Why is having an abstract subclass of concrete class bad design?

After all ANY java abstract is an abstract subclass of Object. Sometimes we need to force the subclass to implement some methods, but may already have a pretty well defined hierarchy with concrete class开发者_开发百科es.

For example: I have a well functioning hierarchy with Vehicle<--- Car

and now I want to add ElectricCar to this hierarchy.

vehicle<--Car<--ElectricCar.

I also want all the different types of electric cars to implement certain behaviors like getBatteryLife or something-

Why would it be a bad idea to make ElectricCar abstract ?

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there's nothing wrong in making it abstract. if your business requires you to make it abstract, it's fine. Like you said, lots of classes in Java lib are abstract and still extending Object.

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It's not bad, per se. Not common, but not bad. The only thing I can think of is understandability: if I saw a concrete class Car that I could instantiate, I would normally assume that any child of it was also instantiable, because 99% of code works this way.Then I'd be confused, for a second, about not being able to instantiate an ElectricCar.

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It could be argued that this pattern breaks the Liskov Substituion Principle since you can't pass "ElectricCar" wherever "Car" is expected if it's declared abstract (you could pass instances of ElectricCar subclasses of course).

In this particular example, the concrete electric cars (hydrogen powered/plug-in/etc?) I would expect to inherit directly from "Car" since they satisfy an "is-a" relationship and are a proper specialisation of "Car". If you wanted to described some common behaviours and traits they should provide then they should also implement an ElectricCar interface.

It seems what you really want is the ability to inherit from Car (since that is what they are) and share/re-use common implementations of electric car related methods. In this case you are looking at a multiple inheritance problem or a need for mixins, neither of which are directly supported in Java.

Providing an abstract class in the middle of a concrete hierarchy may be one way around this, but it's not pretty.

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Personally I would prefer to define an Interface for ElectricCar and then allow the implementing class to define the methods. Then you can share the behavior of getBatteryLife through another mechanism.

I've built some pretty deep hierarchies of Inheritance and I tend to avoid them do to the brittle nature they tend to build up over time. One Base class might make sense, but I would think about how you can compose your object model to share behavior w/o inheritance if possible.

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In you example I would say that supposedly the Car class should be abstract (or an interface), too. But there is nothing wrong with the ElectricCar being abstract.