Why java set length (b) the HashMap is a power of 2

Look at the source

 /**
     * Returns a power of two size for the given target capacity.
     */
    static final 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;
    }

a | = b currency and a = a | b

You can try, if n is a power of 2, it will not change

So why, if the length of a power of 2 it

Hash values ​​range value -2147483648 to 2147483647, probably add up to around 4 billion mapping space. That of course is not directly take.

By mapping only is modulo array. " (n - 1) & hash." (N array representing the length) which is the formula

Modulo (%) the operation if the divisor is a power of 2 is equivalent to minus its divisor with a premise (&) operation (i.e. hash% length == hash & (length -1) is a length of 2 n-th power;). " And a binary bit operation &,% with respect to the operation efficiency can be improved,

This explains why HashMap length is a power of two.