Classes should achieve polymorphic behavior and code reuse by their composition rather than inheritance from a base or parent class. To get the higher design flexibility, the design principle says that composition should be favored over inheritance. Attention reader! Get hold of all the important Java Foundation and Collections concepts with the Fundamentals of Java and Java Collections Course at a student-friendly price and become industry ready.
Reasons to Favour Composition over Inheritance in Java and OOP: The fact that Java does not support multiple inheritances is one reason for favoring composition over inheritance in Java. Since you can only extend one class in Java, but if you need multiple features, such as reading and writing character data into a file, you need Reader and Writer functionality.
It makes your job simple to have them as private members, and this is called Composition. Composition offers better test-ability of a class than Inheritance. If one class consists of another class, you can easily construct a Mock Object representing a composed class for the sake of testing. Inheritance is more rigid as most languages do not allow you to derive from more than one type.
Difference between composition and aggregation if you want to know. Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Collectives on Stack Overflow. Learn more. What is 'aggregation over inheritance'?
Asked 9 years, 4 months ago. Active 9 years, 4 months ago. Viewed 3k times. Improve this question. Community Bot 1 1 1 silver badge. Inheritance is used for polymorphism, where you have a base class and you want to extend or change its functionality. Inheritance allows us to define a class in terms of another class, which makes it easier to create and maintain an application.
This existing class is called the base class, and the new class is referred to as the derived class. The idea of inheritance implements the is a relationship. Most people use multiple-inheritance in the context of applying multiple interfaces to a class. So, you can treat a derived object like any of its base classes. Which among the following is correct for multiple inheritance?
Plus handling all the effects of a Bite in its own separate class could allow me to reuse it in that Frost class, because frost bites but isn't a BiterAnimal, and so on Composition is a very simple and tactile way of building objects Inheritance is relatively easy to understand, but still not as easily demonstrated in real life.
Many objects in real life can be broken down into parts and composed. Say a bicycle can be built using two wheels, a frame, a seat, a chain etc. Easily explained by composition. Whereas in an inheritance metaphor you could say that a bicycle extends a unicycle, somewhat feasible but still much further from the real picture than composition obviously this is not a very good inheritance example, but the point remains the same. Even the word inheritance at least of most US English speakers I would expect automatically invokes a meaning along the lines "Something passed down from a deceased relative" which has some correlation with its meaning in software, but still only loosely fits.
Composition is almost always more flexible Using composition you can always choose to define your own behavior or simply expose that part of your composed parts. This way you face none of the restrictions that may be imposed by an inheritance hierarchy virtual vs. So, it could be because Composition is naturally a simpler metaphor that has less theoretical constraints than inheritance. Furthermore, these particular reasons may be more apparent during design time, or possibly stick out when dealing with some of the pain points of inheritance.
Inheritance is widely used, has lots of benefits and many times is more elegant than composition. These are just some possible reasons one could use when favoring composition. Perhaps you just noticed people saying this in the last few months, but it has been known to good programmers for a lot longer than that. I've certainly been saying it where appropriate for about a decade.
The point of the concept is that there is a large conceptual overhead to inheritance. When you are using inheritance, then every single method call has an implicit dispatch in it. If you have deep inheritance trees, or multiple dispatch, or even worse both, then figuring out where the particular method will dispatch to in any particular call can become a royal PITA.
It makes correct reasoning about the code more complex, and it makes debugging harder. Let me give a simple example to illustrate.
Suppose that deep in an inheritance tree, someone named a method foo. Then someone else comes along and adds foo at the top of the tree, but doing something different. This case is more common with multiple inheritance. Now that person working at the root class has broken the obscure child class and probably doesn't realize it. Admittedly there are ways to write unit tests that will catch this, but there are also cases where you can't easily write tests that way.
By contrast when you use composition, at each call it is usually clearer what you are dispatching the call to. OK, if you're using inversion of control, for instance with dependency injection, then figuring out where the call goes can also get problematic.
But usually it is simpler to figure out. This makes reasoning about it easier. As a bonus, composition results in having methods segregated from each other.
The above example should not happen there because the child class would move off to some obscure component, and there is never a question about whether the call to foo was intended for the obscure component or the main object. Now you are absolutely right that inheritance and composition are two very different tools that serve two different types of things.
