Question

I’m having string consisting of a sequence of digits (e.g. "1234"). How to return the String as an int without using Java’s library functions like Integer.parseInt?

public class StringToInteger {
  public static void main(String [] args){
    int i = myStringToInteger("123");
    System.out.println("String decoded to number " + i);
  }

  public int myStringToInteger(String str){
      /* ... */
  }
}

Answer 1

And what is wrong with this?

int i = Integer.parseInt(str);

EDIT :

If you really need to do the conversion by hand, try this:

public static int myStringToInteger(String str) {
    int answer = 0, factor = 1;
    for (int i = str.length()-1; i >= 0; i--) {
        answer += (str.charAt(i) - '0') * factor;
        factor *= 10;
    }
    return answer;
}

The above will work fine for positive integers, if the number is negative you’ll have to do a little checking first, but I’ll leave that as an exercise for the reader.