Use of Java [Interfaces / Abstract classes]

Question

Hello,

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.

Thank you for your time

-Samuel

Answer 1

I don't think that in your case your interface and class structure aligns well with the reality. In fact, I believe (correct me if I'm wrong) that each zombie can be walking, running, teleporting etc. depending on where it is.

Therefore, you should have a zombie class or interface and have actions which modify the zombie's state. The action would probably be an interface or an abstract class, so that you can apply any action to a zombie without knowing what the exact action does (e.g. action.perform(zobie)).

If you have different kinds of zombies, such as three-legged-zombie and one-armed zombies, you might want to implement different classes which handle the zombie stuff, such as displaying themselfes or validating state changes (e.g. a special kind of zombie may not accept to be teleported).

Answer 2

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.

Answer 3

in terms of your Zombie example, the interface will do well, unless you have common code that you want all zombies to do.

Say you have a Move method, that makes walkingzombies walk, runningzombies run, etc. However, if you want "Move" to make any kind of zombie do something common, then the interface is going to force you to duplicate code, as you cant put a body in an interface.

Answer 4

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 5

Nobody agrees about the use of interfaces over super/abstract classes ;)

The main reason to use interfaces and super/abstract classes is to enable polymorphism. In your case for instance, you have stuff moving on the screen (the player and the zombies and so on). Why not make them all move on the screen using the same method? Maybe inherit everything that's going to move on the screen from an object called "Movable" or something like that.

And if you're really into this stuff you might want to look at mixins as well. It's not something that Java supports directly but there are libraries built for it.

Answer 6

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 7

When in doubt, I choose to follow the GOF paradigm.

Encapsulate what varies: - Define unique behavior in its own class. To refer the above example, implement behaviors for walking, running and teleportation in its separate class. This way, polymorphic behavior is implemented.

Conversely, **Aggregate what is common** - Use Abstract classes to define common behavior in polymorphic associations. I use these principles when designing relationships between objects.

Answer 8

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?

an abstract class will be a super class for your zombies. an interface would also in some sense be a super class (super interface?) for your zombies.

the common properties suggest at least an abstract base class for common properties.

(I am not planning to partially implement functions - therefor my choice for an interface instead of an abstract class)

not sure what you mean by this.

if you had different kinds of monsters (goblins, orcs, etc.) you might find behaviour common to these that would want to belong to different base classes. this would suggest an interface.

i would start with an abstract base class and see what the code tells you as you write it.

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?

your survivor is what is called a player-character (as opposed to a non-player character - someone in a game who will normally not attack your survivor).

most games treat all of these character types as some kind of monster since they will all have many properties in common (health. magic, treasures, weapons, etc.)

so perhaps that's more of an argument for an interface.

see:

http://stackoverflow.com/questions/2768748/using-inheritance-and-polymorphism-to-solve-a-common-game-problem http://stackoverflow.com/questions/1212149/class-diagram-examples-for-rpg-role-playing-game http://stackoverflow.com/questions/2123071/designing-class-hierarchy-for-typical-characters-in-role-playing-game

Answer 9

My opinion is you better use abstract class called Creature as a super class for all type of, well, creatures, and extend it to Zombie for all type of zombies.
And you will also need an interface.. to define what are the things that a creature can do..
like maybe, walk, or claw, or scream...
the reason why you need an abstract class is to disable the instantiation of Creature, you wouldn't want to have a creature without knowing what creature it is, right?

Answer 10

You had better read Head first design patterns. That answers all your questions like this.