String username = "jon.doe@abc.com";
    String password = "12345";

      if(username == null){
        Scanner login = new Scanner(System.in);
        System.out.println("Enter Username: ");
        username = login.nextLine();
        System.out.println("Enter Password: ");
        password = login.nextLine();
    }

    if(password == null && username != null){
        Scanner login = new Scanner(System.in);
        System.out.println("Enter Password: ");
        password = login.nextLine();
    }