Question

I have an abstract class MotherClass and two abstract classes ChildClass1 and ChildClass2 that extend MotherClass.

I wish to make sure that any class extending MotherClass will in fact extend ChildClass1 or ChildClass2. I feel that something is wrong in my tree-based class design. Do you see how to make it properly?

Answer 1

If MotherClass has package visibility and ChildClass1 and ChildClass2 are public and in the same package, you could subclass those two but not MotherClass.

Edit:

Another possibility:

interface Marker {} //note that this is package private

public abstract class Mother<T extends Marker > {}

public class ChildA extends Mother<ChildA> implements Marker {}

public class ChildB extends Mother<ChildB> implements Marker {}

Method:

doSomething(Mother<?> mother() {}

You now couldn’t do

class GrandChild extends Mother<GrandChild> {}

This would compile but give you a warning at least:

class GrandChild extends Mother {} //warning like "Mother is a raw type"

Ways without warning:

class GrandChild extends ChildA {} 
class GrandChild extends ChildB {}