Object Oriented Programming

Programming Paradigm

Object

Objects can be considered as real-world instances of entities like class, that have some characteristics and behaviors.

Class

A blueprint or template for creating similar types of objects. They usually have:

• Attributes or Data

• Behaviors or Methods

Class Relationships

Inheritance

• "is-a" relationship

class Animal            // parent class
{
    public virtual void Move() {
        Console.WriteLine("i can move");
    }
}
class Dog : Animal    // child class    inheritance
{
    public override void Move() {
        Console.WriteLine("i can run");
    }
}

Association->Aggregation->Composition

Inheritance

A mechanism in which one class inherits the properties of another class.

• Single Inheritance

• Multilevel Inheritance

• Multiple Inheritance (Not Supported in C# due to The Diamond Problem)

• Hierarchical Inheritance

• Hybrid Inheritance

C#

class Parent { ... }  

class Child1 : Parent { ... }    // Single, Hierarchical

class Child2 : Parent { ... }    // Hierarchical, Multilevel

class GrandChild : Child2 { ... }    //Muiltilevel

interface Service1 { ... }
interface Service2 { ... }
class Child : Service1, Service2 { ... }    // Multiple

Python

Polymorphism

A concept where an object behaves differently in different situations.

Compile-time Polymorphism / Static Dispatch

• Function/Method/Operator Overloading a.k.a. Early Binding

With overloading you have a function with different sets of parameters. The function that is to be executed is determined using the number and type of the parameters you provide. As these are known at compile time, the compiler already determines the function to use. Because of this, it is called compile time polymorphism.

public class Calculate {  
    public int Add(int a, int b) {
        return a + b;
    }
    public int Add(int a, int b, int c) {    // Function Overloading
        return a + b + c;
    }
}  

Runtime Polymorphism / Dynamic Dispatch

• Method Overriding (virtual/abstract) a.k.a. Late Binding

When you are overriding a virtual function of a base class in one or more derived classes and then call this function from a base class, the actual class of the underlying object is not clear at compile time. Thus it is determined only at runtime which function is executed. That is why it is called runtime polymorphism.

public class Drawing {  
    public virtual double Area() {
        return 0;
    }  
}
  
public class Circle : Drawing {  
    public double Radius { get; set; }  
    public Circle() {  
        Radius = 5;  
    }  
    public override double Area() {        // Method Overriding
        return (3.14) * Math.Pow(Radius, 2);  
    }  
}  
  
public class Square : Drawing {  
    public double Length { get; set; }  
    public Square() {
        Length = 6;  
    }  
    public override double Area() {        // Method Overriding
        return Math.Pow(Length, 2);  
    }  
}  
  
public class Rectangle : Drawing {
    public double Height { get; set; }  
    public double Width { get; set; }  
    public Rectangle() {  
        Height = 5.3;  
        Width = 3.4;  
    }
    public override double Area() {        // Method Overriding  
        return Height * Width;  
    }  
}  

class Program {  
    static void Main(string[] args) {  
        Drawing circle = new Circle();          // Late Binding
        Console.WriteLine("Area :" + circle.Area());  
  
        Drawing square = new Square();          // Late Binding
        Console.WriteLine("Area :" + square.Area());  
  
        Drawing rectangle = new Rectangle();    // Late Binding
        Console.WriteLine("Area :" + rectangle.Area());  
    }  
}  

Abstraction

Abstraction is the process to hide the internal details and show only the functionality.

• The keyword abstract is used before the class or method to declare the class or method as abstract.

  • An Abstract method is a method without a body.

  • The implementation of an abstract method is done by a derived class.

  • When the derived class inherits the abstract method from the abstract class, it must override the abstract method.

Abstraction is the concealment of unnecessary program details so that the user only sees the essential attributes.

The Abstract class and Interface both are used to have abstraction.

class program {  
    abstract class Animal {  
        public abstract void Eat();  
        public void Sound() {  
            Console.WriteLine("dog can sound");  
        }  
    }  
    class Dog : Animal {  
        public override void Eat() {
            Console.WriteLine("dog can eat");  
        }
    }
} 

An Abstract class can have non-abstract Methods(concrete methods) and declare variables while in case of Interface all the methods has to be abstract and there should not be any implementation.

Encapsulation

Encapsulation refers to the bundling of data/properties with the methods that operate on that data, or the restricting of direct access to some of an object's components.

• Encapsulation is used to hide the values or state of a structured data object inside a class, preventing direct access to them by clients in a way that could expose hidden implementation details or violate state invariance maintained by the methods.

• Encapsulation is achieved by taking advantage of the access modifiers that include public, private protected, private, internal, and protected that control the visibility and accessibility of the members of a class.

Encapsulation is the bundling of data, including the methods that operate on that data, into a single, private unit

public class Account {
    private string accountNumber;
    private decimal balance;
    
    public Account(string accountNumber, decimal balance) {
        this.accountNumber = accountNumber;
        this.balance = balance;
    }
    public decimal GetBalance() {
        return balance;
    }
    public void Deposit(decimal amount) {
        balance += amount;
    }
    public void Withdraw(decimal amount) {
        if (balance >= amount)
            balance -= amount;
    }
}

In this example, the Account class has two private members: accountNumber and balance. These members are not accessible from outside the class. The class provides public methods GetBalance(), Deposit(), and Withdraw() to allow outside code to interact with the object.

40+ OOPs Interview Questions and Answers (2025) - InterviewBitInterviewBit

How to Approach

1. Handle Ambiguity

   Depending When being asked an object-oriented design question, you should inquire who is going to use it and how they are going to use it. Depending on the question, you may even want to go through the "six W's": who, what, where, when, how, why

2. Define the Core Objects

3. Analyze Relationships

4. Investigate Actions