30 OOPs interview Questions and Answers (2024)

In the ever-evolving software development landscape, object-oriented programming (OOP) is the dominant model that enables developers to create scalable, modular, and maintainable code Whether you’re an experienced developer or an implementer preparing for an interview, understand the complexity of OOP concepts is important .

In this blog post, we will examine 30 OOP interview questions and provide detailed answers to help you prepare and excel in your next interview.

OOPs Interview Questions

1. What is Object Oriented Programming (OOPs)?

Object Oriented Programming (also known as OOPs) is a programming paradigm where the complete software operates as a bunch of objects talking to each other. An object is a collection of data and the methods which operate on that data.

2. Name the four fundamental principles of OOPs.

There are four basic principles of object-oriented programming (OOP):

Encapsulation: In this theory, data (properties) and methods (functions) that operate on the data are combined into a single object called class Encapsulation helps to hide the content of the object and displays only what is necessary revealed.

Inheritance: Inheritance allows a class (subclass or derived class) to inherit properties and actions from another class (subclass or base class) This enforces rule reuse and generalization.

Polymorphism: Polymorphism enables objects of different classes to be treated as objects of a common base group. It allows you to use an interface for different objects, which allows flexibility and extensibility in code design. There are two types of polymorphism: compile-time (the overwhelming method) and runtime (the overwhelming method).

Abstraction: Abstraction involves simplifying complex systems into instances of classes based on their shared essential attributes and behaviors. It ignores irrelevant details and focuses on relevant parts of the product. Abstractions help manage complexity and improve code design.

3. What is a Class?

Classes are like blueprints or templates. It defines the attributes (data) and methods (functions) that objects created from the class will have. Think of triangles as cookie cutters, and things like cookies cut out of dough with that cutter. A class defines general structures and behaviors, while objects are instances of that class whose properties have specific values.

4. What is an Object?

An object is an instance of a class in programming. Think of a class as a blueprint or template, and an object as a specific realization or instance created from that blueprint. The class defines the attributes (data) and methods (functions), and when you create an object, you’re essentially creating a unique copy of that class with specific values for its attributes.

5. What is a constructor, and why is it used in a class?

A constructor is a special method of a class that calls itself when an object of that class is created. Its primary purpose is to initialize the components of the product or perform any necessary configuration before the product is ready for use. Typically constructors are used to ensure that the object initializes to a valid and consistent state.

In many programming languages, constructors have the same name as classes. When you create a new object with the class, the constructor is automatically called to set the initial state of the object. This often involves assigning values ​​to other important product attributes or behaviors.

6. Explain the concept of method overloading

Method overload is a feature of object-oriented programming languages ​​that allows a class to have multiple methods with the same name but different parameters. In other words, you can define multiple methods in a class with the same name, but each method must have a unique signature, specified by the number of parameters or type.

7. Define the term “interface” in the context of OOP.

For Object-Oriented Programming (OOP), an interface is a template for the methods that a class should implement. It defines a contract that the class agrees to abide by, and specifies the options that must be provided without specifying how they will be used. Basically, the interface shows what an object can do without going into the details of how it does it. It encourages a level of abstraction and helps achieve polymorphism by letting classes implement the same interface in their own way.

8. What are the advantages and disadvantages of OOPs?

9.  What is the difference between Structured Programming and Object Oriented Programming?

10. How does OOP support data abstraction?

Object-oriented programming (OOP) actually supports data abstraction through the concept of abstraction. Abstraction is the process of simplifying complex systems into instances of classes based on the required attributes and behaviors of their partners. In OOP, data is pulled using abstract classes and interfaces.

Abstract classes: These are classes that cannot themselves be instantiated and can have abstract methods (unused methods defined). Abstract classes are templates for specific classes (subclasses) that host their properties and methods. By defining an abstract method, an abstract class specifies the methods that its subclasses should implement, ensures a degree of consistency, and enforces stable order

Interfaces: Interfaces are like abstract classes but declare only method signatures without any implementation. A class can implement multiple interfaces, retrieving method signatures from each. This allows for a high degree of flexibility in designing classes, and encourages a form of abstraction in which the focus is on what the class can do rather than how it is done

The use of abstract classes and interfaces allows OOP developers to abstract away the implementation details of a class, emphasizing what the object does rather than how it achieves its functionality This makes the code modular, maintainable and easy to use , and encourages high levels of data abstraction.

11 . Discuss the role of access modifiers in OOP.

In object-oriented programming (OOP), access modifiers play an important role in managing the visibility and accessibility of class members (attributes and methods). They help promote encapsulation, which is one of the core principles of OOP.

