Java Comparison – Tricky compareTo, Inheritance, Easy to Extend

comparisoninheritancejava

Let's say we have 4 classes A, B, C, D where: A is a superclass of both B and C and C is a superclass of D. I suppose, in a diagram, it should look like this:

   A
 /   \
B     C
       \
        D

Moreover, the class A implements the interface 

Comparable<A>

and also implements a method

public int compareTo(A another)

I'd like to be able to compare every pair of objects x, y such that they are instances of classes from the set {A, B, C, D}. Naturally, I should implement a compareTo method in each of these classes but by default I'm doing this with one sided effect, i.e. defining how to compare object c from C with b from B doesn't tell anything about how to compare b with c.
Now, I could write a lot of code and handle all of those cases (this also means I would have to use 'isInstance' method to handle these, right?) but then every two classes have to be 'aware of' existence of others. So if I wanted to add another class, E, such that D is a superclass of E, I would have to serve all cases in the new class E and moreover I'd have to change something in classes A, B, C, D in order to 'be aware of E'.

So I'm struggling to find an elegant solution to it, i.e. one that doesn't entail changing a lot while adding a new class to the hierarchy.
How can I achieve this?

Thank you in advance for help.

Best Answer

You could write a general compareTo method in class A, which uses a key computed by a method compareKey which is also in A, but can be overridden by each subclass to make as fine a distinction as it wants. If you make it return a float, you can keep subdividing the comparison criterion whenever a new level of inheriting classes is added (within reason, obviously).

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