Stack Based Calculator

I'm attempting to create a program that reads a string full of operations and numbers and prints the result.
So far I only have compatibility with only addition, however that does not seem to hard to add. I'm actually having problems rewriting the string... For example, when I do 5+5+3, I first do 5+5 then 10+3. In order to do this I'm going to have to re-write my string in this way:
5+5+3
10+3
13

So write now, my program does 5+5 then 5+3
returning 10 and 8

So I figure I need to re-write my string somehow... I need to re-write my string to 10+3 then just repeat the process? How would I do this?
All help is appreciated! Thanks.

Code:

---

public class Stack_Based_Calculator {
        public static String input;
        public static void main (String args[]) {
                input = "5+5+3";
                String operation = null;
                //getCharacter(String, letternumber)
                for(int i = 1; i <= input.length(); i++) {
                //System.out.println(getCharacter(input,i));
                       
                if(getCharacter(i).equalsIgnoreCase("+")) {
                        System.out.println(doAddition(i));
                        operation = "+";
               
                }
                }

               
        }

        private static int doAddition(int i) {
                int result = 0;
                int firstnumber = Integer.parseInt(getCharacter(i-1));
                int secondnumber = Integer.parseInt(getCharacter(i+1));
                result = firstnumber + secondnumber;
                return result;
        }

        private static String getCharacter(int i) {
                String letter = input.substring(i-1,i);
                return letter;
        }
       
}

---

Consider for a moment why this is called a stack based calculator. Perhaps a stack type data structure will be helpful. You can pop data off until you get a valid string (two operands and an operator), then push the result back onto the stack.

Thank you for the response, I found it very useful. I decided to take a step back so I don't get to ahead of myself. I'm going to start with 2 additives only. Then attempt to solve the problem using stacks. I'm having trouble checking to see if the current item we're on is not a number (i.e: "+") but i'm having some trouble with the

Code:

---

if stack.peek is "+"

---

I seem to be getting some errors. Thanks for all your help so far and thanks in advance for further explanation.

Errors:
Exception in thread "main" java.util.EmptyStackException
at java.util.Stack.peek(Unknown Source)
at Stack_Based_Calculator.main(Stack_Based_Calculator .java:19) <--- The if statement I was previously talking about.

Code:

---

import java.util.Stack;


public class Stack_Based_Calculator {
        public static String input;
        public static void main (String args[]) {
                Stack stack = new Stack();
                input = "5+3";
                int total = 0;


                for(int i = 1; i <= input.length(); i++) {
                        stack.push (new String(getCharacter(i)));

                }

                if(stack.contains("+") == true) {
                        for(int i = 1; i <= stack.capacity(); i++) {
                                if (stack.peek().toString().equalsIgnoreCase("+")) {
                                        stack.pop();
                                }
                                else
                                {
                                        total = total + Integer.parseInt((String) stack.pop());
                                }
                        }
                }
                System.out.println(total);
        }



        private static String getCharacter(int i) {
                String letter = input.substring(i-1,i);
                return letter;
        }

}

---

One of my blog entries talks about compilation and interpretation of expressions; I think it's usable here.

kind regards,

Jos