ConcurrentHashMap.tableSizeFor(i

ConcurrentHashMap.tableSizeFor(int c)方法：

private static final int tableSizeFor(int c) {
        int n = c - 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;
    }

改进版：

private static final int tableSizeFor(int c) {
        if(c <= 1) {return 1;}
        if(c > (MAXIMUM_CAPACITY >>> 1)) {return MAXIMUM_CAPACITY;}
        int n = c - 1;
        n |= n >>> 1;
        n |= n >>> 2;
        n |= n >>> 4;
        n |= n >>> 8;
        n |= n >>> 16;
        return n + 1;
    }