Interface or an Abstract Class: which one to use?

Question

Please explain when I should use a PHP interface and when I should use an abstract class?

How I can change my abstract class in to an interface?

Answer 1

The differences between an Abstract Class and an Interface:

Abstract Classes

An abstract class can provide some functionality and leave the rest for derived class.

• The derived class may or may not override the concrete functions defined in the base class.

• A child class extended from an abstract class should logically be related.

Interface

An interface cannot contain any functionality. It only contains definitions of the methods.

• The derived class MUST provide code for all the methods defined in the interface.

• Completely different and non-related classes can be logically grouped together using an interface.

Answer 2

Also, just would like to add here that just because any other OO language has some kind of interfaces and abstraction too doesn't mean they have the same meaning and purpose as in PHP. The use of abstraction/interfaces is slightly different while interfaces in PHP actually don't have a real function. They merely are used for semantic and scheme-related reasons. The point is to have a project as much flexible as possible, expandable and safe for future extensions regardless whether the developer later on has a totally different plan of use or not.

If your English is not native you might lookup what Abstraction and Interfaces actually are. And look for synonyms too.

And this might help you as a metaphor:

INTERFACE

Let's say, you bake a new sort of cake with strawberries and you made up a recipe describing the ingredients and steps. Only you know why it's tasting so well and your guests like it. Then you decide to publish your recipe so other people can try that cake as well.

The point here is

- to make it right
- to be careful
- to prevent things which could go bad (like too much strawberries or something)
- to keep it easy for the people who try it out
- to tell you how long is what to do (like stiring)
- to tell which things you CAN do but don't HAVE to

Exactly THIS is what describes interfaces. It is a guide, a set of instructions which observe the content of the recipe. Same as if you would create a project in PHP and you want to provide the code on GitHub or with your mates or whatever. An interface is what people can do and what you should not. Rules that hold it - if you disobey one, the entire construct will be broken.

ABSTRACTION

To continue with this metaphor here... imagine, you are the guest this time eating that cake. Then you are trying that cake using the recipe now. But you want to add new ingredients or change/skip the steps described in the recipe. So what comes next? Plan a different version of that cake. This time with black berries and not straw berries and more vanilla cream...yummy.

This is what you could consider an extension of the original cake. You basically do an abstraction of it by creating a new recipe because it's a lil different. It has a few new steps and other ingredients. However, the black berry version has some parts you took over from the original - these are the base steps that every kind of that cake must have. Like ingredients just as milk - That is what every derived class has.

Now you want to exchange ingredients and steps and these MUST be defined in the new version of that cake. These are abstract methods which have to be defined for the new cake, because there should be a fruit in the cake but which? So you take the black berries this time. Done.

There you go, you have extended the cake, followed the interface and abstracted steps and ingredients from it.

Answer 3

Best practice is to use an interface to specify the contract and an abstract class as just one implementation thereof. That abstract class can fill in a lot of the boilerplate so you can create an implementation by just overriding what you need to or want to without forcing you to use a particular implementation.

Answer 4

The main difference is an abstract class can contain default implementation whereas an interface cannot.

An interface is a contract of behaviour without any implementation.

Answer 5

From a phylosophic point of view :

• An abstract class represents an "is a" relationship. Lets say I have fruits, well I would have a Fruit abstract class that shares common responsabilities and common behavior.

• An interface represents a "should do" relationship. An interface, in my opinion (which is the opinion of a junior dev), should be named by an action, or something close to an action, (Sorry, can't find the word, I'm not an english native speaker) lets say IEatable. You know it can be eaten, but you don't know what you eat.

From a coding point of view :

• If your objects have duplicated code, it is an indication that they have common behavior, which means you might need an abstract class to reuse the code, which you cannot do with an interface.

• Another difference is that an object can implement as many interfaces as you need, but you can only have one abstract class because of the "diamond problem" (check out here to know why! http://en.wikipedia.org/wiki/Multiple_inheritance#The_diamond_problem)

I probably forget some points, but I hope it can clarify things.

PS : The "is a"/"should do" is brought by Vivek Vermani's answer, I didn't mean to steal his answer, just to reuse the terms because I liked them!