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An explanation of Duck Typing vs Polymorphism
// When using Polymorphism, you might define an abstract class to send a message like
// follows, then fully implement it several times in different classes.
abstract class BaseMessage
public void setMessageText messageText
@messageText = messageText
end
public void send;
end
class EmailMessage extends BaseMessage
public void send
// code here to connect to mail server, send an email with @message, etc
end
end
class FacebookMessage extends BaseMessage
public void setMessageText messageText
// We can override the setMessageText definition in BaseMessage in both
// Polymorphism and Duck Typing
@messageText = messageText + "\n"
end
public void send
// code here to connect to Facebook and send the message
end
end
class NotAValidMessage // this class does not extend BaseMessage
public void setMessageText messageText
@messageText = messageText
end
public void send
// code here to do send this message somewhere
end
end
// If you then want to use any of these classes in a function, you do it like
// this. Whether the first argument is valid or not depends on it extending
// BaseMessage, thus passing NotAValidMessage would cause an error.
public void setMessageAndSend (BaseMessage message, messageText)
message.setMessageText(messageText)
message.send()
end
// If using duck typing, you'd define setMessageAndSend as follows:
public void setMessageAndSend (message, messageText)
message.setMessageText(messageText)
message.send()
end
// Notice that we don't specify any type for the first argument, merely
// assuming that what you pass does have functions named setMessageText
// and send. As such all of EmailMessage, FacebookMessage and
// NotAValidMessage can be passed to this alternative setMessageAndSend
// implementation.
________________________________________________________
// It helps to consider why an example like this is useful.
// If you are working on a large application, you might allow users to
// send emails and Facebook messages but also you might allow your computer
// servers to message each other to perform different tasks. These two use
// cases might be rather different things, both conceptually and in the code
// base, so forcing them to extend the same class could seem inappropriate.
// If you want to let users schedule "Happy Christmas" messages for Christmas
// Day, then you'll want to have code that can cause a message to send without
// writing it twice for Emails and Facebook messages. Polymorphism is sufficient
// for this use case.
// However, if you now also want to schedule messages between your computer
// servers, you have the choice of duplicating your scheduling code used for
// user messages, or using duck typing to just make your message scheduling work
// for any type that can be sent.
// Questions? https://46bit.com/contact/
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