The table, initialized on first use, and resized as necessary. When allocated, length is always a power of two. (We also tolerate length zero in some operations to allow bootstrapping mechanics that are currently not needed.)
number
the number of times that an amount is to be multiplied by itself 乘方；幂
4 to the power of 3 is 43 (= 4 × 4 × 4 = 64).
4 的 3 次方是 43。
transient Node<K,V>[] table; // 放node节点的数组The next size value at which to resize (capacity * load factor).
// (The javadoc description is true upon serialization.
// Additionally, if the table array has not been allocated, this
// field holds the initial array capacity, or zero signifying
// DEFAULT_INITIAL_CAPACITY.)
int threshold;