Top 40 OOPs Interview Questions and Answers (2024)

Table of Content

Basic OOPS Interview Questions

Q.1 What is Object-Oriented Programming (OOP)?

Object-Oriented Programming (OOP) is a programming paradigm based on the concept of “objects”, which can contain data in the form of fields (often known as attributes or properties) and code, in the form of procedures (often known as methods). OOP languages use class definitions to create objects, allowing for code and data to be structured in a way that models complex systems more naturally.

Q.2 What are the main principles of OOP?

The main principles of OOP are:

Encapsulation: Bundling the data (attributes) and methods that operate on the data into a single unit or class.
Inheritance: Allowing a class to inherit characteristics (methods and properties) from another class.
Polymorphism: Enabling a method to do different things based on the object it is acting upon.
Abstraction: Hiding complex reality while exposing only the necessary parts of an object.

Q.3 What is a class in OOP?

A class in OOP is a blueprint or template for creating objects. It provides initial values for state (member variables or attributes), and implementations of behavior (member functions or methods). A class defines the kinds of data and the functionality their objects will have.

Q.4 What is an object in OOP?

An object in OOP is an instance of a class. When a class is defined, no memory is allocated until an object of that class is created. An object has a physical presence in memory, and holds data and methods declared in the class. Each object can have unique attributes and common behaviors.

Q.5 Can you explain the concept of “inheritance”?

Inheritance is a fundamental principle of OOP that allows a new class, known as a subclass, to inherit data and behaviors (methods) from an existing class, referred to as a superclass. This helps in reusing code and creating a hierarchical classification of classes that builds upon the functionality of existing components.

Q.6 What is encapsulation?

Encapsulation is the mechanism of restricting access to some of an object’s components, which means that the object’s internal state cannot be accessed directly, and its representation is invisible from outside of the object’s definition. Access to the data and methods is tightly controlled by an interface. Encapsulation helps to prevent accidental manipulation and safeguard the object’s integrity.

Q.7 How does polymorphism work in OOP?

Polymorphism in OOP refers to the ability of different classes to respond to the same message (method call) in different ways. This is typically achieved through method overriding where a subclass redefines a method of its superclass, allowing the method to have behaviors tailored to the subclass. Polymorphism enhances flexibility and integration by allowing the same interface to be used for different underlying forms (data types).

Q.8 What is the difference between a class and an object?

The key difference is that a class is a blueprint or template for creating objects, defining the data and behavior of the objects, whereas an object is an instance of a class. Objects are the concrete entities that are created using the structure provided by their respective class.

Q.9 What are constructors and destructors?

Constructors and destructors are special methods in a class:

Constructor: A constructor is a method that is automatically called when an object is created. It is used to initialize objects, setting initial values for member variables and performing other setup operations.
Destructor: A destructor is a method that is automatically called when an object is destroyed or de-allocated. It is used for cleanup operations, such as releasing resources, closing file handles, or freeing memory that was dynamically allocated.

Q.10 What is method overloading?

Method overloading is the ability to create multiple methods in the same class with the same name but different parameters. It allows a class to perform the same action in different ways depending on the arguments passed. This is a form of polymorphism where different methods have the same name but different signatures (i.e., the number, type, or order of parameters).

Intermediate OOPS Interview Questions

Q.11 What is method overriding?

Method overriding occurs in object-oriented programming when a subclass provides a specific implementation of a method that is already defined in its superclass. This allows the subclass to tailor or modify the existing behavior of the method inherited from the superclass.

Q.12 Can you explain the concept of abstraction with an example?

Abstraction in OOP is the concept of hiding the complex reality while exposing only the necessary parts of an object. It helps in reducing programming complexity and increasing efficiency. For example, a car is an abstraction of a complex system of components like an engine, wheels, brakes, etc. Drivers interact with a simple interface (steering wheel, pedals) without needing to understand the details of the components.

Q.13 What is multiple inheritance and what are its potential issues?

Multiple inheritance is a feature of some OOP languages where a class can inherit behaviors and attributes from more than one parent class. This can make the program more flexible but also leads to complexities and problems such as the “Diamond Problem,” where an ambiguity arises when two classes B and C inherit from A, and class D inherits from both B and C. If there is a method in A that B and C have overridden, and D does not override it, which version of the method does D inherit: that of B, or C?

Q.14 How do interfaces differ from abstract classes?

