public class LoginSystem {
    private static LoginSystem instance = null;
    private ArrayList<User> userList;
    private User loggedUser = null;

    protected LoginSystem(){
        userList = new ArrayList<User>();
    }
    public static LoginSystem getInstance(){
        if(instance == null){
            instance = new LoginSystem();
        }
        return instance;
    }

    public boolean Login(String login, String password){
        User temp = null;
        for (User user : userList) {
            if(user.username.equals(login)){
                temp = user;
                break;
            }
        }
        if(temp == null)
            return false;
        if(!LoginSystem.byteArrayToHexString((login + "!" + password).getBytes()).equals(temp.password))
            return false;
        loggedUser = temp;
        return true;
    }

    public static String byteArrayToHexString(byte[] b) {
        String result = "";
        for (int i=0; i < b.length; i++) {
            result += Integer.toString( ( b[i] & 0xff ) + 0x100, 16).substring( 1 );
        }
        return result;
    }

    public static boolean Register(String login, String password)
    {
        instance.userList.add(new User(login, password));
        return true;
    }
}

class User {
    String username;
    String password;

    public User(String username, String password){
        this.username = username;
        this.password = LoginSystem.byteArrayToHexString((username + "!" + password).getBytes());
    }
}