Sure inheritance carries conceptual overhead, but when it is the right tool for the job, it carries less conceptual overhead than trying to not use it and do by hand what it does for you. Nobody who knows what they are doing would say that you should never use inheritance. But be sure it is the right thing to do. Unfortunately many developers learn about object oriented software, learn about inheritance, and then go out to use their new axe as often as possible.
Which means that they try to use inheritance where composition was the right tool. Hopefully they will learn better in time, but frequently this does not happen until after a few removed limbs, etc. Telling them up front that it is a bad idea tends to speed up the learning process and reduce injuries.
In one of the comment, Mason mentionned that one day we would be speaking about Inheritance considered harmful. The problem with inheritance is at once simple, and deadly, it does not respect the idea that one concept should have one functionality. This is not the only approach, though languages are mostly stuck with it. Also, the lack of ease for doing otherwise contribute to making it largely used: frankly, even knowing this, would you inherit from an interface and embed the base class by composition, delegating most of the method calls?
It would be considerably better though, you'd even be warned if suddenly a new method pops in the interface and would have to choose, consciously, how to implement it. As a counter-point, Haskell only allow to use the Liskov Principle when "deriving" from pure interfaces called concepts 1. You cannot derive from an existing class, only composition allow you to embed its data.
It's a reaction to the observation that OO beginners tend to use inheritance when they don't need to. Inheritance is certainly not a bad thing, but it can be overused. If one class just needs functionality from another, then composition will probably work. In other circumstances, inheritance will work and composition won't. Inheriting from a class implies a lot of things. It implies that a Derived is a type of Base see the Liskov Substitution principle for the gory details , in that whenever you use a Base it would make sense to use a Derived.
It gives the Derived access to the protected members and member functions of Base. It's a close relationship, meaning it has high coupling, and changes to one are more likely to require changes to the other.
Coupling is a bad thing. It makes programs harder to understand and modify. Other things being equal, you should always select the option with less coupling. Therefore, if either composition or inheritance will do the job effectively, choose composition, because it's lower coupling. If composition will not do the job effectively, and inheritance will, choose inheritance, because you have to. IMHO, It comes down to the fact that most programmers don't really get inheritance, and end up overdoing it, partially as a result of how this concept is taught.
This concept exists as a way to try to dissuade them from overdoing it, instead of focusing on teaching them how to do it right. Anyone who has spent time teaching or mentoring knows that this is what often happens, especially with new developers who have experience in other paradigms:.
These developers initially feel that inheritance is this scary concept. So they end up creating types with interface overlaps e. Then often as a result of overzealous teaching , they learn about the benefits of inheritance, but it's often taught as the catch-all solution to reuse.
They end up with a perception that any shared behavior must be shared through inheritance. This is because the focus is often on implementation inheritance rather than subtyping.
For the sake of avoiding rewriting and for making sure they have taken advantage of sharing implementation, they start designing their hierarchy around the intended implementation rather than the underlying abstractions. The "Stack inherits from doubly-linked list" is a good example of this.
In Java, many teachers do not distinguish the "no multiple inheritance" or "multiple inheritance is evil" from the importance of interfaces. All this is compounded by the fact that we've learned so much of the beauty of not having to write superfluous code with implementation inheritance, that tons of straightforward delegation code looks unnatural.
So composition looks messy, which is why we need these rules of thumbs to push new programmers to use them anyway which they overdo as well. The simple answer is: inheritance has greater coupling than composition. Given two options, of otherwise equivalent qualities, choose the one that is less coupled. I think this sort of advice is like saying "prefer driving to flying". That is, planes have all sorts of advantages over cars, but that comes with a certain complexity.
Whereas when you do need to fly, it's generally supposed to be obvious. Likewise, inheritence can do things composition can't, but you should use that when you need it, and not when you don't. So if you're never tempted to just assume you need inheritence when you don't, then you don't need the advice of "prefer composition".
But many people do , and do need that advice. It's supposed to be implicit that when you really DO need inheritance, it's obvious, and you should use it then. Inheritance isn't inherently bad, and composition isn't inherently good. They are merely tools that an OO programmer can use to design software.
When you look at a class, is it doing more than it absolutely should SRP? Is it duplicating functionality unnecessarily DRY , or is it overly interested in the properties or methods of other classes Feature Envy?.
0コメント