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

I wrote an article about that:

Abstract classes and interfaces

Summarizing:

When we talk about abstract classes we are defining characteristics of an object type; specifying what an object is.

When we talk about an interface and define capabilities that we promise to provide, we are talking about establishing a contract about what the object can do.

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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

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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:

A way of comparing two items (what item should go first) 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.

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If you are looking at java as OOP language,

"interface does not provide method implementation" is no longer valid with Java 8 launch. Now java provides implementation in interface for default methods.

In simple terms, I would like to use

interface: To implement a contract by multiple unrelated objects. It provides "HAS A" capability.

abstract class: To implement the same or different behaviour among multiple related objects. It establishes "IS A" relation.

Oracle website provides key differences between interface and abstract class.

Consider using abstract classes if :

You want to share code among several closely related classes. You expect that classes that extend your abstract class have many common methods or fields, or require access modifiers other than public (such as protected and private). You want to declare non-static or non-final fields.

Consider using interfaces if :

You expect that unrelated classes would implement your interface. For example,many unrelated objects can implement Serializable interface. You want to specify the behaviour of a particular data type, but not concerned about who implements its behaviour. You want to take advantage of multiple inheritance of type.

Example:

Abstract class ( IS A relation)

Reader is an abstract class.

BufferedReader is a Reader

FileReader is a Reader

FileReader and BufferedReader are used for common purpose : Reading data, and they are related through Reader class.

Interface ( HAS A capability )

Serializable is an interface.

Assume that you have two classes in your application, which are implementing Serializable interface

Employee implements Serializable

Game implements Serializable

Here you can't establish any relation through Serializable interface between Employee and Game, which are meant for different purpose. Both are capable of Serializing the state and the comparasion ends there.

Have a look at these posts :

How should I have explained the difference between an Interface and an Abstract class?

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My two cents:

An interface basically defines a contract, that any implementing class must adhere to(implement the interface members). It does not contain any code.

On the other hand, an abstract class can contain code, and there might be some methods marked as abstract which an inheriting class must implement.

The rare situations I've used abstract classes is when i have some default functionality that the inheriting class might not be interesting in overriding, in say an abstract base class, that some specialized classes inherit from.

Example(a very rudimentary one!):Consider a base class called Customer which has abstract methods like CalculatePayment(), CalculateRewardPoints() and some non-abstract methods like GetName(), SavePaymentDetails().

Specialized classes like RegularCustomer and GoldCustomer will inherit from the Customer base class and implement their own CalculatePayment() and CalculateRewardPoints() method logic, but re-use the GetName() and SavePaymentDetails() methods.

You can add more functionality to an abstract class(non abstract methods that is) without affecting child classes which were using an older version. Whereas adding methods to an interface would affect all classes implementing it as they would now need to implement the newly added interface members.

An abstract class with all abstract members would be similar to an interface.

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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 classes that implement it - but doesn't specify how you should implement it. Also, a class can implement MANY interfaces, but can only extend ONE Abstract class. 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.

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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.

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1.If you are creating something that provides common functionality to unrelated classes, use an interface.

2.If you are creating something for objects that are closely related in a hierarchy, use an abstract class.

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When to prefer an abstract class over interface?

If one plans on updating a base class throughout the life of a program/project, it is best to allow that the base class be an abstract class If one is trying to build a backbone for objects that are closely related in a hierarchy, it is highly beneficial to use an abstract class

When to prefer an interface over abstract class?

If one is not dealing with a massive hierarchical type of framework, interfaces would be a great choice Because multiple inheritance is not supported with abstract classes(diamond problem), interfaces can save the day

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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/

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Classes may inherit from only one base class, so if you want to use abstract classes to provide polymorphism to a group of classes, they must all inherit from that class. Abstract classes may also provide members that have already been implemented. Therefore, you can ensure a certain amount of identical functionality with an abstract class, but cannot with an interface.

Here are some recommendations to help you to decide whether to use an interface or an abstract class to provide polymorphism for your components.

If you anticipate creating multiple versions of your component, create an abstract class. Abstract classes provide a simple and easy way to version your components. By updating the base class, all inheriting classes are automatically updated with the change. Interfaces, on the other hand, cannot be changed once created in that way. If a new version of an interface is required, you must create a whole new interface.

If the functionality you are creating will be useful across a wide range of disparate objects, use an interface. Abstract classes should be used primarily for objects that are closely related, whereas interfaces are best suited for providing common functionality to unrelated classes.

If you are designing small, concise bits of functionality, use interfaces. If you are designing large functional units, use an abstract class.

If you want to provide common, implemented functionality among all implementations of your component, use an abstract class. Abstract classes allow you to partially implement your class, whereas interfaces contain no implementation for any members.

Copied from:
http://msdn.microsoft.com/en-us/library/scsyfw1d%28v=vs.71%29.aspx

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I think the most succinct way of putting it is the following:

Shared properties => abstract class. Shared functionality => interface.

And to put it less succinctly...

