Parsing Operator and Operand

Question

OK, so I am writing a console calculator and I am stuck. I have everything working but the actual part where a user enters say 3*2+8 and then the program tell the user the answer. I can, however, enter a single number like 3 and it return 3.0. I think I need to parse the expression entered by the user to perform the math. Can you help me?

    public static void main(String[] args) {
        String input = ""; // initalize the string
        boolean isOn = true; // used for the while loop...when false, the program will exit.
        String exitCommand = "Exit"; // exit command



        System.out.print("Enter a math problem"); // dummy test
        Scanner keyboard = new Scanner(System.in);
        //input = keyboard.nextLine();
        String[] token = input.split(("(?<=[-+*/])|(?=[-+*/])"));
        while (isOn) {
            for (int i = 0; i < token.length; i++) {
                //System.out.println(token[i]);
                Double d = Double.parseDouble(keyboard.nextLine()); //This causes an error
                //String[] token = d.split(("(?<=[-+*/])|(?=[-+*/])"));
                //input = keyboard.nextLine();

                if (input.equalsIgnoreCase(exitCommand)) {
                    // if the user enters exit(ignored case) the boolean goes to false, closing the application
                    isOn = false;
                }

               System.out.print(token[0] + d); // shows the math problem(which would by the end of the coding should show the
                //answer to the entered math problem.

            }


        }
    }
    public void validOperator() {
        ArrayList<String> operator = new ArrayList<String>();
        operator.add("+");
        operator.add("-");
        operator.add("*");
        operator.add("/");

    }
    public void validOperands(){
        ArrayList<String> operand = new ArrayList<String>();
        operand.add("0");
        operand.add("1");
        operand.add("2");
        operand.add("3");
        operand.add("4");
        operand.add("5");
        operand.add("6");
        operand.add("7");
        operand.add("8");
        operand.add("9");
    }
}

Thanks

Answer 1

You do not take in account the precedence of math operators. And what if a user will use brackets, like (2 + 3) * 4 + 5/2

I recommend to convert the initial expression to the postfix notation before evaluating it.

Here is the example with explanation for C++, I think it will be easy to apply this explanation to the java language.