Shift Operator Multiplier

Question

Please suggest a way to optimize the below code. I want to reduce the execution time. This is a program to multiply using the shift operator.

import java.util.Scanner;
public class Test2 {
 public static void main(String args[] ) throws Exception {
 Scanner in = new Scanner(System.in);
 int N = in.nextInt();
 long[] [] list=new long[N][2];
 for(int i=0;i<N;i++){
 list[i][0]=in.nextLong();
 list[i][1]=in.nextLong();
 }
 for(long[] s:list){
 System.out.println(calc(s[0], s[1]));
 }
 }
 public static String calc(long n1, long n2) {
 long temp, i = 0, result = 0;
 String s=null;
 while (n2 != 0) {
 if ((n2 & 1) == 1) {
 temp = n1;
 if(s==null){
 s="("+i+"-"+i+") ";
 }else{
 s=s+"("+i+"-"+i+") + ";
 }
 result += (temp<<=i); 
 }
 n2 >>= 1; 
 i++;
 }
 return s;
 }
}

• user34073: I changed the title to state what the code does to better follow site rules, and cleaned up a couple other minor issues. I hope you get a good answer.
• tim: can you expand a little bit on how your program works, exactly? If I input the numbers 4 and 3, I get (0-0) (1-1) + . If I input 2 and 3, I get the same result. If I input 1001 and 0110, I get (3-3) (6-6) + , which is also what I get with 101 and 0110. I'm quite unsure what is expected as input, and what is delivered as output.
• Rahul ray: @tim sorry for let reply here main thing is result that is multiplication of two number using shift operator,I was printing value of i just for convenience to see how loop is running .

Answer 1

The first thing I see is concatenating a string in a loop. This will create a new string every loop. You don't want that.

Instead use a StringBuilder:

public static String calc(long n1, long n2) {
 long temp, i = 0, result = 0;
 StringBuilder s=new StringBuilder();
 while (n2 != 0) {
 if ((n2 & 1) == 1) {
 temp = n1;
 if(s.length()!=0)s.append("+ ");
 s.append("(").append(i).append("-").append(i).append(") ");
 result += (temp<<=i); 
 }
 n2 >>= 1; 
 i++;
 }
 return s.toString();
}