Are grandparent classes also parents?

Question

Let's say I have three classes, Animal, Bird and Pigeon. If Bird extends Animal and Pigeon extends Bird, is Animal the parent class for both or is Bird?

So, for instance, if the constructor for Animal said "hi I am animal" and the constructor for Bird said "hi am I bird", which will it say in class Pigeon if I call the super constructor?

Answer 1

You cannot answer this question unambiguously. Different OOP languages implement OOP paradigm differently.

In Ruby you will get only "Hi, I am bird!"

class Animal
  def initialize
    puts "Hi, I am animal!"
  end
end

class Bird < Animal
  def initialize
    puts "Hi, I am bird!"
  end
end

class Pigeon < Bird
  def initialize
    super # <= call to the superclass
  end
end

pg = Pigeon.new

In PHP we get only "Hi, I am bird!"

<?php

   class Animal {

      public function __construct()
      {
        echo "I'm Animal";
      }

    }

    class Bird extends Animal {

       public function __construct()
       {
         echo "I'm Bird";
       }  

    }

   class Pigeon extends Bird {

      public function __construct()
      {
         parent::__construct();
      }

    }

   $Pigeon = new Pigeon();

  ?>

While in C++ you will get both "Hi I am animal!" and "Hi I am Bird!"

#include <iostream>
using namespace std;

class Animal
{
   public:
   Animal()
   {
     cout << "Hi I am animal!" << endl;
   }
};

class Bird : public Animal
{
  public:
  Bird()
  {
    cout << "Hi I am Bird!" << endl;
  }
};

class Pigeon : public Bird
{
  public:
  Pigeon():Bird(){} // <= call to the base constructor
};

int main()
{
  Pigeon pg;
}

In Swift (Apple) we get both, too

class Animal {
  public init() {
    print("I'm Animal")
  }
}

class Bird: Animal {
   public override init() {
     print("I'm Bird")
   }
}

class Pigeon: Bird {
   public override init() {
    super.init()
   }
}

var pigeon = Pigeon()

To understand why, you should get into details of how C++ implements OOP (vtables and etc.) and how Ruby classes lookup its methods and constants in the hierarchy, and other languages.

In addition if you want to see Object Oriented "Nightmare" then have a look at Lua, though I like it.

As to your question in the title: "Are grandparent classes also parents", of course subclasses are related to all super-classes up in the hierarchy chain, but that relationship are implemented differently in different programming languages.

Answer 2

You have a type hierarchy here. Animal is parent for Bird and Bird is for Pigeon. Since the inheritance relation is transitive, Animal is a "parent" for Pigeon as well.

In real-world language, of course, lots of watering down and extension of core principles happen. For instance, which behaviour overrides which can be different due to different method dispatching rules.

Specifically for constructors, conceptually Pigeon() should call Bird(), and Bird() should call Animal(); otherwise we don't know that the object is set up correctly. Hence, Pidgeon() will "say" both! (Which it says first depends on where in the constructors the super() calls happen in relation to the "saying".)

Note that some programming languages may allow you to leave out calls to the super constructor at your own risk (specifically breaking Liskov's substitution principle if you're not careful).