To give some context to this topic, the idea was brought forward to me by Alex Eagle. I was happily coding away when Alex sprung his idea for Composition over Inheritance for Noop – a language we are developing with testability and dependency injection in mind. My gut reaction was that this was blasphemy, and it couldn’t be done. You can’t just do away with inheritance, its one of the building blocks of OO based programming languages. But now, after I have let the idea digest for a few days, it doesn’t seem so far fetched any more. And here’s why.
Let me first talk about the biggest problems with vanilla inheritance as we have it in Java. Joshua Bloch hits it on the nail in his Effective Java book item about “Favoring composition over inheritance.” But lets do a quick recap anyway.
The biggest problem is that inheritance often ends up breaking encapsulation. This is because the child class depends on the implementation of the parent class. But between releases, something in the parent class implementation can change and can break all child classes without even touching its code. Another common gotcha is in how protected fields and members are used. Often, the parent class changes the value of fields depending on how methods are called. Not understanding this behavior often leads to buggy or simply wrong behavior from the subclasses.
Another problem with a subclass – especially from the point of view of unit testing – is that there is no way to create an instance of the subclass in isolation. By this, I mean that everytime I create an instance of the subclass, I am forced to have the parent class as well. In most cases, this shouldn’t be a problem, but I have run into situations where the parent class is just a landmine waiting to explode, with the default constructor not being explicit in stating its dependencies. So instant Kablaam!!! Or the parent class will load things you don’t really care about and make things slow in a test. There was this insidious test I ran into once, which extended a base test case, which did the same thing. About 7 layers deep. And the test itself didn’t really care about 3 or 4 of those layers, but had to jump through all the hoops and get everything because it was a parent class.
There are a few more issues, which are well documented in Effective Java item 16, “Favor composition over inheritance.”. I won’t bore you further on this, assuming I have convinced the skeptics about the problems with inheritance. If not, go read that book, and you shall be convinced. But then, I wanted to postulate on whether it was at all possible to have a programming language which does away with inheritance (As Noop proposes).
So when do we use inheritance ? To me, Polymorphism is about the only time when inheritance and subclassing is deemed appropriate. Be it having different subtypes or just plain old code reuse. So unless you want to have a base abstract class which has some methods defined (Like Shape with draw() method and Circles and Rectangles), inheritance is not really needed.
In Java, interfaces allow you to perform polymorphic operations with abandon, and convert between types. And interfaces don’t straddle you down with the requirement that you get the base class for every instance.
Also, if you use composition, then you can reuse code by using delegation. For example, you could define a Shape interface with a DefaultShape implementation. Now rather than subclassing a concrete type Shape, you could have a Rectangle which implements Shape. And if you wanted to reuse some code, let Rectangle take in a DefaultShape instance and just delegate to it when necessary. This offers multiple benefits. One, you are not tied down to getting things from the base class. In your test, you could pass in a mock, a null, whatever you want. The only problem is that this option is not viable if you don’t have an interface. If that is the case (or the thing you are subclassing is in a package outside of your control), then you are stuck doing inheritance the old fashioned way.
And this is (atleast the last time I heard the proposal) what Noop aims to solve. When you want to subclass, you tell the class what you want to compose. Regardless of whether it is an interface or not, it will create that class with an instance of your composition type. By default, all methods in the composition type will be available in the subclass, and it will delegate automatically, unless you override it. You get complete control over object creation, and this could potentially support multiple inheritance through this approach.
What do other people think ? It this feasible ? Am I missing something obvious when inheritance is the only approach and composition just doesn’t cut it (both right now and in the Noop proposal) ? Are you interested in Noop ? Drop me a line.