FinalSolution.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 "-": {
                    double firstNumber = numbers.pop();
                    double secondNumber = numbers.pop();
                    result = numbers.push(secondNumber - firstNumber);
                    break;
                }
                case "*": {
                    result = numbers.push(numbers.pop() * numbers.pop());
                    break;
                }
                case "/": {
                    double firstNumber = numbers.pop();
                    double secondNumber = numbers.pop();
                    result = numbers.push(secondNumber / firstNumber);
                    break;
                }
                default:
                    numbers.push(Double.parseDouble(c));
                    break;
            }
        }
        return result;
    }