Question

Got a task to make a program that registers animals and the object is to get familiar with inheritance, polymorphism and so on.

One thing that pussles me is no matter how much I read about it just seems pointless.

I create my main class which is animal with some generic fields that apply to all animals lets say name, age and species.

So far so good all animals has this info but every animal has a unique field aswell so ill create my cat as public class cat : animal and give the cat the field teeth for example.

Now I want to make a new animal which is a cat, im taking data from several listboxes so I would need a constructor that takes those parameters and this is what I dont get, do I have to declare them in every child class aswell?

I know that my animal should have 3 parameters from the animal class plus another from the cat class so the new cat should accept (name, age, species, teeth) but it seems that I have to tell the constructor in the cat class to accept all of these and theres my question, what purpose does the animal class serve? If I still need to write the code in all subclasses why have the base class? Probably me not getting it but the more I read the more confused I become.

Answer 1

Like Sergey said, its not only about constructors. It saves you having to initialize the same fields over and over. For example,

Without inheritance

class Cat
{
    float height;
    float weight;
    float energy;
    string breed;

    int somethingSpecificToCat;

    public Cat()
    {
        //your constructor. initialize all fields
    }

    public Eat()
    {
        energy++;
        weight++;
    }

    public Attack()
    {
        energy--;
        weight--;
    }   

}

class Dog
{
    float height;
    float weight;
    float energy;
    string breed;

    int somethingSpecificToDog;

    public Dog()
    {
        //your constructor. initialize all fields
    }

    public Eat()
    {
        energy++;
        weight++;
    }

    public Attack()
    {
        energy--;
        weight--;
    }   

}

With Inheritance

Everything common to animals gets moved to the base class. This way, when you want to setup a new animal, you don’t need to type it all out again.

abstract class Animal
{
    float height;
    float weight;
    float energy;
    string breed;

    public Eat()
    {
        energy++;
        weight++;
    }

    public Attack()
    {
        energy--;
        weight--;
    }   
}
class Cat : Animal
{   
    int somethingSpecificToCat;

    public Cat()
    {
        //your constructor. initialize all fields
    }   
}

class Dog : Animal
{   
    int somethingSpecificToDog;

    public Dog()
    {
        //your constructor. initialize all fields
    }   
}

Another advantage is, if you want to tag every animal with a unique ID, you don’t need to include that in each constructor and keep a global variable of the last ID used. You can easily do that in the Animal constructor since it will be invoked everytime the a derived class is instantiated.

Example

abstract class Animal
{
    static int sID = 0;

    float height;
    float weight;
    int id;

    public Animal()
    {
        id = ++sID;
    }
}

Now when you do;

Dog lassie = new Dog();  //gets ID = 1
Cat garfield = new Cat(); // gets ID = 2

If you want a list of all Animals in your ‘farm’,

without inheritance

List<Cat> cats = new List<Cat>();   //list of all cats
List<Dog> dogs = new List<Dog>(); //list of all dogs
...etc

With inheritance

List<Animal> animals = new List<Animal>();  //maintain a single list with all animals
animals.Add(lassie as Animal);
animals.Add(garfield as Animal);

This way, if you want to see if you have an animal called Pluto, you just need to iterate over a single list (animals) rather than multiple lists (Cats, Dogs, Pigs etc.)

EDIT in response to your comment

You don’t need to instantiate Animal. You simply create an object of whichever Animal you want to. In fact, since an Animal will never be a generic Animal, you can create Animal as an abstract class.

abstract class Animal
{
    float height;
    float weight;
    float energy;
    string breed;

    public Eat()
    {
        energy++;
        weight++;
    }

    public Attack()
    {
        energy--;
        weight--;
    }   
}
class Cat : Animal
{   
    int somethingSpecificToCat;

    public Cat()
    {
        //your constructor. initialize all fields
    }   
}

class Dog : Animal
{   
    int somethingSpecificToDog;

    public Dog()
    {
        //your constructor. initialize all fields
    }   
}

Cat garfield = new Cat();
garfield.height = 24.5;
garfield.weight = 999; //he's a fat cat
//as you can see, you just instantiate the object garfield
//and instantly have access to all members of Animal

Animal jerry = new Animal(); //throws error
//you cannot create an object of type Animal
//since Animal is an abstract class. In this example
//the right way would be to create a class Mouse deriving from animal and then doing

Mouse jerry = new Mouse();

Edit to your comment

If you store it in a list of Animals, you still have access to all fields. You just have to cast it back to its original type.

List<Animal> animals = new List<Animal>();
animals.Add(garfield as Animal);
animals.Add(lassie as Animal);

//if you do not cast, you cannot access fields that were specific to the derived class.
Console.WriteLine(animals[0].height);   //this is valid. Prints Garfield's height
Console.WriteLine(animals[0].somethingSpecificToCat); //invalid since you haven't casted
Console.WriteLine((animals[0] as Cat).somethingSpecificToCat); //now it is valid

//if you want to do it in a loop

foreach(Animal animal in animals)
{
    //GetType() returns the derived class that the particular animal was casted FROM earlier

    if(animal is Cat)
    {
        //the animal is a cat
        Cat garfield = animal as Cat;
        garfield.height;
        garfield.somethingSpecificToCat;
    }
    else if (animal is Dog)
    {
        //animal is a dog
        Dog lassie = animal as Dog;
        lassie.height;
        lassie.somethingSpecificToDog;
    }   
}