HashMap源码解析（简单易懂）

/*
  每一个key-value存储在Node<K,V>中，HashMap由Node<K,V>[]数
  组组成。
 */
 static class Node<K,V> implements Map.Entry<K,V> {
         final int hash;
         final K key;
         V value;
         Node<K,V> next;

         //hash:每个节点插入时先计算hash
         Node(int hash, K key, V value, Node<K,V> next) {
             this.hash = hash;
             this.key = key;
             this.value = value;
             this.next = next;
         }

         public final K getKey()        { return key; }
         public final V getValue()      { return value; }
         public final String toString() { return key + "=" + value; }

         public final int hashCode() {
             return Objects.hashCode(key) ^ Objects.hashCode(value);
         }

         public final V setValue(V newValue) {
             V oldValue = value;
             value = newValue;
             return oldValue;
         }

         public final boolean equals(Object o) {
             if (o == this)
                 return true;
             if (o instanceof Map.Entry) {
                 Map.Entry<?,?> e = (Map.Entry<?,?>)o;
                 if (Objects.equals(key, e.getKey()) &&
                     Objects.equals(value, e.getValue()))
                     return true;
             }
             return false;
         }
     }
 HashMap常量与属性：
    常量：
    static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; //默认初始化容量是16  
    static final int MAXIMUM_CAPACITY = 1 << 30;//数组的最大容量 2的30次方
    static final float DEFAULT_LOAD_FACTOR = 0.75f; //默认加载因子 ，阈值=capacity*DEFAULT_LOAD_FACTOR
    static final int TREEIFY_THRESHOLD = 8; //默认链表长度大于8时，链表转化成红黑数
    static final int UNTREEIFY_THRESHOLD = 6; //默认元素个数小于6时，红黑数退化成链表
    static final int MIN_TREEIFY_CAPACITY = 64;//在转变成树之前还会有一次判断，只有键值对数量大于64才会发生转换，
                                               //这是为了避免在哈希表建立初期，多个键值对恰好被放入同一个链表中导致不必要的转化

    属性:

transient Node<K,V>[] table;

transient Set<Map.Entry<K,V>> entrySet;

transient int size;

transient int modCount;//扩容次数

int threshold;//阈值，超过这个值就扩容

final float loadFactor; //加载因子

方法：

 static final int hash(Object key) {
         int h;
         return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
     }

public V put(K key, V value) {
        return putVal(hash(key), key, value, false, true);
    }

/**
* Implements Map.put and related methods
*
* @param hash hash for key
* @param key the key
* @param value the value to put
* @param onlyIfAbsent if true, don't change existing value 
* @param evict if false, the table is in creation mode.
* @return previous value, or null if none
*/

final V putVal(int hash, K key, V value, boolean onlyIfAbsent,

boolean evict) {//onlyIfAbsent==true:如果key值已存在，则不改变其value值
        Node<K,V>[] tab; Node<K,V> p; int n, i;
        if ((tab = table) == null || (n = tab.length) == 0)
            n = (tab = resize()).length;//null或者长度为零，需要初始化大小，或者扩容
        if ((p = tab[i = (n - 1) & hash]) == null) //(n - 1) & hash:确定index,因为n是2的次幂，n-1各进制位都为1，
                                                   // &：按位与 任何数与n-1做 &运算结果小于n-1,所以保证任何hash的下标都在n-1中
            tab[i] = newNode(hash, key, value, null);//p==null表明：该下标的数组节点未被使用，直接赋值
        else {//hash在 Node<K,V>[] 数组产生冲突，则在该节点的链表或红黑树查找
            Node<K,V> e; K k;
            if (p.hash == hash &&
                ((k = p.key) == key || (key != null && key.equals(k))))//要插入的元素和冲突节点的第一个元素相等
                e = p;//p:保存根据hash值找到的节点，e保存实时查找的最后一次找到的元素
            else if (p instanceof TreeNode)//桶中的元素按红黑树排列
                e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
            else {//桶中的元素按链表排列
                for (int binCount = 0; ; ++binCount) {
                    if ((e = p.next) == null) {//将元素添加到p的后面
                        p.next = newNode(hash, key, value, null);
                         //元素个数大于8，链表转红黑树
                        if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                            treeifyBin(tab, hash);
                        break;
                    }

                    //遍历到了key相等的元素，退出遍历
                    if (e.hash == hash &&
                        ((k = e.key) == key || (key != null && key.equals(k))))
                        break;
                    p = e;//e=p.next;p=e;往后遍历
                }
            }
            if (e != null) { // existing mapping for key
                V oldValue = e.value;
                if (!onlyIfAbsent || oldValue == null)//判断是否赋值
                    e.value = value;
                afterNodeAccess(e);
                return oldValue;
            }
        }
        ++modCount;//添加元素，modCount++
        if (++size > threshold)//判断是否需要扩容
            resize();
        afterNodeInsertion(evict);
        return null;//默认值
    }

final Node<K,V>[] resize() {
        Node<K,V>[] oldTab = table;
        int oldCap = (oldTab == null) ? 0 : oldTab.length;
        int oldThr = threshold;
        int newCap, newThr = 0;
        if (oldCap > 0) {
            if (oldCap >= MAXIMUM_CAPACITY) {
                threshold = Integer.MAX_VALUE;
                return oldTab;
            }
            else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
                     oldCap >= DEFAULT_INITIAL_CAPACITY)//容量2倍，阈值2倍，
                newThr = oldThr << 1; // double threshold
        }
        //oldCap>=0&& oldThr>0
        else if (oldThr > 0) // initial capacity was placed in threshold
            newCap = oldThr;//使用原来的阈值作为新的容量
        else {               // zero initial threshold signifies using defaults
            newCap = DEFAULT_INITIAL_CAPACITY;
            newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);//终于用到阈值的计算方法
        }
        //以上判断，确定了新的容量
        if (newThr == 0) {
            float ft = (float)newCap * loadFactor;
            newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
                      (int)ft : Integer.MAX_VALUE);
        }
        threshold = newThr;
        @SuppressWarnings({"rawtypes","unchecked"})
            Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
        table = newTab;
        if (oldTab != null) {
            for (int j = 0; j < oldCap; ++j) {
                Node<K,V> e;
                if ((e = oldTab[j]) != null) {
                    oldTab[j] = null;
                    if (e.next == null)
                        newTab[e.hash & (newCap - 1)] = e;
                    else if (e instanceof TreeNode)
                        ((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
                    else { // preserve order
                        Node<K,V> loHead = null, loTail = null;
                        Node<K,V> hiHead = null, hiTail = null;
                        Node<K,V> next;
                        do {
                            next = e.next;
                            if ((e.hash & oldCap) == 0) {
                                if (loTail == null)
                                    loHead = e;
                                else
                                    loTail.next = e;
                                loTail = e;
                            }
                            else {
                                if (hiTail == null)
                                    hiHead = e;
                                else
                                    hiTail.next = e;
                                hiTail = e;
                            }
                        } while ((e = next) != null);
                        if (loTail != null) {
                            loTail.next = null;
                            newTab[j] = loHead;
                        }
                        if (hiTail != null) {
                            hiTail.next = null;
                            newTab[j + oldCap] = hiHead;
                        }
                    }
                }
            }
        }
        return newTab;
    }