Evaluating Posfix using Stack (LinkedList)

Question

I need to evaluate postfix expressions using a linked list stack. I think i need some help with the algorithm. I write 13+ as input but i get 100 as output.

PostfixCalculator Class:

public class PostfixCalculator{
    String expression;
    MyStack stack = new MyStack<Double>();

    public PostfixCalculator(String postFixExpression)
    {
         expression = postFixExpression;
    }

    public String calculate()
    {
        String output = "";
        char character = ' ';
        double digit = 0;

        for(int x = 0; x < expression.length(); x++)
        {
            if(Character.isDigit(expression.charAt(x))) {
                    digit = expression.charAt(x);
            }
            character = expression.charAt(x);
            if(expression.charAt(x) == digit)
            {
                stack.push(digit);
            }
            else if(character == '*')
            {
                double tmp = (double) stack.pop() * (double) stack.pop();
                stack.push(tmp);
            }
            else if(character == '/')
            {
                double tmp = (double) stack.pop() / (double) stack.pop();
                stack.push(tmp);
            }
            else if(character == '+')
            {
                double tmp = (double) stack.pop() + (double) stack.pop();
                stack.push(tmp);
            }
            else if(character == '-')
            {
                double tmp = (double) stack.pop() - (double) stack.pop();
                stack.push(tmp);
            }
        }

        while(!stack.isEmpty())
        {
            output = output + (double) stack.pop();
        }

        return output;
    }
}

PostfixCalculatorTest Class:

import java.util.Scanner;

public class PostfixCalculatorTest
{

    public static void main(String[] args)
    {
        Scanner input = new Scanner(System.in);
        System.out.println("Type the postfix expression that you want to evaluate");
        String expression = input.next();
        PostfixCalculator calculator = new PostfixCalculator(expression);
        System.out.println(calculator.calculate());
    }
}

Answer 1

First this

if(Character.isDigit(expression.charAt(x))) {
     digit = expression.charAt(x);
}

saves the decimal ASCII value of the char at the position x as double, for the char '1' it is 49, for '3' it is 51, hence you get 100 as result

it should be

digit = Double.parseDouble("" + expression.charAt(x));

i.e. parse the char to get the double value it represents.

Here is the small change

character = expression.charAt(x);
if(Character.isDigit(character)) {
    digit = Double.parseDouble("" + character);
    stack.push(digit);
}

then it would work for 13+ and give 4 as result.

Those line can be removed:

character = expression.charAt(x);
if(expression.charAt(x) == digit)
{
    stack.push(digit);
}