What is the difference between an interface and abstract class?

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 methodsfrom 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).

 

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.

Class 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;
    }
}

Abstract 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");
    }
}