4 ms·

Is "Spaceship extends GameObject" really that different than "Duck extends Animal"?

haberman 15y ago
Yes, because drawing a bunch of game objects on a screen is something a real program might actually want to do, and therefore opens up opportunities to talk about real-world design tradeoffs. Making a program print "quack" when you call the "speak()" method is something that only happens in textbooks, and since there's no reason to actually do that there's no easy way to discuss the alternatives and why polymorphism is a win.
- fleitz 15y ago
  So what you're saying is that Farmville is the ultimate result of reading OO textbooks, and that if they had spent more time on a realistic model such as spaceships and gameobjects that a game such as Knights of the Old Republic could be produced by Zynga?
  - wlievens 15y ago
    That has nothing to do with what haberman is saying. Most people know games, and a simple game model can be expressed in inheritance terms in such a way that's illustrative and less contrived than conventional tutorial metaphors. And I agree with that.
repsilat 15y ago
When people say "Duck extends Animal" is bad they typically mean that it's trying to model real-world relationships rather than computational relationships. The example is meant to demonstrate that inheritance should be based on what inheritance means for your program, to make your codebase "nicer", and not to simply categorise your objects. "Spaceship extends GameObject" exists mostly to facilitate polymorphism and code reuse - virtual function resolution, probably, along with all of the non-virtual data and behaviour associated with every GameObject. A "GameObject" isn't a real thing, it's just a convenience. Out of context, "Duck extends Animal" could obviously do the same things for the same reasons, but that's not really the point of the example. It's implied that the relationship was used because real ducks are real animals, not because Duck inheriting from Animal is a good idea.
- marshray 15y ago
  I'll buy that. But don't try to convince any OO purists that inheritance "exists mostly to facilitate polymorphism and code reuse". :-)
- billybob 15y ago
  "It's implied that the relationship was used because real ducks are real animals, not because Duck inheriting from Animal is a good idea." I just do not understand this objection. If you wrote Sim Farm, it's quite possible you'd want Duck to inherit from Bird and Cow and Pig to inherit from Mammal, and both Bird and Mammal to inherit from Animal. In the course of caring for your farm, you have to fix tractors, grow crops, and care for animals. All Animals must be fed and will starve if they don't. Why would you code "needs food" on every individual animal class? Birds can get avian flu, and Mammals can get rabies; the pigs can even get rabies from the dogs. Why would you code "can get rabies" on Dog and Pig? Maybe you don't always want to use Sim Farm as your example for teaching OO, but a lot of working code does model real-world items. Animals and cars and such are no worse than files as objects to model, and have the advantage that non-programmers already know something about them. Hence they can focus on the OO concepts and not on the domain logic.

Yes, because drawing a bunch of game objects on a screen is something a real program might actually want to do, and therefore opens up opportunities to talk about real-world design tradeoffs. Making a program print "quack" when you call the "speak()" method is something that only happens in textbooks, and since there's no reason to actually do that there's no easy way to discuss the alternatives and why polymorphism is a win.

fleitz 15y ago
So what you're saying is that Farmville is the ultimate result of reading OO textbooks, and that if they had spent more time on a realistic model such as spaceships and gameobjects that a game such as Knights of the Old Republic could be produced by Zynga?
- wlievens 15y ago
  That has nothing to do with what haberman is saying. Most people know games, and a simple game model can be expressed in inheritance terms in such a way that's illustrative and less contrived than conventional tutorial metaphors. And I agree with that.

repsilat 15y ago

When people say "Duck extends Animal" is bad they typically mean that it's trying to model real-world relationships rather than computational relationships. The example is meant to demonstrate that inheritance should be based on what inheritance means for your program, to make your codebase "nicer", and not to simply categorise your objects. "Spaceship extends GameObject" exists mostly to facilitate polymorphism and code reuse - virtual function resolution, probably, along with all of the non-virtual data and behaviour associated with every GameObject. A "GameObject" isn't a real thing, it's just a convenience. Out of context, "Duck extends Animal" could obviously do the same things for the same reasons, but that's not really the point of the example. It's implied that the relationship was used because real ducks are real animals, not because Duck inheriting from Animal is a good idea.

marshray 15y ago
I'll buy that. But don't try to convince any OO purists that inheritance "exists mostly to facilitate polymorphism and code reuse". :-)
billybob 15y ago
"It's implied that the relationship was used because real ducks are real animals, not because Duck inheriting from Animal is a good idea." I just do not understand this objection. If you wrote Sim Farm, it's quite possible you'd want Duck to inherit from Bird and Cow and Pig to inherit from Mammal, and both Bird and Mammal to inherit from Animal. In the course of caring for your farm, you have to fix tractors, grow crops, and care for animals. All Animals must be fed and will starve if they don't. Why would you code "needs food" on every individual animal class? Birds can get avian flu, and Mammals can get rabies; the pigs can even get rabies from the dogs. Why would you code "can get rabies" on Dog and Pig? Maybe you don't always want to use Sim Farm as your example for teaching OO, but a lot of working code does model real-world items. Animals and cars and such are no worse than files as objects to model, and have the advantage that non-programmers already know something about them. Hence they can focus on the OO concepts and not on the domain logic.