How to perform multiplication, using bitwise operators?

Question

I am working through a problem which i was able to solve, all but for the last piece - i am not sure how can one do multiplication using bitwise operators:

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Answer 1

public static int multi(int x, int y){
        boolean neg = false;
        if(x < 0 && y >= 0){
            x = -x;
            neg = true;
        }
        else if(y < 0 && x >= 0){
            y = -y;
            neg = true;
        }else if( x < 0 && y < 0){
            x = -x;
            y = -y;
        }

        int res = 0;
        while(y!=0){
            if((y & 1) == 1) res += x;
            x <<= 1;
            y >>= 1;
        }
        return neg ? (-res) : res;
    }