Question

It seems to me that both interface and abstract function are quite similar,

it’s like an order that some method must be implemented,

so what’s the difference?

Answer 1

Have a look at this.

Quoting: (Very Good Explantion by e-satis)

Interface

An interface is a contract: the guy writing the interface say “hey, I accept things looking that way”, and the guy using the interface say “OK, the class I write looks that way”.

And interface is an empty shell, there are only the signatures (name / params / return type) of the methods. The methods do not contain anything. The interface can’t do anything. It’s just a pattern.

E.G (pseudo code):

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

    int getFuel();
}

// my team mate complies and write vehicle looking that way
class Car implements MotoVehicle
{

    int fuel;

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


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

Implementing an interface consume very little CPU, because it’s not a class, just a bunch of names, and therefor there is no expensive lookup to do. It’s great when it matters such as in embedded devices.

Abstract classes

Abstract classes, unlike interfaces, are classes. There are more expensive to use because there is a lookup 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 guy saying “these classes should look like that, and they got that in common, so fill in the blanks !”.

e.g:

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

    int fuel;

    // they ALL have fuel, so why let others implement that ?
    // let's make it for everybody
    int getFuel()
    {
         return this.fuel;
    }

    // that can be very different, force them to provide their
    // implementation
    abstract void run();


}

// my team mate complies and write vehicle looking that way
class Car extends MotorVehicule
{
    void run()
    {
        print("Wrroooooooom");
    }
}

By: https://stackoverflow.com/users/9951/e-satis