postfix evaluate

double evalute() {
    OurStack<String> postFixStack = new OurStack<String>();

    while (!postFix.isEmpty())
    {
        curr = postFix.remove();

        if (!isoperand(curr)) {
            // curr is +, -, *, /

            double operand2 = postFixStack.pop();
            double operand1 = postFixStack.pop();

            if curr == "+"
                postFixStack.push(operand1 + operand2);
            else if curr == "-"
                postFixStack.push(operand1 - operand2);
            else if curr == "*"
                postFixStack.push(operand1 * operand2);
            else
                postFixStack.push(operand1 / operand2);
        } else {
            // curr is a number

            postFixStack.push(Double.parseDouble(curr));
        }
    }

    return postFixStack.peek(); // last number remaining on stack
}