calculator - Computing negative numbers in Java

Okay... I want to get around the concept of computing negative numbers. I am self-learning and I am running to a lot of difficulty. How do implement negative input?

My code:

public class MainSystem {


    public static void main(String[] args) {
        try (Scanner console = new Scanner(System.in)) {
            String input;

            System.out.print(">>> ");
            input = console.nextLine();
            splitEntry(input);

            System.out.println("The console is now closed.");
        }

    }

    private static void splitEntry(String input) {

        String function = "[+\\-*/]+"; //placing them in an index
        String[] token = input.split(function);//and this
        double num1 = Double.parseDouble(token[0]);
        double num2 = Double.parseDouble(token[1]);
        //double answer;
        String operator = input.toCharArray()[token[0].length()] + "";


        if (operator.matches(function) && (token[0] + token[1] + operator).length() == input.length()) {
            System.out.println("Operation is " + operator + ", your first number is " + token[0] + " your second number is " + token[1]);
        } else {
            System.out.println("Your entry of " + input + " is invalid");
        }
        if (operator.matches(function)
                && (token[0] + token[1] + operator).length() == input.length()) {
            double result = 0;
            if (operator.equals("+")) { // this is simplified by using formatters 

                result = num1 + num2;
            } else if (operator.equals("-")) {
                result = num1 - num2;
            } else if (operator.equals("*")) {
                result = num1 * num2;
            } else if (operator.equals("/")) {
                result = num1 / num2;
            }
            System.out.printf("Your first number %.2f %s by your second number %.2f = makes for %.2f%n", num1, operator, num2,
                    result);

        }
    }
}

How do I allow my calculator to place -2 + 5 and get the answer 3?

Every time I try to do that, the program crashes? What to do to make negative numbers to be computed