In our case, the MessageApp interface represents an abstract product type. The implementation of each messaging app would reside in their respective concrete classes, which are concrete product types, such as SMSMessage, EmailMessage, and WhatsAppMessage. This relationship is described with the following diagram:

The product (abstract type) and all of the concrete product classes should look as follows:

// Product (abstract type)
public interface MessageApp {
  void sendMessage(String message);
}

//Concrete Product
public class EmailMessage implements MessageApp {
  @Override
  public void sendMessage(String message) {
    //Mail specific implementation
    System.out.println("Sending eMail message ...."+message);
  }
}

//Concrete Product
public class SMSMessage implements MessageApp {
  @Override
  public void sendMessage(String message) {
    //SMS specific implementation.
    System.out.println("sending SMS message ..."+message);
  }
}

//Concrete Product
public class WhatsAppMessage implements MessageApp {
  @Override
  public void sendMessage(String message) {
    //Whatsapp specific implementation
    System.out.println("Sending Whatsapp message ..."+message);
  }
}