public V put(K key, V value) {
        if (key == null)
            return putForNullKey(value);
        int hash = hash(key);  //获取当前key的hash值
        int i = indexFor(hash, table.length);  //返回在hash桶里面的位置
        for (Entry<K,V> e = table[i]; e != null; e = e.next) {  //遍历当前hansh桶后面的元素
            Object k;
            if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {  //如果有相同的key，那么需要替换value
                V oldValue = e.value;
                e.value = value;
                e.recordAccess(this);
                return oldValue;  //返回以前的value
            }
        }

        modCount++;
        addEntry(hash, key, value, i);  //放入entry
        return null;
    }