3、取get

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;
    //若是table数组不为空 且 该键值对对应的位置存有node节点
    
    if ((tab = table) != null && (n = tab.length) > 0 && (first = tab[(n - 1) & hash]) != null) {
        if (first.hash == hash && ((k = first.key) == key || (key != null && key.equals(k))))
            return first;
        //若是table中存储的键值对不是要查找的这对，就从该桶的链中查找
        if ((e = first.next) != null) {
        
            if (first instanceof TreeNode)
                return ((TreeNode<K,V>)first).getTreeNode(hash, key);
            do {
                if (e.hash == hash && ((k= e.key) == key || (key != null && key.equals(k))))
                    return e;
            } while ((e = e.next) != null);
        }
    }
    return null;//该键值对不存在
}