>> entrySet;
    // 返回“key的集合”，實(shí)際上返回一個(gè)“KeySet對(duì)象”
    public Set<K> keySet() {
        Set<K> ks = keySet;
        if (ks == null) {
            ks = new KeySet();
            keySet = ks;
        }
        return ks;
    }
    // Key對(duì)應(yīng)的集合
    // KeySet繼承于AbstractSet，說(shuō)明該集合中沒(méi)有重復(fù)的Key。
    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);
        }
    }
    // 返回“value集合”，實(shí)際上返回的是一個(gè)Values對(duì)象
    public Collection<V> values() {
        Collection<V> vs = values;
        if (vs == null) {
            vs = new Values();
            values = vs;
        }
        return vs;
    }
    // “value集合”
    // Values繼承于AbstractCollection，不同于“KeySet繼承于AbstractSet”，
    // Values中的元素能夠重復(fù)。因?yàn)椴煌膋ey可以指向相同的value。
    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);
        }
    }
    // 返回“WeakHashMap的Entry集合”
    // 它實(shí)際是返回一個(gè)EntrySet對(duì)象
    public Set<Map.Entry<K,V>> entrySet() {
        Set<Map.Entry<K,V>> es = entrySet;
        return es != null ? es : (entrySet = new EntrySet());
    }
    // EntrySet對(duì)應(yīng)的集合
    // EntrySet繼承于AbstractSet，說(shuō)明該集合中沒(méi)有重復(fù)的EntrySet。
    private class EntrySet extends AbstractSet<Map.Entry<K,V>> {
        public Iterator<Map.Entry<K,V>> iterator() {
            return new EntryIterator();
        }
        // 是否包含“值(o)”
        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);
        }
        // 刪除“值(o)”
        public boolean remove(Object o) {
            return removeMapping(o);
        }
        // 返回WeakHashMap的大小
        public int size() {
            return WeakHashMap.this.size();
        }
        // 清空WeakHashMap
        public void clear() {
            WeakHashMap.this.clear();
        }
        // 拷貝函數(shù)。將WeakHashMap中的全部元素都拷貝到List中
        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;
        }
        // 返回Entry對(duì)應(yīng)的Object[]數(shù)組
        public Object[] toArray() {
            return deepCopy().toArray();
        }
        // 返回Entry對(duì)應(yīng)的T[]數(shù)組(T[]我們新建數(shù)組時(shí)，定義的數(shù)組類型)
        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);
        }
    }
}

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;
    }
    ...
    ...
    ...

static class Node<K,V> implements Map.Entry<K,V> {
    final int hash;
    final K key;
    V value;
    Node<K,V> next;
    Node(int hash, K key, V value, Node<K,V> next) {
        this.hash = hash;
        this.key = key;
        this.value = value;
        this.next = next;
    }
    ...
    ...
    ...