在HashMap源码中看到使用了位移运算符，所以在此记录这个公式。
/**
* The default initial capacity - MUST be a power of two.
*/
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16
/**
* The maximum capacity, used if a higher value is implicitly specified
* by either of the constructors with arguments.
* MUST be a power of two <= 1<<30.
*/
static final int MAXIMUM_CAPACITY = 1 << 30;
左移运算符（<<）
记住公式：a << b = a * 2^b
右移运算符（>>）
记住公式：a >> b = a / 2^b