There are typically three main access modifiers:

1. Public:
   • Members declared as public are accessible from any part of the program.
   • This is the least restrictive access level.
   • Public members can be accessed by instances of the class, as well as by other classes.
2. Private:
   • Members declared as private are only accessible within the class itself.
   • This provides a high level of encapsulation, as the internal implementation details are hidden from external classes.
   • Private members cannot be accessed directly by instances of other classes.
3. Protected:
   • Members declared as protected are accessible within the class and its subclasses (derived classes).
   • This allows for a certain level of visibility to subclasses while still restricting access to the outside world.
   • It promotes the concept of inheritance and helps in building a hierarchy of classes.

By using access modifiers, developers can control the exposure of class members, preventing unintended external access and ensuring that the internal implementation remains secure and encapsulated. This promotes code maintainability, readability, and reduces the risk of unintended interference or misuse of class members.

12. Explain the concept of abstract classes.

Abstract classes are a key concept in object-oriented programming (OOP) that allows you to create a blueprint for other classes. An abstract class cannot be instantiated on its own; instead, it serves as a base class for other classes to inherit from. The primary purpose of abstract classes is to define a common interface and set of behaviors that derived classes must implement.

Here are some key features of abstract classes:

1. Cannot be instantiated: You cannot create an instance (object) of an abstract class. It exists solely for the purpose of providing a common structure for its subclasses.
2. May contain abstract methods: Abstract classes can have abstract methods, which are declared without providing an implementation. Subclasses that inherit from an abstract class must provide concrete implementations for these abstract methods.
3. May contain concrete methods: Abstract classes can also have regular (concrete) methods with an implementation. Subclasses can choose to override these methods, but it is not mandatory.
4. May have fields/attributes: Abstract classes can have attributes or fields, which are characteristics or data members that the subclasses inherit.

13.  What are some commonly used Object Oriented Programming Languages?

There are several widely used Object-Oriented Programming (OOP) languages. Some of the most common ones include:

1. Java: Known for its platform independence and used extensively in web development and enterprise applications.
2. Python: A versatile language used in web development, data science, artificial intelligence, and more.
3. C++: A powerful language often used in system/software development, game development, and performance-critical applications.
4. C#: Developed by Microsoft, commonly used for building Windows applications and games using the .NET framework.
5. Ruby: Known for its simplicity and productivity, often used in web development with frameworks like Ruby on Rails.
6. JavaScript: Primarily a scripting language for web development, it supports both object-oriented and functional programming paradigms.

These languages provide different features and are suitable for various application domains.

14. What are the different types of Polymorphism?

Polymorphism in programming languages generally comes in two flavors: compile-time (or static) polymorphism and runtime (or dynamic) polymorphism.

1. Compile-time Polymorphism:
   • Function Overloading: It occurs when multiple functions in the same scope have the same name but different parameter lists. The compiler determines which function to call based on the number and types of arguments.
   • Operator Overloading: This involves defining multiple behaviors for a specific operator based on the types of its operands. For example, you can overload the “+” operator for both numerical and string concatenation.
2. Runtime Polymorphism:
   • Function Overriding (Method Overriding): In object-oriented programming, when a subclass provides a specific implementation for a method that is already defined in its superclass. The decision on which method to call is made at runtime based on the actual object type.
   • Interface-based Polymorphism: When classes implement the same interface or abstract class, they can be treated interchangeably through the common interface, allowing for polymorphic behavior.

Both types of polymorphism contribute to making code more flexible, modular, and easier to understand.

15.  What is the difference between overloading and overriding?

Overloading occurs when you have multiple methods in the same class with the same name but different parameters. In other words, Overriding occurs when a subclass provides a specific implementation of a particular method already defined in its superclass.

16. Are there any limitations on Inheritance?

Yes, there are more challenges when you have more authority. Although inheritance is a very strong OOPs feature, it also has significant drawbacks.

• As it must pass through several classes to be implemented, inheritance takes longer to process.
• The base class and the child class, which are both engaged in inheritance, are also closely related to one another (called tightly coupled). Therefore, if changes need to be made, they may need to be made in both classes at the same time.
• Implementing inheritance might be difficult as well. Therefore, if not implemented correctly, this could result in unforeseen mistakes or inaccurate outputs.

