What is the difference between an interface and abstract class?

What exactly is the difference between an interface and an abstract class?

Solution 1:

Interfaces

An interface is a contract: The person writing the interface says, "hey, I accept things looking that way", and the person using the interface says "OK, the class I write looks that way".

An interface is an empty shell. There are only the signatures of the methods, which implies that the methods do not have a body. The interface can't do anything. It's just a pattern.

For example (pseudo code):

// I say all motor vehicles should look like this:
interface MotorVehicle
{
    void run();

    int getFuel();
}

// My team mate complies and writes vehicle looking that way
class Car implements MotorVehicle
{

    int fuel;

    void run()
    {
        print("Wrroooooooom");
    }


    int getFuel()
    {
        return this.fuel;
    }
}

Implementing an interface consumes very little CPU, because it's not a class, just a bunch of names, and therefore there isn't any expensive look-up to do. It's great when it matters, such as in embedded devices.

Abstract classes

Abstract classes, unlike interfaces, are classes. They are more expensive to use, because there is a look-up to do when you inherit from them.

Abstract classes look a lot like interfaces, but they have something more: You can define a behavior for them. It's more about a person saying, "these classes should look like that, and they have that in common, so fill in the blanks!".

For example:

// I say all motor vehicles should look like this:
abstract class MotorVehicle
{

    int fuel;

    // They ALL have fuel, so lets implement this for everybody.
    int getFuel()
    {
         return this.fuel;
    }

    // That can be very different, force them to provide their
    // own implementation.
    abstract void run();
}

// My teammate complies and writes vehicle looking that way
class Car extends MotorVehicle
{
    void run()
    {
        print("Wrroooooooom");
    }
}

Implementation

While abstract classes and interfaces are supposed to be different concepts, the implementations make that statement sometimes untrue. Sometimes, they are not even what you think they are.

In Java, this rule is strongly enforced, while in PHP, interfaces are abstract classes with no method declared.

In Python, abstract classes are more a programming trick you can get from the ABC module and is actually using metaclasses, and therefore classes. And interfaces are more related to duck typing in this language and it's a mix between conventions and special methods that call descriptors (the __method__ methods).

As usual with programming, there is theory, practice, and practice in another language :-)

Solution 2:

The key technical differences between an abstract class and an interface are:

• Abstract classes can have constants, members, method stubs (methods without a body) and defined methods, whereas interfaces can only have constants and methods stubs.

• Methods and members of an abstract class can be defined with any visibility, whereas all methods of an interface must be defined as public (they are defined public by default).

• When inheriting an abstract class, a concrete child class must define the abstract methods, whereas an abstract class can extend another abstract class and abstract methods from the parent class don't have to be defined.

• Similarly, an interface extending another interface is not responsible for implementing methods from the parent interface. This is because interfaces cannot define any implementation.

• A child class can only extend a single class (abstract or concrete), whereas an interface can extend or a class can implement multiple other interfaces.

• A child class can define abstract methods with the same or less restrictive visibility, whereas a class implementing an interface must define the methods with the exact same visibility (public).

Solution 3:

An Interface contains only the definition / signature of functionality, and if we have some common functionality as well as common signatures, then we need to use an abstract class. By using an abstract class, we can provide behavior as well as functionality both in the same time. Another developer inheriting abstract class can use this functionality easily, as they would only need to fill in the blanks.

Taken from:

http://www.dotnetbull.com/2011/11/difference-between-abstract-class-and.html

http://www.dotnetbull.com/2011/11/what-is-abstract-class-in-c-net.html http://www.dotnetbull.com/2011/11/what-is-interface-in-c-net.html

Solution 4:

An explanation can be found here: http://www.developer.com/lang/php/article.php/3604111/PHP-5-OOP-Interfaces-Abstract-Classes-and-the-Adapter-Pattern.htm

An abstract class is a class that is only partially implemented by the programmer. It may contain one or more abstract methods. An abstract method is simply a function definition that serves to tell the programmer that the method must be implemented in a child class.

An interface is similar to an abstract class; indeed interfaces occupy the same namespace as classes and abstract classes. For that reason, you cannot define an interface with the same name as a class. An interface is a fully abstract class; none of its methods are implemented and instead of a class sub-classing from it, it is said to implement that interface.

Anyway I find this explanation of interfaces somewhat confusing. A more common definition is: An interface defines a contract that implementing classes must fulfill. An interface definition consists of signatures of public members, without any implementing code.

Solution 5:

I don't want to highlight the differences, which have been already said in many answers ( regarding public static final modifiers for variables in interface & support for protected, private methods in abstract classes)

In simple terms, I would like to say:

interface: To implement a contract by multiple unrelated objects

abstract class: To implement the same or different behaviour among multiple related objects

From the Oracle documentation

Consider using abstract classes if :

1. You want to share code among several closely related classes.
2. 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).
3. You want to declare non-static or non-final fields.

Consider using interfaces if :

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

abstract class establishes "is a" relation with concrete classes. interface provides "has a" capability for classes.

If you are looking for Java as programming language, here are a few more updates:

Java 8 has reduced the gap between interface and abstract classes to some extent by providing a default method feature. An interface does not have an implementation for a method is no longer valid now.

Refer to this documentation page for more details.

Have a look at this SE question for code examples to understand better.

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