When to use an interface instead of an abstract class and vice versa?

Question

This may be a generic OOP question. I wanted to do generic comparison between an interface and an abstract class on the basis of their usage. When would one want to use and interface and when would on want to use an abstract class?

Answer 1

Use an abstract class if you want to provide some basic implementations.

Answer 2

in java you can inherit from one (abstract) class to "provide" functionality and you can implement many interfaces to "ensure" functionality

Answer 3

An abstract class can have shared state or functionality. An interface is only a promise to provide the state or functionality. A good abstract class will reduce the amount of code that has to be rewritten because it's functionality or state can be shared. The interface has no defined information to be shared

Answer 4

This can be a very difficult call to make...

One pointer I can give: An object can implement many interfaces, whilst an object can only inherit one base class( in a modern OO language like c#, I know C++ has multiple inheritance - but isn't that frowned upon?)

Answer 5

An abstract class can have implementations.

An interface doesn't have implementations, it simply defines a kind of contract.

There can also be some language-dependent differences: for example C# does not have multiple inheritance, but multiple interfaces can be implemented in a class.

Answer 6

The answers vary between languages. For example, in Java a class can implement (inherit from) multiple interfaces but only inherit from one abstract class. So interfaces give you more flexibility. But this is not true in C++.

Answer 7

Purely on the basis of inheritance, you would use an Abstract where you're defining clearly descendant, abstract relationships (i.e. animal->cat) and/or require inheritance of virtual or non-public properties, especially shared state (which Interfaces cannot support).

You should try and favour composition (via dependency injection) over inheritance where you can though, and note that Interfaces being contracts support unit-testing, separation of concerns and (language varying) multiple inheritance in a way Abstracts cannot.

Answer 8

I wrote an article about that:

http://www.thecoldsun.com/en/content/01-2009/abstract-classes-and-interfaces

Summarizing: When we talk about abstract classes we are defining characteristics of an object type, specifying what an object is but in the case of an interface we define a capability and we bond to provide that capability, we are talking about establishing a contract about what the object can do.

Answer 9

Personally, I almost never have the need to write abstract classes.

Most times I see abstract classes being (mis)used, it's because the author of the abstract class is using the "Template method" pattern.

The problem with "Template method" is that it's nearly always somewhat re-entrant - the "derived" class knows about not just the "abstract" method of its base class that it is implementing, but also about the public methods of the base class, even though most times it does not need to call them.

(Overly simplified) example:

abstract class QuickSorter
{
    public void Sort(object[] items)
    {
        // implementation code that somewhere along the way calls:
        bool less = compare(x,y);
        // ... more implementation code
    }
    abstract bool compare(object lhs, object rhs);
}

So here, the author of this class has written a generic algorithm and intends for people to use it by "specializing" it by providing their own "hooks" - in this case, a "compare" method.

So the intended usage is something like this:

class NameSorter : QuickSorter
{
    public bool compare(object lhs, object rhs)
    {
        // etc.
    }
}

The problem with this is that you've unduly coupled together two concepts:

1. A way of comparing two items (what item should go first)
2. A method of sorting items (i.e. quicksort vs merge sort etc.)

In the above code, theoretically, the author of the "compare" method can re-entrantly call back into the superclass "Sort" method... even though in practise they will never want or need to do this.

The price you pay for this unneeded coupling is that it's hard to change the superclass, and in most OO languages, impossible to change it at runtime.

The alternative method is to use the "Strategy" design pattern instead:

interface IComparator
{
    bool compare(object lhs, object rhs);
}

class QuickSorter
{
    private readonly IComparator comparator;
    public QuickSorter(IComparator comparator)
    {
        this.comparator = comparator;
    }

    public void Sort(object[] items)
    {
        // usual code but call comparator.Compare();
    }
}

class NameComparator : IComparator
{
    bool compare(object lhs, object rhs)
    {
        // same code as before;
    }
}

So notice now: All we have are interfaces, and concrete implementations of those interfaces. In practise, you don't really need anything else to do a high level OO design.

To "hide" the fact that we've implemented "sorting of names" by using a "QuickSort" class and a "NameComparator", we might still write a factory method somewhere:

ISorter CreateNameSorter()
{
    return new QuickSorter(new NameComparator());
}

Any time you have an abstract class you can do this... even when there is a natural re-entrant relationship between the base and derived class, it usually pays to make them explicit.

One final thought: All we've done above is "compose" a "NameSorting" function by using a "QuickSort" function and a "NameComparison" function... in a functional programming language, this style of programming becomes even more natural, with less code.

Answer 10

When to do what is a very simple thing if you have the concept clear in your mind.

Abstract classes can be Derived whereas Interfaces can be Implemented. There is some difference between the two. When you derive an Abstract class, the relationship between the derived class and the base class is 'is a' relationship. e.g., a Dog is an Animal, a Sheep is an Animal which means that a Derived class is inheriting some properties from the base class.

Whereas for implementation of interfaces, the relationship is "can be". e.g., a Dog can be a spy dog. A dog can be a circus dog. A dog can be a race dog. Which means that you implement certain methods to acquire something.

I hope I am clear.

Answer 11

One interesting location where interfaces fare better than abstract classes is when you need to add extra functionality to a group of (related or unrelated) objects. If you cannot give them a base abstract class (e.g., they are sealed or already have a parent), you can give them a dummy (empty) interface instead, and then simply write extension methods for that interface.

Answer 12

I wrote an article of when to use an abstract class and when to use an interface. There is a lot more of a difference between them other than "one IS-A... and one CAN-DO...". To me, those are canned answers. I mention a few reasons when to use either of them. Hope it helps.

http://codeofdoom.com/wordpress/2009/02/12/learn-this-when-to-use-an-abstract-class-and-an-interface/

Answer 13

OK, having just "grokked" this myself - here it is in layman's terms (feel free to correct me if I am wrong) - I know this topic is oooooold, but someone else might stumble across it one day...

Abstract classes allow you to create a blueprint, and allow you to additionally CONSTRUCT (implement) properties and methods you want ALL its descendants to possess.

An interface on the other hand only allows you to declare that you want properties and/or methods with a given name to exist in all descendants - regardless of implementation. Also, a class can implement MANY interfaces, but can only extend ONE Abstract class.

Even though both get "extended", they serve rather different purposes once you think about it. An Interface is more of a high level architectural tool (which becomes clearer if you start to grasp design patterns) - an Abstract has a foot in both camps and can perform some of the dirty work too.

Why use one over the other? The former allows for a more concrete definition of descendants - the latter allows for greater polymorphism. This last point is important to the end user/coder, who can utilise this information to implement the A.P.I(nterface) in a variety of combinations/shapes to suit their needs.

I think this was the "lightbulb" moment for me - think about interfaces less from the author's perpective and more from that of any coder coming later in the chain who is adding implementation to a project, or extending an API.