Why do I need inheritance in OOP? Real-world examples.

The problem in general is that it is sometimes unclear from books why do we need some particular technology. In this case we are going to discuss why do you need inheritance and where it is used in real-life applications. Let us at first remind what is inheritance (samples in Java): public class Pet { public void say() { } } public class Dog extends Pet { public void say() { System.out.println("I am a dog."); } } public class Cat extends Pet { public void say() { System.out.println("I am a cat."); } } public class Test{ public static void main(String[] args) { Pet pet1 = new Dog(); Pet pet2 = new Cat(); pet1.say(); pet2.say(); } } This program will output: I am a dog. I am a cat. The idea is very simple: despite pet1 and pet2 are of type Pet, pet1 is pointing to object of class Dog and pet is pointing to object of class Cat. The common question which is usually raised is: why we don’t have just Dog pet1 = new Dog(); and Cat pet2 = new Cat(); ? Why do we need to access it via Pet? Let me give you some real-world examples where do we need it: 1. Servlets. When we create a servlet we inherit base servlet class and override method doGet() or doPost()to add our functionality to the servlet. The server (for example

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Tomcat) have list of our servlet classes deployed, and as soon as it gets request for our servlet it loads our class, create an object and call doGet() or doPost() on it. As soon as server have no idea what classes do we have it address object of our class via variable of type HttpServlet. 2. The similar idea was used in early versions of Struts library. 3. In .NET as well as in Java you override Exception class or one of it’s successors to create exception specific to your application. The system (Java or .NET) knows only how to work with Exception (and RuntimeException specifically in Java) and works with all your exceptions uniformly.


Author: Artem's Blog

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10 thoughts on “Why do I need inheritance in OOP? Real-world examples.”

  1. You are actually explaining about Dynamic Method Dispatch. Its is better if you go for any Real time Implementations of Inheritance.


  2. in c# after
    Pet pet1 = new Dog();
    cat pet22 = new cat();
    when i called
    pet1.say();// it shows the say method of Pet class not of dog class.
    and pent22.say();//shows the Cat class’s say method.



  3. Awaisdar:
    The reason why the same code won’t work without correction in C# is the following:
    in Java all methods are virtual by default, compare to C# where in order to make them virtual you have to specify “virtual” keyword. The following program is C# version of the example code:

    using System;

    namespace Test
    public class Pet {
    public virtual void say() { }

    public class Dog : Pet {
    public override void say() { Console.WriteLine(“I am a dog.”); }

    public class Cat : Pet {
    public override void say() { Console.WriteLine(“I am a cat.”); }

    class Program
    static void Main(string[] args)
    Pet pet1 = new Dog();
    Pet pet2 = new Cat();


  4. what is missing in your example, and what is on the main reasons to have polymorphism in the first place is this class:

    public class PetTrainer {
    public void makeMyPetTalk(Pet p) {

    As you can see, PetTrainer can make any Pet talk – cats, dogs or even tigers that might come in the future.


  5. What you have shown in your blog is a way to implement inheritance.In my view inheritance is concept where one brand new object use some or all property of another object i.e. reuse the property of existing object rather than creating new.

    Some Real World Example:
    Battery and charger of Mobile Phones and Mobile of Nokia-Battery and charger is made ones and being used in different different models of Nokia mobile phones.

    Pencil Battery-an object——>used in radio,torch,toys.Etc(i.e. reuse of existing object in another object)Companies which are making torch,radio,toy they are just using the battery rather to make a new battery.



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