Java calculator

import java.util.Scanner;

public class Calculator {
    static double num1, num2, returnValue;
    static boolean debug;

    public static void main(String[] args) {
        if (args.length == 3) {
            withArgs(args[0], Double.parseDouble(args[1]), Double.parseDouble(args[2]));
        } else if (args.length <= 0) {
            withoutArgs();
        } else if (args.length == 1 && args[0].equalsIgnoreCase("--debug")) {
            System.out.println("Debug mode enabled");
            debug = true;
            withoutArgs();
        }else {
            error();
        }
    }

    private static void withArgs(String type, double num1, double num2) {
        switch (type.toLowerCase()) {
            case "a":case "add":case "addition":
                returnValue = addition(num1, num2);
                System.out.println(num1 + " + " + num2 + " = " + returnValue);
                break;
            case "s":case "sub":case "subtraction":
                returnValue = subtraction(num1, num2);
                System.out.println(num1 + " - " + num2 + " = " + returnValue);
                break;
            case "m":case "mul":case "mult":case "multi":case "multiplication":
                returnValue = multiplication(num1, num2);
                System.out.println(num1 + " * " + num2 + " = " + returnValue);
                break;
            case "d":case "div":case "division":
                returnValue = division(num1, num2);
                System.out.println(num1 + " / " + num2 + " = " + returnValue);
                break;
            default:
                System.out.println("Invalid math operation."
                    +"\nValid operations: Addition, Subtraction, Multiplication, Division.");
                System.exit(0);
                break;
        }
    }

    private static void withoutArgs() {
        try (Scanner in = new Scanner(System.in)) {
            String type;

            System.out.print("Math Operation: ");
            type = in.nextLine();

            System.out.print("First Number: ");
            num1 = in.nextDouble();

            System.out.print("Second Number: ");
            num2 = in.nextDouble();

            withArgs(type, num1, num2);
        } catch (Exception e) {
            if (debug == true) {
                e.printStackTrace();
            } else {
                System.out.println("Invalid input. Inputs types:"
                    +"\n    Math Operation: String"
                    +"\n    First Number: Double"
                    +"\n    Second Number: Double");
            }
        }
    }

    private static void error() {
        System.out.println("Invalid usage."
            +"\nValid usage: Calculator [<operation>] [<first number>] [<second number>]");
    }

    private static double addition(double num1, double num2) {
        return num1 + num2;
    }

    private static double subtraction(double num1, double num2) {
        return num1 - num2;
    }

    private static double multiplication(double num1, double num2) {
        return num1 * num2;
    }

    private static double division(double num1, double num2) {
        return num1 / num2;
    }
}