Multiplication of two integers using bitwise operators

Question

How can I multipy two integers using bitwise operators?

I found an implementation here. Is there a better way of implementing multiplication?

For example: 2

Answer 1

This one is purely with bit-wise operations.

public int bitwiseMultiply(int a, int b) {
    if (a ==0  || b == 0) {
        return 0;
    }

    if (a == 1) {
        return b;
    }
    else
        if (b == 1) {
            return a;
        }


    int result = 0; // Not needed, just for test
    int initA = a;
    boolean isORNeeded = false;
    while (b != 0 ) {

        if (b == 1) {
            break;
        }

        if ((b & 1) == 1) { // Carry needed, odd number
            result += initA; // Test, not needed
            isORNeeded = true;
        }

        a <<= 1; // Double the a
        b >>= 1; // Half the b
        System.out.println("a=["+a+"], b=["+b+"], result=["+result+"]");
    }

    return (isORNeeded ? (a | initA) : a); // a + result;
}