UML Class Diagram – Choosing Between Abstract Class and Interface

abstract classclass-diagraminterfacesuml

I am modeling a class diagram and have spotted an opportunity to simplify it slightly. What I want to know is, would this it be better to implement an abstract class or an interface?

The scenario is this, I have the classes:

Artist
Genre
Album
Song

All of which share the methods getName, setName, and getCount (playcount that is). Would it be best to create an abstract 'Music' class with the aforementioned abstract methods, or should I create an interface, since the classes that implement the interface have to include all of the interface's methods (I think, correct me if I'm wrong).

I hope I've given enough detail, please ask questions if I haven't.

Thanks!

Best Answer

You should create an abstract class.

Generally speaking, a song can consist of a Title / Artist / Album / Genre. All of those are just a string of data which can be handled the same way. By using an abstract class you don't have to write mentioned "getName", "setName" and "getCount" more than once, which sounds pretty neat.

Using an interface would just tell you that the functions should exist in each class, but you'd still have to write them separately. However, if you come up with methods they all have in common, but which work differently for each class, I suppose an interface would make more sense.

What bugs me though is that you want them to be classes. For example, since you would want an album to have it's own "Times played", you'd have to make sure that there's only one instance of that specific album. Each time a new song is added, you'd either have to search through all existing classes with that name and set a pointer or something similar to the already existing one.

Then again, the good part about using an album as an instance of a class, is that it's possible to have several albums with the same "name" without counting them as the same album. Err.. anyways, my mind has drifted away and this is not really related to the question.

Related Solutions

C# Design – When to Use Abstract Classes vs Interfaces with Extension Methods

When you need a part of the class to be implemented. The best example I've used is the template method pattern.

public abstract class SomethingDoer
{
    public void Do()
    {
        this.DoThis();
        this.DoThat();
    }

    protected abstract void DoThis();
    protected abstract void DoThat();
}

Thus, you can define the steps that will be taken when Do() is called, without knowing the specifics of how they will be implemented. Deriving classes must implement the abstract methods, but not the Do() method.

Extensions methods don't necessarily satisfy the "must be a part of the class" part of the equation. Additionally, iirc, extension methods cannot (appear to) be anything but public in scope.

edit

The question is more interesting than I had originally given credit for. Upon further examination, Jon Skeet answered a question like this on SO in favour of using interfaces + extension methods. Also, a potential downside is using reflection against an object hierarchy designed in this way.

Personally, I am having trouble seeing the benefit of altering a currently common practice, but also see few to no downsides in doing it.

It should be noted that it is possible to program this way in many languages via Utility classes. Extensions just provide the syntactic sugar to make the methods look like they belong to the class.

Are Abstract Classes and Methods Obsolete?

No.

Interfaces cannot provide default implementation, abstract classes and method can. This is especially usefull to avoid code duplication in many cases.

This is also a really nice way to reduce sequential coupling. Without abstract method/classes, you cannot implement template method pattern. I suggest you look at this wikipedia article : http://en.wikipedia.org/wiki/Template_method_pattern

Best Answer

Related Solutions

C# Design – When to Use Abstract Classes vs Interfaces with Extension Methods

Are Abstract Classes and Methods Obsolete?

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