private void launch(){
        
        // variables
        String input = "";
        Fraction value = new Fraction(1,1);
        
        // create Scanner
        Scanner scan = new Scanner(System.in);
        
        // welcome message
        System.out.println("Welcome to James Hills' Fraction Calculator.");
        System.out.println("Please enter a calculation below:");
        
        // obtain intital input
        input = scan.nextLine();
        
        // take commands from the user and display the result
        do  {
            
            value = evaluate(value, input);
            System.out.println(" = " + value.toString());
            input = scan.nextLine();
            
            } while (!input.equalsIgnoreCase("Goodbye"));
        
        // closing
        System.out.println();
        System.out.println("Fraction Calculator now closing.");
    }