Understanding BigInteger constructor

I have the following code from my textbook for figuring out factorial:

 import java.math.*;
 public class LargeFactorial {
 public static void main(String[] args) {

  System.out.println("50! is \n" + factorial(50));
  }  public static BigInteger factorial(long n) {
      BigInteger result = BigInteger.ONE;
      for (int i = 1; i <= n; i++)
          result = result.multiply(new BigInteger(i +""));
      return result;
      }

However, I really don't understand new BigInteger(i +""). Why they put +"" in the constructor?I mean we are not multiplying an empty String and it does not make any sense either.Please explain.