/**
 * Write a description of class CalculatorStack here.
 * 
 * @author John Paxton
 * @version 1.0
 */

import java.util.Stack;
import java.util.EmptyStackException;

public class CalculatorStack
{
    CalculatorStack()
    {
        stack = new Stack<Integer> ();
    }
    
    public void setString (String what)
    {
        toEvaluate = what;
    }
    
    public void evaluate()
    {
        char current;
        Integer number1;
        Integer number2;
        
        stack.clear();
        error = false;
        
        try
        {
            for (int i = 0; i < toEvaluate.length(); i++)
            {
                current = toEvaluate.charAt(i);
                if ((current == '+') || (current == '-'))
                {
                    number2 = stack.pop();
                    number1 = stack.pop();
                    if (current == '+')
                    {
                        stack.push ( number1 + number2 );
                    }
                    else
                    {
                        stack.push( number1 - number2 );
                    }
                }   
                else
                {
                    stack.push(Character.getNumericValue(current));
                }
            }
            result = stack.pop();
        }
        catch (EmptyStackException e)
        {
            error = true;
        }
        
        if (!stack.empty())
        {
            error = true;
        }
    }
    
    public void printResult()
    {
        System.out.print("Evaluating " + toEvaluate + " yields ");
        if (error)
        {
            System.out.println("an ERROR");
        }
        else
        {
            System.out.println(result);
        }
    }
    
    private Stack<Integer> stack;
    private String toEvaluate;
    private int result;
    private boolean error;
}