Evaluator.java

public class Evaluator {
    public class ParseException extends Exception {
      public ParseException () {
        super("Error parsing string");
      }

      public ParseException(String msg) {
        super(msg);
      }
    }

    public int eval(String expr) throws EmptyStackException, ParseException {
        return 0;
    }
}

RPN.java

public class RPN extends Evaluator {

    Stack<Integer> stack = null;

    public int eval(String expr) throws EmptyStackException, ParseException {
        stack = new Stack<Integer>();
        for (int i = 0; i < expr.length(); i++) {
            char c = expr.charAt(i);
            if (c >= '0' && c <= '9') {
                int num = c - '0';
                i++;
                while ( i < expr.length() && (expr.charAt(i) >= '0' && expr.charAt(i) <= '9')) {
                    num = num*10 + expr.charAt(i) - '0';
                    i++;
                }
                i--;
                stack.push(num);
            } else if (c == ' ') {
                // Ignore it!
            } else if (c == '*' || c == '/' || c == '%' || c == '+' || c == '-') {
                apply(c);
            } else {
                throw ParseException("Unknown symbol: " + c);
            }
        }
        return stack.pop();
    }

    private void apply(char op) throws EmptyStackException {
        int v2 = stack.pop();
        int v1 = stack.pop();
        int ans = 0;
        switch (op) {
            case '*':
            ans = v1 * v2;
            break;
            case '/':
            ans = v1 / v2;
            break;
            case '%':
            ans = v1 % v2;
            break;
            case '+':
            ans = v1 + v2;
            break;
            case '-':
            ans = v1 - v2;
            break;
        }
        stack.push(ans);
    }
}