BadFruit | Ruby Gem for Rotten Tomatoes | Application Framework library

While technical alternatives have been suggested in comments, I'd like to add another perspective. What we're seeing here is a class design problem which comes up when attempting to use inheritance for anything other than a true generalization/specialization relationship.

The example declares:

1. Each Fruit must be able to consume another Fruit.
2. An Apple is a kind of Fruit.

Then the idea is:

• An Apple must not consume any kind of Fruit, but an Apple only. 🚫

If an Apple were really a Fruit, it would fully adhere to the Fruit's declaration and be able to consume another Fruit of any kind. As the intended apple Apple violates rule 1, is not really a Fruit and the language prevents you from declaring it as such.

Trying to work around this (e.g. via runtime checks in overridden methods) masquerades the underlying problem and introduces surprises to those using such classes.

Solution:

• Use inheritance for true generalization/specialization relationships only. If it is 100% certain, no strings attached, that an apple is a fruit, inheritance is a perfect fit. Otherwise it is not.
• In this case: Rethink the intended semantics:
  • What's the real meaning of consume?
  • Is there a notion of a fruit consuming an arbitrary (potentially incompatible specialization of another) fruit?
  • Or is it rather individual specializations of fruit which each have their own independent notion of consuming? Then there would be no common consume method at the Fruit level.

Answering the additional question in the comment:

how can I make sure this will copy properties of both SourFruit and CoreFruit?

I'd rather not express SweetFruit and SourFruit as specializations of a CoreFruit. Flavors such as sweet and sour are traits of a fruit and better expressed as properties.

But I could extend your example a bit and then suggest a class design which includes a clone() function providing a deep copy functionality on a base class Flavor. Note that the output shows different hash codes for cloned objects: