1. cap = 6，返回结果 8
2. cap = 7，返回结果 8
3. cap = 8，返回结果 8

int tableSizeFor(int cap) 
{
        int n = cap - 1;
        n |= n >>> 1;
        n |= n >>> 2;
        n |= n >>> 4;
        n |= n >>> 8;
        n |= n >>> 16;
        return (n < 0) ? 1 : (n >= MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : n + 1;
    }