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What is the difference between an interface and abstract class?

Hi guys, I am without internet in my house until 19th 😦
Because of this, I will post some interesting things, but nothing that I wrote. (Because I am without time/internet). Sorry, for this !

Interface

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 (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 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 is no 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 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");
}
}

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.

See you next week !

Font: http://stackoverflow.com/questions/1913098/what-is-the-difference-between-an-interface-and-abstract-class

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