Interfaces and abstract classes both can be used to enforce certain methods to be implemented in the classes that inherit them. However, interfaces do not contain any concrete methods (methods with body) while abstract classes may contain a mix of methods with and without an implementation. Moreover, a class can implement multiple interfaces but usually inherits from only one abstract class.

Q.15 What is the use of the ‘this’ keyword in programming?

The ‘this’ keyword is used in many programming languages to refer to the current instance of the class in which it is used. It is a reference to the current object, whose method or constructor is being called. The keyword helps in distinguishing between class fields and parameters with the same name, and for invoking other constructors in a constructor chaining.

Q.16 What are access specifiers?

Access specifiers (or access modifiers) define the accessibility of classes, methods, and other members. They control the scope of access: whether the components of a class can be accessed from other classes and methods. Common access specifiers include public, private, and protected.

Q.17 Can you explain static methods and static variables?

Static methods and variables belong to the class, rather than any instance of the class. Static variables are shared among all instances of a class; changing the variable affects all instances. Static methods can be called on the class itself, without an instance, and can only access other static members of the class.

Q.18 What is an abstract method?

An abstract method is a method that is declared in an abstract class, or in an interface, and does not have an implementation. Subclasses are required to provide an implementation for these methods. Abstract methods help in providing a template for further implementation.

Q.19 How does method overloading differ from method overriding?

Method overloading occurs when two or more methods in the same class have the same method name but different parameters (different type or number of parameters). Method overriding, on the other hand, is when a method in a subclass has the same name, same parameters, and same return type as a method in its superclass, but provides a specific implementation.

Q.20 What is the significance of the final keyword in Java?

The final keyword in Java makes a variable unchangeable (constant), a method not overrideable, and a class not inheritable. It is used to provide immutable capabilities to variables, to prevent methods from being overridden for security or design reasons, and to prevent further inheritance.

Q.21 Explain the concept of exception handling in OOP.

Exception handling is a programming construct in OOP languages that handles runtime errors, providing a way to transfer control from one part of the program to another. OOP languages typically use a try-catch-finally structure where the program tries to execute code, catches errors if they occur, and then finally executes code regardless of whether an error occurred, ensuring that necessary cleanup and other essential statements are executed.

Q.22 What is object cloning?

Object cloning refers to the creation of an exact copy of an object, replicating its state. It is used when you need a duplicate object with the same properties and state as the original. Cloning is important in situations where object creation is a costly operation.

Q.23 What are virtual functions and how do they work?

Virtual functions in OOP languages like C++ are functions that you expect to be overridden in derived classes. When a class declares a function as virtual, it indicates to the runtime to support late binding on this method. Virtual functions allow a program to call methods that don’t necessarily exist at the moment the code is compiled.

Q.24 How can encapsulation be used for data hiding?

Encapsulation involves bundling the data (variables) and code (methods) that operate on the data into a single unit or class, and restricting access to some of the object’s components. This is usually done by making variables private within a class, and providing public setter and getter methods to modify and read the values of the variables. This hides the internal state of the object from the outside world, protecting it from unauthorized access and misuse.

Q.25 Explain the Liskov Substitution Principle.

The Liskov Substitution Principle is one of the five SOLID principles of object-oriented design, which states that objects of a superclass should be replaceable with objects of its subclasses without affecting the functioning of the program. This means a subclass should override the parent class methods in a way that does not break functionality from a client’s point of view.

Q.26 What is the importance of constructors in object cloning?

Constructors play a critical role in object cloning as they ensure that a new object instance is correctly initialized when a clone is created. Depending on the cloning method, constructors in the cloned object can help in setting up or modifying initial state that differs from the original object.

Q.27 What is operator overloading?

Operator overloading is a feature in some OOP languages that allows existing operators to be redefined for user-defined data types (classes). This means that operators like +, -, * can be used not just for primitive types, like integers and doubles, but also for objects, allowing for more intuitive code.

Q.28 How does the garbage collector work in OOP languages like Java or C#?

In languages like Java and C#, garbage collection is an automatic memory management feature that frees up memory that is no longer in use. The garbage collector runs in the background and identifies objects that are no longer accessible or referenced by any part of the program, and then deallocates the memory occupied by these objects, thus preventing memory leaks.

Q.29 What is a memory leak and how can OOP help prevent it?

