Question

Say I have a class pet and it is composed of two variables:

Class Pet {
Category category; // be amphibian, fish, mammal, dog
Diet diet; //meat, vegetarian, omnivore
...

If category = dog I want to restrict diet to omnivore only (and vice versa). My actual problem is somewhat more complex, whats the best way to code this ? I started with nested enums and then inner builder class …

Is their a common pattern I can employ ?

Answer 1

You have this trouble because you are not taking advantage of inheritance here. Dog IS A Pet. So let Dog extend Pet. Pet will have abstract setDiet() and Dog will implement this with applying the diet rules applicable for Dog.

class Pet
{
  Diet diet;

  abstract boolean satisfiesDietRules();

  public Pet()
  {
    if(!satisfiesDietRules())
    {
      throw CannotCreatePetException();
    }
  }
}

class Dog extends Pet
{
  boolean satisfiesDietRules()
  {
    //rules
  }
}

Now, you can encapsulate the diet rules in their respective enum (your current approach) or leverage inheritance which is natural. If you go composition approach, then you can’t leverage polymorphism. So it depends upon your business requirements to understand which one to use. If you make sure that you doesn’t violate LSV then inheritance seems to be a natural solution for this.