17. What different types of inheritance are there?

In object-oriented programming, inheritance is a mechanism that allows a class to inherit properties and behaviors from another class. There are several types of inheritance, each serving different purposes. The most common types include:

1. Single Inheritance:
   • A class can inherit properties and behaviors from only one class.
   • Example: Class B inherits from Class A.
2. Multiple Inheritance:
   • A class can inherit properties and behaviors from more than one class.
   • Example: Class C inherits from both Class A and Class B.
3. Multilevel Inheritance:
   • A class can inherit from another class, and then another class can inherit from the derived class.
   • Example: Class C inherits from Class B, and Class B inherits from Class A.
4. Hierarchical Inheritance:
   • Multiple classes inherit from a single base class.
   • Example: Both Class B and Class C inherit from Class A.
5. Hybrid (Virtual) Inheritance:
   • Combination of multiple and single or multilevel inheritance.
   • It involves the use of both class-level and interface-level inheritance.
   • Example: Class D inherits from both Class A and Interface I.
6. Interface Inheritance:
   • A class can implement one or more interfaces, which define a contract for the methods the class must implement.
   • Example: Class X implements Interface Y and Interface Z.
7. Mixin Inheritance:
   • A form of multiple inheritance where a class can inherit the characteristics and behaviors of multiple classes (mixins).
   • Mixins are typically small, focused classes that provide a specific functionality.
   • Example: Class P with Mixin Q and Mixin R.

18. What is an interface?

In object-oriented programming (OOP), an interface is a programming construct that describes the conventions about the behavior of a class. This is a way of specifying which methods a class should implement, without providing usage information. An interface defines a collection of abstract methods that must be implemented by any concrete class that claims to be attached to that interface.

19.  What is a constructor?

In object-oriented programming (OOP), a constructor is a special method or function that is automatically called when an object is created. Its primary purpose is to initialize the object’s attributes or properties and set them to default values. Constructors ensure that the object is in a valid and usable state upon its creation.

20. What are the various types of constructors in C++?

In C++, constructors are special member functions that are automatically called when an object is created. They are responsible for initializing the object’s data members. There are several types of constructors in C++:

Default Constructor:

• A constructor that takes no parameters.
• Automatically invoked when an object is created without any arguments.
• If no constructor is defined for a class, the compiler provides a default constructor.

class MyClass {
public:
    MyClass() {
        // Default constructor
    }
};

Parameterized Constructor:

• A constructor that takes one or more parameters.
• Used to initialize the object with specific values during creation.

class MyClass {
public:
    MyClass(int x, double y) {
        // Parameterized constructor
        // Initialization logic using parameters
    }
};

Copy Constructor:

• A constructor that takes an object of the same class as a parameter.
• Used to create a new object as a copy of an existing object.

class MyClass {
public:
    MyClass(const MyClass& other) {
        // Copy constructor
        // Initialization logic to copy data from 'other'
    }
};

Destructor:

• A special member function that is called when an object goes out of scope or is explicitly deleted.
• Used for cleanup tasks like releasing resources acquired by the object.

class MyClass {
public:
    ~MyClass() {
        // Destructor
        // Cleanup logic
    }
};

Initializer List:

• Not a separate type of constructor, but worth mentioning.
• Used to initialize data members in the constructor’s initializer list rather than in the body of the constructor.

class MyClass {
private:
    int x;
    double y;

public:
    MyClass(int a, double b) : x(a), y(b) {
        // Initialization using initializer list
    }
};

21. What is a copy constructor?

Copy Constructor is a type of constructor, whose purpose is to copy an object to another. What it means is that a copy constructor will clone an object and its values, into another object, is provided that both the objects are of the same class.

22. What is a destructor?

Contrary to constructors, which initialize objects and specify space for them, Destructors are also special methods. But destructors free up the resources and memory occupied by an object. Destructors are automatically called when an object is being destroyed.

23. Can we overload the constructor in a class?

In some programming languages, like C++ and Java, it is possible to overload constructors in a class. Constructor overloading refers to the ability to define multiple constructors with different parameter lists within the same class.

24. Can we overload the destructor in a class?

No. A destructor cannot be overloaded in a class. The can only be one destructor present in a class.

25. What is a subclass?

The subclass is a part of Inheritance. The subclass is an entity, which inherits from another class. It is also known as the child class.

26. Define a superclass?

Superclass is also a part of Inheritance. The superclass is an entity, which allows subclasses or child classes to inherit from itself.

27. What is an abstract class?

An abstract class in object-oriented programming is a class that cannot be instantiated on its own and is typically created to serve as a base or blueprint for other classes. It may contain abstract methods, which are methods without a defined implementation in the abstract class. Subclasses that inherit from the abstract class must provide concrete implementations for these abstract methods.

28.  What is pure virtual function?

A pure virtual function, also known as an abstract function is a member function that doesn’t contain any statements. This function is defined in the derived class if needed.

29. How is data abstraction accomplished?

Data abstraction is accomplished with the help of abstract methods or abstract classes.

30. What is meant by exception handling?

No one wants its software to fail or crash. Exceptions are the major reason for software failure. The exceptions can be handled in the program beforehand and prevent the execution from stopping. This is known as exception handling.
So exception handling is the mechanism for identifying the undesirable states that the program can reach and specifying the desirable outcomes of such states.
Try-catch is the most common method used for handling exceptions in the program.