A memory leak occurs when a computer program incorrectly manages memory allocations, resulting in reduced performance or system failure. OOP helps prevent memory leaks through encapsulation and abstraction, which encourage tighter control over which parts of a program can access specific pieces of memory. Proper destructor usage in OOP can also ensure that memory is freed when objects are no longer needed.

Q.30 Can you explain the concept of dependency injection?

Dependency Injection is a design pattern used in OOP to make classes less dependent on each other by reducing hard-coded dependencies. Instead of instantiating dependencies inside the class, they are injected at runtime, typically by a framework. This makes the system easier to test, maintain, and extend.

Advanced OOPS Interview Questions

Q.31 What is the diamond problem in OOP and how can it be resolved?

The diamond problem occurs in object-oriented programming when two classes B and C inherit from A, and class D inherits from both B and C. If there is a method in A that B and C have overridden, and D does not override it, there is an ambiguity as to which version of the method D inherits—B’s or C’s. This problem can be resolved using multiple inheritance mechanisms such as virtual inheritance in C++, which ensures that only one instance of the base class A exists in the inheritance hierarchy, thus resolving the ambiguity.

Q.32 Explain the Composite Reuse Principle.

The Composite Reuse Principle is an OOP principle that advocates for composition over inheritance as a method of reusing functionality. Rather than inheriting from another class to gain its functionality, a class should contain instances of other classes that implement the desired functionality. This leads to better system modularity and reduces the tight coupling that often occurs with extensive inheritance.

Q.33 Discuss the SOLID principles in OOP.

The SOLID principles are a set of five guidelines meant to improve software design and promote code understandability and maintainability:

Single Responsibility Principle: A class should have only one reason to change, meaning it should have only one job or responsibility.
Open/Closed Principle: Objects or entities should be open for extension but closed for modification.
Liskov Substitution Principle: Objects of a superclass should be replaceable with objects of its subclasses without altering the correctness of the program.
Interface Segregation Principle: No client should be forced to depend on methods it does not use; larger interfaces should be split into smaller ones.
Dependency Inversion Principle: High-level modules should not depend on low-level modules. Both should depend on abstractions (e.g., interfaces).

Q.34 What is a design pattern? Give an example.

A design pattern is a general, reusable solution to a commonly occurring problem within a given context in software design. It is not a finished design that can be directly converted into code but a template for how to solve a problem that can be used in many different situations. An example is the Singleton pattern, which ensures a class has only one instance and provides a global point of access to it.

Q.35 How do proxy patterns work in OOP?

A proxy pattern in OOP provides a surrogate or placeholder object for another object to control access to it. This can be used for security reasons, handling the cost of instantiation of a heavyweight object, or adding additional behaviors when an object is accessed. Typical uses include virtual proxies, protection proxies, and smart references.

Q.36 What are mixins in object-oriented design?

Mixins are a class that offers functionality to other classes but is not meant to stand on its own (i.e., you do not instantiate a mixin). They enable the sharing of methods across multiple classes, which can help to avoid the use of deep inheritance hierarchies and promote function reuse. They are similar to interfaces but can contain an implementation.

Q.37 Explain the difference between deep copy and shallow copy.

A shallow copy of an object copies all of the member field values. This means that if a field value is a reference to an object, it only copies the reference, not the referred object. A deep copy copies all fields, and makes copies of dynamically allocated memory pointed to by the fields. A deep copy duplicates everything directly reachable from the fields of the object, resulting in a complete copy.

Q.38 What are the different ways to implement polymorphism in C++?

In C++, polymorphism can be implemented in several ways:

Compile-time polymorphism: Achieved through function and operator overloading.
Runtime polymorphism: Achieved through method overriding using virtual functions, allowing the method being called to be determined at runtime.

Q.39 How can the Observer pattern be implemented and used in software design?

The Observer pattern is a design pattern where an object, called the subject, maintains a list of its dependents, called observers, and notifies them automatically of any state changes, usually by calling one of their methods. It is primarily used in implementing distributed event-handling systems. The pattern promotes loose coupling since the subject doesn’t need to know anything about the observers.

Q.40 What are the implications of mutable and immutable objects in a multi-threaded environment?

In a multi-threaded environment, immutable objects are naturally thread-safe because their state cannot change after they are created, which means they can be shared between threads without synchronization. Mutable objects, however, can be modified after creation, which can lead to issues such as race conditions if not properly managed. Using immutable objects can greatly simplify the development of concurrent applications.

Top OOPs Interview Questions and Answers