Use of Java [Interfaces / Abstract classes] [duplicate]

Question

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

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 2

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.