HashMap的put和get操作

• PUT
  1. 先取key的hashCode()做hash,计算index
  2. 若没碰撞直接放到bucket中
  3. 若碰撞，以链表的形式存在buckets后
  4. 如果碰撞导致链表过长，将链表转换成红黑树
  5. 如果节点已存在则替换old value（保证key的唯一性）
  6. 如果bucket满了(超过load factor*current capacity)，就resize

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;
    // tab为空则创建
    if ((tab = table) == null || (n = tab.length) == 0)
        n = (tab = resize()).length;
    // 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;
        // 节点存在
        if (p.hash == hash &&
            ((k = p.key) == key || (key != null && key.equals(k))))
            e = p;
        // 该链为树
        else if (p instanceof TreeNode)
            e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
        // 该链为链表
        else {
            for (int binCount = 0; ; ++binCount) {
                if ((e = p.next) == null) {
                    p.next = newNode(hash, key, value, null);
                    if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                        treeifyBin(tab, hash);
                    break;
                }
                if (e.hash == hash &&
                    ((k = e.key) == key || (key != null && key.equals(k))))
                    break;
                p = e;
            }
        }
        // 写入
        if (e != null) { // existing mapping for key
            V oldValue = e.value;
            if (!onlyIfAbsent || oldValue == null)
                e.value = value;
            afterNodeAccess(e);
            return oldValue;
        }
    }
    ++modCount;
    // 超过load factor*current capacity，resize
    if (++size > threshold)
        resize();
    afterNodeInsertion(evict);
    return null;
}

• GET
  1. bucket里的第一个节点，直接命中
  2. 如果有冲突，则通过key.equals(k)去查找对应的entry
  3. 若为树，则在树中通过key.equals(k)查找 ，O(logn)
  4. 若为链表，则在链表中通过key.equals(k)查找，O(n)

public V get(Object key) {
  Node<K,V> e;
  return (e = getNode(hash(key), key)) == null ? null : e.value;
}
final Node<K,V> getNode(int hash, Object key) {
  Node<K,V>[] tab; Node<K,V> first, e; int n; K k;
  if ((tab = table) != null && (n = tab.length) > 0 &&
      (first = tab[(n - 1) & hash]) != null) {
      // 直接命中
      if (first.hash == hash && // always check first node
          ((k = first.key) == key || (key != null && key.equals(k))))
          return first;
      // 未命中
      if ((e = first.next) != null) {
          // 在树中get
          if (first instanceof TreeNode)
              return ((TreeNode<K,V>)first).getTreeNode(hash, key);
          // 在链表中get
          do {
              if (e.hash == hash &&
                  ((k = e.key) == key || (key != null && key.equals(k))))
                  return e;
          } while ((e = e.next) != null);
      }
  }
  return null;
}

转自：HashMap的put和get操作https://my.oschina.net/134596/blog/1800725