Hashmap简单原理

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

        public void remove() {
            if (lastRet < 0)
                throw new IllegalStateException();
            checkForComodification();

            try {
                ArrayList.this.remove(lastRet);
                cursor = lastRet;
                lastRet = -1;
                expectedModCount = modCount;
            } catch (IndexOutOfBoundsException ex) {
                throw new ConcurrentModificationException();
            }
        }

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

方法一：
static final int hash(Object key) {   //jdk1.8 & jdk1.7
     int h;
     // h = key.hashCode() 为第一步 取hashCode值
     // h ^ (h >>> 16)  为第二步 高位参与运算
     return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}
方法二：
static int indexFor(int h, int length) {  
    //jdk1.7的源码，jdk1.8没有这个方法，而是直接写入到各个方法里
     return h & (length-1);  //第三步 取模运算
}

    public V put(K key, V value) {
         // 对key的hashCode()做hash
        return putVal(hash(key), key, value, false, true);
    }


    final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
                   boolean evict) {
        Node<K,V>[] tab; Node<K,V> p; int n, i;
         // 步骤1：tab为空则创建
        if ((tab = table) == null || (n = tab.length) == 0)
            n = (tab = resize()).length;
          // 步骤2：计算index，并对null做处理 
        if ((p = tab[i = (n - 1) & hash]) == null)
            tab[i] = newNode(hash, key, value, null);
        else {
            Node<K,V> e; K k;
            // 步骤3：节点key存在，直接覆盖value
            if (p.hash == hash &&
                ((k = p.key) == key || (key != null && key.equals(k))))
                e = p;
            // 步骤4：判断该链为红黑树
            else if (p instanceof TreeNode)
                e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
            // 步骤5：该链为链表
            else {
                for (int binCount = 0; ; ++binCount) {
                    if ((e = p.next) == null) {
                        p.next = newNode(hash, key, value, null);
                         //链表长度大于8转换为红黑树进行处理
                        if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                            treeifyBin(tab, hash);
                        break;
                    }
                    // key已经存在直接覆盖value
                    if (e.hash == hash &&
                        ((k = e.key) == key || (key != null && key.equals(k))))
                        break;
                    p = e;
                }
            }
            if (e != null) { // existing mapping for key
                V oldValue = e.value;
                if (!onlyIfAbsent || oldValue == null)
                    e.value = value;
                afterNodeAccess(e);
                return oldValue;
            }
        }
        ++modCount;
        // 步骤6：超过最大容量 就扩容
        if (++size > threshold)
            resize();
        afterNodeInsertion(evict);
        return null;
    }

  void resize(int newCapacity) {   //传入新的容量
      Entry[] oldTable = table;    //引用扩容前的Entry数组
      int oldCapacity = oldTable.length;         
      if (oldCapacity == MAXIMUM_CAPACITY) {  //扩容前的数组大小如果已经达到最大(2^30)了
          threshold = Integer.MAX_VALUE; //修改阈值为int的最大值(2^31-1)，这样以后就不会扩容了
          return;
      }
   
      Entry[] newTable = new Entry[newCapacity];  //初始化一个新的Entry数组
      transfer(newTable);                         //！！将数据转移到新的Entry数组里
      table = newTable;                           //HashMap的table属性引用新的Entry数组
      threshold = (int)(newCapacity * loadFactor);//修改阈值
 }

  void transfer(Entry[] newTable) {
      Entry[] src = table;                   //src引用了旧的Entry数组
      int newCapacity = newTable.length;
      for (int j = 0; j < src.length; j++) { //遍历旧的Entry数组
          Entry<K,V> e = src[j];             //取得旧Entry数组的每个元素
          if (e != null) {
              src[j] = null;//释放旧Entry数组的对象引用（for循环后，旧的Entry数组不再引用任何对象）
              do {
                  Entry<K,V> next = e.next;
                  int i = indexFor(e.hash, newCapacity); //！！重新计算每个元素在数组中的位置
                  e.next = newTable[i]; //标记[1]
                  newTable[i] = e;      //将元素放在数组上
                  e = next;             //访问下一个Entry链上的元素
            } while (e != null);
         }
     }
 } 

  e.next = newTable[i]; //1
  newTable[i] = e;      //2
  e = next;             //3

                        Node<K,V> loHead = null, loTail = null;//lo链表复制索引不变的元素
                        Node<K,V> hiHead = null, hiTail = null;//hi链表复制索引+n的元素
                        Node<K,V> next;
                        do {
                            next = e.next;
                            if ((e.hash & oldCap) == 0) {//用按位与判断节点的索引是否变化
                                if (loTail == null)//这里就是尾插入了
                                    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;//lo链表直接复制到新数组的原位置
                        }
                        if (hiTail != null) {
                            hiTail.next = null;
                            newTab[j + oldCap] = hiHead;//hi链表则复制到新数组的+n位置
                        }