public class HashMap<K,V> extends AbstractMap<K,V> implements Map<K,V>, Cloneable, Serializable{

     //*******内部类

     //Enrty<K,V> 单向链表

     static class Entry<K,V> implements Map.Entry<K,V> {

        final K key;

        V value;

        Entry<K,V> next;

        int hash;

        /**

         * Creates new entry.

         */

        Entry(int h, K k, V v, Entry<K,V> n) {

            value = v;

            next = n;

            key = k;

            hash = h;

        }

     }

     //*******属性

     static final Entry<?,?>[] EMPTY_TABLE = {};

     //Entry<K,V>[]数组实现

     transient Entry<K,V>[] table = (Entry<K,V>[]) EMPTY_TABLE;

     final float loadFactor;

     static final float DEFAULT_LOAD_FACTOR = 0.75f;

     static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16

     //*******构造器

     public HashMap() {

        this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR);

    }

    public HashMap(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);

        this.loadFactor = loadFactor;

        threshold = initialCapacity;

        init();

    }

    public HashMap(int initialCapacity) {

        this(initialCapacity, DEFAULT_LOAD_FACTOR);

    }

    //********方法

    //hash方法

    final int hash(Object k) {

        int h = hashSeed;

        if (0 != h && k instanceof String) {

            return sun.misc.Hashing.stringHash32((String) k);

        }

        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);

    }    

     //put方法

     public V put(K key, V value) {

        //在put操作时，判断数组是否为空，再数组初始化容量

        if (table == EMPTY_TABLE) {

            inflateTable(threshold);

        }

        if (key == null)

            return putForNullKey(value);

        int hash = hash(key);

        //定位数组索引i

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

        //如果table[i]存在，然后使用单向链表解决冲突

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

            Object k;

            //如2个key 的hash值相等，并且  equals也相等，则覆盖旧值

            if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {

                V oldValue = e.value;

                e.value = value;

                e.recordAccess(this);

                return oldValue;

            }

        }

        modCount++;

        //如果table[i]没有，添加元素

        addEntry(hash, key, value, i);

        return null;

    }

    //数组容量初始化

    private void inflateTable(int toSize) {

        // Find a power of 2 >= toSize

        int capacity = roundUpToPowerOf2(toSize);

        threshold = (int) Math.min(capacity * loadFactor, MAXIMUM_CAPACITY + 1);

        //根据计算容量初始化数组

        table = new Entry[capacity];

        initHashSeedAsNeeded(capacity);

    }

    //定位数组索引i

    static int indexFor(int h, int length) {

        // assert Integer.bitCount(length) == 1 : "length must be a non-zero power of 2";

        return h & (length-1);

    }

    void addEntry(int hash, K key, V value, int bucketIndex) {

        if ((size >= threshold) && (null != table[bucketIndex])) {

            //扩容为原来的2倍

            resize(2 * table.length);

            hash = (null != key) ? hash(key) : 0;

            bucketIndex = indexFor(hash, table.length);

        }

        createEntry(hash, key, value, bucketIndex);

    }

    //扩容

    void resize(int newCapacity) {

        Entry[] oldTable = table;

        int oldCapacity = oldTable.length;

        if (oldCapacity == MAXIMUM_CAPACITY) {

            threshold = Integer.MAX_VALUE;

            return;

        }

        Entry[] newTable = new Entry[newCapacity];

        transfer(newTable, initHashSeedAsNeeded(newCapacity));

        table = newTable;

        threshold = (int)Math.min(newCapacity * loadFactor, MAXIMUM_CAPACITY + 1);

    }

    //旧数组数据迁移至新数组（此处会引起  环形链表  ）

    void transfer(Entry[] newTable, boolean rehash) {

        int newCapacity = newTable.length;

        //遍历旧数组中的每一项

        for (Entry<K,V> e : table) {

            while(null != e) {

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

                if (rehash) {

                    e.hash = null == e.key ? 0 : hash(e.key);

                }

                int i = indexFor(e.hash, newCapacity);

                e.next = newTable[i];

                newTable[i] = e;

                e = next;

            }

        }

    }

    void createEntry(int hash, K key, V value, int bucketIndex) {

        Entry<K,V> e = table[bucketIndex];

        table[bucketIndex] = new Entry<>(hash, key, value, e);

        size++;

    }

    public V get(Object key) {

        if (key == null)

            return getForNullKey();

        Entry<K,V> entry = getEntry(key);

        return null == entry ? null : entry.getValue();

    }

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

        if (size == 0) {

            return null;

        }

        int hash = (key == null) ? 0 : hash(key);

        for (Entry<K,V> e = table[indexFor(hash, table.length)];

             e != null;

             e = e.next) {

            Object k;

            if (e.hash == hash &&

                ((k = e.key) == key || (key != null && key.equals(k))))

                return e;

        }

        return null;

    }

}