Method addUser

java/Chapter 8/Question8_5/UserManager.java:8–15

(int id, String details, int accountType)

	private Hashtable<Integer, User> users;
	
	public User addUser(int id, String details, int accountType) {
		if (users.containsKey(id)) {
			return null;
		}
		User user = new User(id, details, accountType);
		users.put(id, user);
		return user;
	}
	
	public boolean remove(User u) {
		return remove(u.getID());