Why I am getting java.lang.StringIndexOutOfBounds Exception?

Question

I have a program which is taking two String parameters and treating them as int's according to their indexes and try to do addition like we do on the paper but I am getting java.lang.StringOutOfBounds Exception , I couldn't solve it any help is appreciated Thanks in advance

import java.util.Arrays;

public class Algorithm {
    boolean flag=false;

    public void topla(String str1,String str2){

        String temp="";
        int sum1=0,sum2=0;

        String sonuc1="";
        if(str2.length()>str1.length()){
            temp=str1;
            str1=str2;
            str2=temp;

        }

         for (int i=0; i<str1.length(); i++)
         {
             sum1 = Integer.parseInt(str1.substring(str1.length()-1-i, 1));
             sum2=0;
             if(str2.length()-i>0){
                 sum2=Integer.parseInt(str2.substring(str2.length()-1-i,1));
             }
             else
                 sum2 = 0;
             sum1 = sum1 + sum2;
             if(flag)
             {
                 sum1 += 1;   
                 flag = false;
             }
             if (sum1 > 9)    
             {
                 flag = true;
                 sum1 -= 10;
             }
             sonuc1 += sum1;
         }
         char[] sonucu_duzelt = sonuc1.toCharArray();
         String string =new String(sonucu_duzelt);
         System.out.println(string);
         }

Answer 1

Lets have a look at substring method's implementation:

public String substring(int beginIndex, int endIndex) {
    if (beginIndex < 0) {
        throw new StringIndexOutOfBoundsException(beginIndex);
    }
    if (endIndex > value.length) {
        throw new StringIndexOutOfBoundsException(endIndex);
    }
    int subLen = endIndex - beginIndex;
    if (subLen < 0) {
        throw new StringIndexOutOfBoundsException(subLen);
    }
    return ((beginIndex == 0) && (endIndex == value.length)) ? this
            : new String(value, beginIndex, subLen);
}

And this code is important for you:

int subLen = endIndex - beginIndex;
if (subLen < 0) {
    throw new StringIndexOutOfBoundsException(subLen);
}

You must change your below lines:

Integer.parseInt(str1.substring(str1.length()-1-i, 1));
Integer.parseInt(str2.substring(str2.length()-1-i,1));

str1.length()-1-i can't be greater than 1 . When i equals to (length - 1) , str1.length()-1-i equals to 0. Because of 0 < 1, Jvm throws new StringIndexOutOfBoundsException() .