Question

I want to compare two multiplication methods implemented in Java which use shift operations on big numbers. Thus I need sufficiently large BigIntegers.

Since I want to compare them bit-wise, what would be the best approach to generate BigIntegers with n bits which are fully used in the multiplication operation.

My approach so far is this:

byte[] bits = new byte[bitLength];

BigInteger number = new BigInteger(bits).flipBit(bitLength);

Answer 1

How about this:

import java.math.BigInteger;

public class Test
{
    public static void main(String[] args)
    {
        int bits = 3;

        BigInteger value = BigInteger.ZERO
                                     .setBit(bits)
                                     .subtract(BigInteger.ONE);
        System.out.println(value);  // Prints 7 == 111 in binary
    }
}

In other words, set the bit which is one higher than you want, then subtract one to get a value which uses all the lower bits.