AllOperators.java

public double evaluate(String s) {
    String[] elements = getStringArray(s);
    Stack<Double> numbers = new Stack();
    double result = 0.0;
    for (String c : elements) {
        switch (c) {
            case "+":
                result = numbers.push(numbers.pop() + numbers.pop());
                break;
            case "-":
                result = numbers.push(numbers.pop() - numbers.pop());
                break;
            case "*":
                result = numbers.push(numbers.pop() * numbers.pop());
                break;
            case "/":
                result = numbers.push(numbers.pop() / numbers.pop());
                break;
            default:
                numbers.push(Double.parseDouble(c));
                break;
        }
    }
    return result;
}