static class Entry<K,V> implements Map.Entry<K,V> {
        final K key;    //key 引用
        V value;         //value  引用
        Entry<K,V> next;   //下一个Entry 节点的引用
}

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

    /**
     * The default initial capacity - MUST be a power of two.
     */
    static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 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.
     */
    static final int MAXIMUM_CAPACITY = 1 << 30;

    /**
     * The load factor used when none specified in constructor.
     */
    static final float DEFAULT_LOAD_FACTOR = 0.75f;

    /**
     * An empty table instance to share when the table is not inflated.
     */
    static final Entry<?,?>[] EMPTY_TABLE = {};

    /**
     * The table, resized as necessary. Length MUST Always be a power of two.
     */
    transient Entry<K,V>[] table = (Entry<K,V>[]) EMPTY_TABLE;

    /**
     * The number of key-value mappings contained in this map.
     */
    transient int size;

    /**
     * The next size value at which to resize (capacity * load factor).
     * @serial
     */
    // If table == EMPTY_TABLE then this is the initial capacity at which the
    // table will be created when inflated.
    int threshold;

    /**
     * The load factor for the hash table.
     *
     * @serial
     */
    final float loadFactor;
}

public HashMap() {
        this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR);
    }

 public HashMap(int initialCapacity, float loadFactor) {        //initialCapacity:16   loadFactor 0.75f
        if (initialCapacity < 0)
            throw new IllegalArgumentException("Illegal initial capacity: " +
                                               initialCapacity);
        if (initialCapacity > MAXIMUM_CAPACITY)     //MAXIMUM_CAPACITY   1073741824  false

             initialCapacity = MAXIMUM_CAPACITY;
        if (loadFactor <= 0 || Float.isNaN(loadFactor))   false
            throw new IllegalArgumentException("Illegal load factor: " +
                                               loadFactor);

        this.loadFactor = loadFactor;      //赋值给loadFactor=0.75
        threshold = initialCapacity;       //赋值给threshold=16  当为16是自动扩容
        init();
    }

public V put(K key, V value) {    //如插入  key="city"   value="http://www.mamicode.com/shanghai"
        if (table == EMPTY_TABLE) {     //true
            inflateTable(threshold);   //参数为16
        }
        if (key == null)        // false
            return putForNullKey(value);
        int hash = hash(key);    //返回key值的hash码； 比如返回为 337
        int i = indexFor(hash, table.length);  //将hash 取模与16 获得的结果为 1
        for (Entry<K,V> e = table[i]; e != null; e = e.next) {  //遍历 Entry[1] 中的链表节点对象 包含 原先有的节点和新增进去的节点
            Object k;
            if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {  //当Entry中包含 相同的hash码的key 并且key和要添加的key相等即可以是否重复  则进入以下逻辑：新节点替换重复的节点
                V oldValue = http://www.mamicode.com/e.value;>

/**
     * Inflates the table.
     */
    private void inflateTable(int toSize) {   //toSize 16
        // Find a power of 2 >= toSize
        int capacity = roundUpToPowerOf2(toSize);  //capacity=16

        threshold = (int) Math.min(capacity * loadFactor, MAXIMUM_CAPACITY + 1); // threshold=16*0.75
        table = new Entry[capacity];     //创建 Entry[] 数组长度为16   
        initHashSeedAsNeeded(capacity);   //这个方法可以暂时不用深究
    }　　

private static int roundUpToPowerOf2(int number) {    //  number=16
        // assert number >= 0 : "number must be non-negative";
        return number >= MAXIMUM_CAPACITY   //fase  返回 16
                ? MAXIMUM_CAPACITY
                : (number > 1) ? Integer.highestOneBit((number - 1) << 1) : 1;   //number=16>1  返回 16
    }　

public V get(Object key) {    //如 key="name"
        if (key == null)      //false
            return getForNullKey();
        Entry<K,V> entry = getEntry(key);

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

 final Entry<K,V> getEntry(Object key) {   //key=name
        if (size == 0) {    //false
            return null;
        }

        int hash = (key == null) ? 0 : hash(key);   //例如 返回hash=337
        for (Entry<K,V> e = table[indexFor(hash, table.length)];  //indexFor(hash, table.length)上面分析过 返回值为 1；遍历 table[1] 中的节点
             e != null;
             e = e.next) {
            Object k;
            if (e.hash == hash &&
                ((k = e.key) == key || (key != null && key.equals(k))))   //如果存在key的hash码相等，并且对象也相等则返回 对应的Entry 节点
                return e;
        }
        return null;   //否则返回null
    }