public Class MailConfiguration{

  public static String userName;
  public static String password;
  public static EmailServiceProvider emailService;
}

public Class MailInit{

  public void initialConfiguration(String userName, String password, String emailService ){
    MailConfiguration.userName = userName;
    MailConfiguration.password = password;

    if(emailService == "google"){
      MailConfiguration.emailService = Class.forName('GmailEmailProvider').getInstance();
    }

    // Register new service in the system, no cascading changes
    else if (emailService == "hotmail"){

      MailConfiguration.emailService = Class.forName('HotEmailProvider').getInstance();
    }

    else{
      MailConfiguration.emailService = Class.forName('YahooEmailProvider').getInstance();
    }
  }
}


public Interface EmailServiceProvider{

  public Connection connect(String userName, String password);
  public void sendEmail(String recipient, String subject, String messageBody);
  public String receiveEmail();
}

public Class GoogleEmailProvider implements EmailServiceProvider{

  private GMailLowLevelClass gmailLowLevelConnection;

  @Override
  public connect(String userName, String password){

    GMailLowLevelClass gmailLowLevelClassObject = new GMailLowLevelClass();
    this.gmailLowLevelConnection = gmailLowLevelClassObject.connect(String userName, String password);
  }

  @Override
  public sendEmail(String recipient, String subject, String messageBody){

    this.gmailLowLevelConnection.send(recipient, subject , messageBody );
  }

  @Override
  public String receiveEmail(){

    return this.gmailLowLevelConnection.receiveEmail();
  }
}

public Class YahooEmailService implements EmailService{

  private YahooMailLowLevelClass yahooLowLevelConnection;

  @Override
  public connect(String userName, String password){

    YahooLowLevelClass yahooLowLevelClassObject = new YahooLowLevelClass();
    this.yahooLowLevelConnection = yahooLowLevelClassObject.connect(String userName, String password);
  }

  @Override
  public sendEmail(String recipient, String subject, String messageBody){

    this.yahooLowLevelConnection.send(recipient, subject , messageBody );
  }

  @Override
  public String receiveEmail(){

    return this.yahooLowLevelConnection.receiveEmail();
  }
}

// Adding new feature by extension i.e open closed principle
public Class HotEmailService implements EmailService{

  private HotMailLowLevelClass hotMailLowLevelConnection;

  @Override
  public connect(String userName, String password){

    HotMailLowLevelClass hotmailLowLevelClassObject = new HotMailLowLevelClass();
    this.hotMailLowLevelConnection = hotmailLowLevelClassObject.connect(String userName, String password);
  }

  @Override
  public sendEmail(String recipient, String subject, String messageBody){

    this.hotMailLowLevelConnection.send(recipient, subject , messageBody );
  }

  @Override
  public String receiveEmail(){

    return this.hotMailLowLevelConnection.receiveEmail();
  }
}

// MainClient class remains intact.
public Class MailClient{

  public void sendEmail(String recipient, String subject, String messageBody){

    MailConfiguration.emailService.connect(MailConfiguration.userName, MailConfiguration.password);
    MailConfiguration.emailService.send(recipient, subject,messageBody);
  }

  public String receiveEmail(){

    MailConfiguration.emailService.connect(MailConfiguration.userName, MailConfiguration.password);
    return MailConfiguration.emailService.receive();
  }
}