Abstract Class Example:

public abstract class BaseAnimal
{
    public int NumberOfLegs { get; set; }

    protected BaseAnimal(int numberOfLegs)
    {
        NumberOfLegs = numberOfLegs;
    }
}

public class Dog : BaseAnimal
{
    public Dog() : base(4) { }
}

public class Human : BaseAnimal 
{
    public Human() : base(2) { }
}

Since animals have a shared property - number of legs in this case - it makes sense to make an abstract class containing this shared property. This also allows us to write common code that operates on that property. For example:

public static int CountAllLegs(List<BaseAnimal> animals)
{
    int legCount = 0;
    foreach (BaseAnimal animal in animals)
    {
        legCount += animal.NumberOfLegs;
    }
    return legCount;
}

Interface Example:

public interface IMakeSound
{
    void MakeSound();
}

public class Car : IMakeSound
{
    public void MakeSound() => Console.WriteLine("Vroom!");
}

public class Vuvuzela : IMakeSound
{
    public void MakeSound() => Console.WriteLine("VZZZZZZZZZZZZZ!");        
}

Note here that Vuvuzelas and Cars are completely different things, but they have shared functionality: making a sound. Thus, an interface makes sense here. Further, it will allow programmers to group things that make sounds together under a common interface -- IMakeSound in this case. With this design, you could write the following code:

List<IMakeSound> soundMakers = new List<ImakeSound>();
soundMakers.Add(new Car());
soundMakers.Add(new Vuvuzela());
soundMakers.Add(new Car());
soundMakers.Add(new Vuvuzela());
soundMakers.Add(new Vuvuzela());

foreach (IMakeSound soundMaker in soundMakers)
{
    soundMaker.MakeSound();
}

Can you tell what that would output?

Lastly, you can combine the two.

Combined Example:

public interface IMakeSound
{
    void MakeSound();
}

public abstract class BaseAnimal : IMakeSound
{
    public int NumberOfLegs { get; set; }

    protected BaseAnimal(int numberOfLegs)
    {
        NumberOfLegs = numberOfLegs;
    }

    public abstract void MakeSound();
}

public class Cat : BaseAnimal
{
    public Cat() : base(4) { }

    public override void MakeSound() => Console.WriteLine("Meow!");
}

public class Human : BaseAnimal 
{
    public Human() : base(2) { }

    public override void MakeSound() => Console.WriteLine("Hello, world!");
}

Here, we're requiring all BaseAnimals make a sound, but we don't know its implementation yet. In such a case, we can abstract the interface implementation and delegate its implementation to its subclasses.

One last point, remember how in the abstract class example we were able to operate on the shared properties of different objects and in the interface example we were able to invoke the shared functionality of different objects? In this last example, we could do both.

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Consider using abstract classes if any of these statements apply to your situation:

You want to share code among several closely related classes. You expect that classes that extend your abstract class have many common methods or fields or require access modifiers other than public (such as protected and private). You want to declare non-static or non-final fields. This enables you to define methods that can access and modify the state of the object to which they belong.

Consider using interfaces if any of these statements apply to your situation:

You expect that unrelated classes would implement your interface. For example, the interfaces Comparable and Cloneable are implemented by many unrelated classes. You want to specify the behavior of a particular data type, but not concerned about who implements its behavior. You want to take advantage of multiple inheritances.

Source

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

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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++.

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For me, I would go with interfaces in many cases. But I prefer abstract classes in some cases.

Classes in OO generaly refers to implementation. I use abstract classes when I want to force some implementation details to the childs else I go with interfaces.

Of course, abstract classes are useful not only in forcing implementation but also in sharing some specific details among many related classes.

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

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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?)

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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.

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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.

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The short answer: An abstract class allows you to create functionality that subclasses can implement or override. An interface only allows you to define functionality, not implement it. And whereas a class can extend only one abstract class, it can take advantage of multiple interfaces.

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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.

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Basic thumb rule is: For "Nouns" use Abstract class and for "Verbs" use interface

E.g: car is an abstract class and drive, we can make it an interface.

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If we have an implementation that will be the same for all the derived classes and at that time it is better to use the abstract class over an interface. when we have an interface, we can move our implementation to any class that implements interface. In abstract class, it avoids code duplication and share the implementation for all derived class. The interfaces allow to develop loosely coupled systems which helps for better testing.

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Both are contract for class definition:

Conclusion 1: Both intent is object generalization

In defining abstract classes, they can have default implementation also.

Conclusion 2: distinguish is in Behavioral generalization Design

In utilizing abstract classes, classes can inherit from just one abstract class

Conclusion 3: abstract class has limitation in utilizing. It means limitation in Behavioral generalization.

Final Conclusion-When to use which: Distinguish is in behavioral generalization level

In designing behavioral of classes, If functionality is just conceptually limited among determined classes or in other word, is share among determined class, use abstract class. but if functionality is more general than determined classes or we can/want add functionality to other classes, use interface as a contract.

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