Can you re-make a method abstract in the inheritance tree?

Question

EDIT:

To be clear: The fact that the design is quite ugly is not the point. The point is, that the design is there and I am in the situation to have to add another sub-c

Answer 1

Yes you can. In fact, it even says you can explicitly in the language spec:

An instance method that is not abstract can be overridden by an abstract method.

The problem you are facing is described a few paragraphs up:

The declaration of an abstract method mmust appear directly within an abstract class (call it A) unless it occurs within an enum declaration (§8.9); otherwise a compile-time error occurs.

So, the problem is that your class is not abstract, as others pointed out already; it may just be useful to know the specific parts of the spec which describe it.

Answer 2

You want children of MotorizedVehicle to have a default implementation of bar, but not so for the children of the FlyingMotorizedVehicle.

abstract class BasicMotorizedVehicle
    // no bar
    ... // Rest of old MotorizedVehicle

class MotorizedVehicle extends BasicMotorizedVehicle
    Foo bar(...) { ... }

class FlyingMotorizedVehicle extends BasicMotorizedVehicle

Answer 3

Brother, study Interfaces too.

If you want some class, such as Vehicle to only provide function prototypes, then you should always use Interface.

And make your FlyingMotorizedVehicle as abstract class.

• Abstract class can have both types of functions, either prototype only (abstrac functions) or fully implemented functions.
• Interfaces have only function prototypes, they can't contain function implementations, Interfaces are required to be implemented.

For further study, you can find many useful links, including this one.

=============================CODE-EXAMPLE=======================================

For Vehicle

public interface Vehicle {

    Something doStuff(...);   
    SomethingElse doOtherStuff(...);
    Foo bar(...);

}

For FlyingMotorizedVehicle

public abstract class FlyingMotorizedVehicle extends MotorizedVehicle {

    SomethingElse doOtherStuff(...) {
        return new SomethingElse();
    } 
}

===============================================================================

Happy OOP-ing!

Answer 4

Yes, You have to redefine the bar(...) as abstract method. Then you have to declare public class FlyingMotorizedVehicle as a abstract class as well

public abstract class FlyingMotorizedVehicle extends MotorizedVehicle {

    @Override
    SomethingElse doOtherStuff(...) {
        return new SomethingElse();
    }

    abstract Foo bar(...);

}