Show the basics of inheritance and polymorphism in object-oriented programming. Introductory. Part of Industrial Logic's Object-Oriented Design Workshop. http://industriallogic.com/shop

1. Industrial Logic, Inc.
Twitter: @IndustrialLogic
Polymorphism
Crafting Interchangeable Parts

6. Interface

7. Polymorphism Example

8. Duplicated Code
public class SmsMessager
{
private string[] _numbers;
public void Numbers(string[] numbers)
{
_numbers = numbers;
}
public void SendMessage(string subject, string text)
{
SmsConnector.SendMessage(subject, text).To(_numbers).Execute();
}
}
public class EmailMessager
{
private List<string> _recipients;
public void Recipients(List<string> recipients)
{
_recipients = recipients;
}
public void SendEmail(string subject, string body)
{
EmailGateway.Recipients(_recipients).Subject(subject).Body(body).Send();
}
}

9. Caller repeats itself
public class MessagerCaller
{
public static void BorrowFromPhoneFriends(SmsMessager sms, string[] friends)
{
sms.Numbers(friends);
sms.SendMessage("Hello!", "Can I borrow $100.00?");
}
public static void BorrowFromEmailFriends(EmailMessager email, List<string> friends)
{
email.Recipients(friends);
email.SendEmail("Hello!", "Can I borrow $100.00?");
}
}

10. Duplicated Code

11. Make them the same
public class SmsMessager
{
private List<string> _recipients;
public void Recipients(List<string> recipients)
{
_recipients = recipients;
}
public void SendMessage(string subject, string text)
{
SmsConnector.SendMessage(subject, text).To(_recipients.ToArray()).Execute();
}
}
public class EmailMessager
{
private List<string> _recipients;
public void Recipients(List<string> recipients)
{
_recipients = recipients;
}
public void SendMessage(string subject, string text)
{
EmailGateway.Recipients(_recipients).Subject(subject).Body(text).Send();
}
}

12. Make them the same

13. Not really better yet
public class MessagerCaller
{
public static void BorrowFromPhoneFriends(SmsMessager sms, List<string> friends)
{
sms.Recipients(friends);
sms.SendMessage("Hello!", "Can I borrow $100.00?");
}
public static void BorrowFromEmailFriends(EmailMessager email, List<string> friends)
{
email.Recipients(friends);
email.SendMessage("Hello!", "Can I borrow $100.00?");
}
}

14. Extract a base class
abstract class BaseMessager
{
protected List<string> _recipients;
public void Recipients(List<string> recipients)
{
_recipients = recipients;
}
public abstract void SendMessage(string subject, string text);
}
public class SmsMessager : BaseMessager
{
public override void SendMessage(string subject, string text)
{
SmsConnector.SendMessage(subject, text).To(_recipients.ToArray()).Execute();
}
}
public class EmailMessager : BaseMessager
{
public override void SendMessage(string subject, string text)
{
EmailGateway.Recipients(_recipients).Subject(subject).Body(text).Send();
}
}

15. Extract a base class

16. Extract an interface
public interface Messager
{
void Recipients(List<string> recipients);
void SendMessage(string subject, string text);
}
abstract class BaseMessager : Messager
{
protected List<string> _recipients;
public void Recipients(List<string> recipients)
{
_recipients = recipients;
}
public abstract void SendMessage(string subject, string text);
}

17. Extract an interface

18. Caller uses Interface
public class MessagerCaller
{
public static void Borrow(Messager messager, List<string> friends)
{
messager.Recipients(friends);
messager.SendMessage("Hello!", "Can I borrow $100.00?");
}
}

19. Polymorphism
Advantages and
Disadvantages

20. Duplication

21. Before
public class MessagerCaller
{
public static void BorrowFromPhoneFriends(SmsMessager sms, string[] friends)
{
sms.Numbers(friends);
sms.SendMessage("Hello!", "Can I borrow $100.00?");
}
public static void BorrowFromEmailFriends(EmailMessager email, List<string> friends)
{
email.Recipients(friends);
email.SendEmail("Hello!", "Can I borrow $100.00?");
}
}
After
public class MessagerCaller
{
public static void Borrow(Messager messager, List<string> friends)
{
messager.Recipients(friends);
messager.SendMessage("Hello!", "Can I borrow $100.00?");
}
}

22. Extension

23. Message my Facebook
friends
public class FacebookMessager : BaseMessager
{
public override void SendMessage(string subject, string text)
{
List<Friend> friends = Facebook.FriendsFrom(_recipients);
foreach(var friend in friends)
{
Post message = new FacebookMessage(friend, subject, text);
Facebook.Message(message);
}
}
}

24. Message my Facebook friends

25. Speculative Generality

26. Combinatorial Explosion

27. Summary
• Use polymorphism to craft interchangeable
parts by:
• Implementing interfaces
• Putting shared code in a base class
• Avoid speculative generality and combinatorial
explosions

28. Polymorphism Exercise