Stack Based Calculator

[Jeremy5]

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.

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;
	}
	
}

[sunde887]

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.

[Jeremy5]

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

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.

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;
	}

}

[JosAH]

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

kind regards,

Jos