Creating many object types/classes just to use instanceof

Question

I'm wondering if I'm creating/breaking down object types into meaningless classes and if there is a better way to do it.

Say I've got the following classes (--> extends the higher class):

Vehicle class - takes care of movement and a default speed
-->Car class - adds passengers
-->-->ElectricCar class - a default constructor
-->-->GasolineCar class - a default constructor
-->-->HybridCar class - a default constructor
-->Motorcycle class - a default constructor
-->Bike class - a default constructor, overrides speed
-->Segway class - a default constructor, overrides speed

As you can see, they're mostly default constructors. So I'm only creating them in case I ever need to use an instanceof conditional statement. Most of my code ends up in the Vehicle class because I'm trying to avoid repeating the same code in every class. Also, I should make Vehicle abstract, correct? I never create a Vehicle object.

Is this OK or is there a better way to do it?

Answer 1

First, as long as the code functions and solves the task at hand I would say it is "correct" enough (so yes, it is okay). That being said, some people prefer to use interface(s) instead of abstract classes (because you can implement multiple interfaces but only directly extend from one ancestor). Further, if you're using Java 8+ you can add default methods to interface(s). Other alternatives might include having one Vehicle class but a VehicleType enum field for the Vehicle class.

Answer 2

There's never such a thing as "too many classes" as long as your classes make sense and serve a function. There is however such a thing as pointless abstraction in my opinion.

If you're 100% sure that you will only ever use these classes to determine the vehicle type (quote from your question),

I ever need to use an instanceof conditional statement

Then maybe having sub classes qualifies as pointless abstraction. Just use an enum instead which describes your vehicles type and move on.

If however you anticipate or plan on using your sub classes for more than just the type, it's completely fine to do it by having sub classes and it's in fact preferred to having lots of if/else or switch statements in your Vehicle class.