Interface vs Abstract class

Interfaces

An interface is a contract: the guy writing the interface says, “hey, I accept things looking that way“, and the guy 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.
Implementing an interface consumes very little CPU, because it’s not a class, just a bunch of names, and therefore there is no expensive look-up to do. It’s great when it matters such as in embedded devices.

// 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 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 guy saying, “these classes should look like that, and they have that in common, so fill in the blanks!”.

e.g:

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

    int fuel;

    // they ALL have fuel, so why not let others implement this?
    // 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 writes vehicle looking that way
class Car extends MotorVehicle
{
    void run()
    {
        print("Wrroooooooom");
    }
}
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