Use of Java [Interfaces / Abstract classes] [dupli

Question

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• Interface vs Abstract Class (general OO) 33 answers

Lately i decided to take a look at Java so i am still pretty new to it and also to the approach of OO programming, so i wanted to get some things straight before learning more, (i guess it's never to soon to start with good practices).

I am programming a little 2D game for now but i think my question applies to any non trivial project. For the simplicity I'll provide examples from my game.

I have different kinds of zombies, but they all have the same attributes (x, y, health, attack etc) so i wrote an interface Zombie which i implement by WalkingZombie, RunningZombie TeleportingZombie etc. Is this the best thing to do? Am i better of with an abstract class? Or with a super class? (I am not planning to partially implement functions - therefor my choice for an interface instead of an abstract class)

I have one class describing the main character (Survivor) and since it is pretty big i wanted to write an interface with the different functions, so that i can easily see and share the structure of it. Is it good practice? Or is it simply a waste of space and time?

I hope this question will not be rated as subjective because i thought that experienced programmers won't disagree about this kind of topic since the use of interfaces / super classes / abstract classes follows logical rules and is thereby not simply a personal choice.

Answer 1

Yes, I think you're heading down the right track with interfaces over abstract classes.

Any concrete Zombie you might want to make could possess any combination of the Walking, Running or Teleporting features you care to implement.

I think modern programming theory discourages inheritance as much as possible, because it inhibits reusability and flexibility in the long-run. Rather, use interfaces and composition to achieve flexibility without 'tight coupling'.

One methodology to re-use code without inheritance, you could apply the 'Favour composition over inheritance' paradigm.

I like to think Josh Bloch's 'Effective Java' (2nd edition) can be taken as "current thinking" ...

http://books.google.com/books?id=ZZOiqZQIbRMC&pg=RA1-PA71&lpg=RA1-PA71&dq=%22Bloch%22+%22Effective+java:+programming+language+guide%22+&hl=de&sig=RxlDlRBWUvNAzsAFzqOcftrYI5E#v=onepage&q&f=false

So, you could implement all your behaviours as independent classes, and then give each zombie implementation its own combination of behaviours, through implementation & composition..

Hope that makes sense & helps ...

Answer 2

I would have written Zombie as an abstract class to avoid the redefinition of the fields x, y, health, etc...

For the Survivor class, I would simply have declare public the functions to be used externally. I declare public functions at the top of the class. Declaring an interface when there is only one class implementing it uselessly adds a file to maintain. Avoid it.

Answer 3

You can think of an interface as a "contract". You are defining a set of methods that classes which implement this interface must implement.

An abstract class, on the other hand, is used when you have some code that could be common to all the child classes you want to implement. So you might have an abstract class called Shape that has some common code, and in your derived classes (Circle, Square, etc.) you could have the code that is specific to those shapes (getArea would be an example). But something like color might be common to all shapes, so you could put a getColor method in your Shape abstract class.

And you can combine the two ideas. You can have abstract classes which implement interfaces, and this gives you the best of both worlds.

These concepts are used over and over again in OO, so it's important to understand them. You seem to be well on your way :).

So if your zombie class has some common behavior that applies to all types of zombies, it sounds like a good candidate to be an abstract class. You could also consider creating an interface (maybe a GameCharacter interface) if you have other characters in your game (maybe UndeadMice or something :)). Then your Zombie abstract class and UndeadMouse abstract class would implement the GameCharacter interface.