Top 85+ OOPs Interview Questions in Java with Answers

Here, we have listed the most important tricky OOPs interview questions in java with the best possible answers.

These realtime OOPs interview questions in java can be asked in any technical interviews from freshers and experienced 3 to 5 years.

If you are a developer or tester and want to crack the interview, you should prepare answers to these OOPs interview questions.

We have listed all OOPs interview questions and answers topic-wise so that you do not have any problem understanding.

We have also covered some coding programming questions based on OOPs concepts that will help you to crack the technical tests in different companies.

Frequently Asked OOPs Interview Questions in Java

OOPs Interview Questions in Java

1. What are the core OOPs concepts in Java?
Or, What are the basic principles of OOPs concept in Java?
Or, What is the main fundamental principles (main pillars) of the OOPs concepts in Java?

Ans: There are four basic principles that make a language Object Oriented. They are as follows:

Encapsulation: It is the mechanism of hiding data implementation by restricting access to public methods.

Abstraction: It is a technique by which we can hide the data that is not required to a user.

Inheritance: It is a process by which a child class acquires all the properties and behaviors of the parent class.

Polymorphism: Polymorphism means many forms. It is the technique by which we can perform a single task in different ways. It is further divided into two types – static and dynamic.

Static polymorphism is achieved using method overloading and dynamic polymorphism using method overriding.

2. What Is Object-Oriented Programming in Java?

Ans: Object-oriented programming (OOP) in Java is a programming methodology or paradigm (model) to design a computer program using classes and objects.

It is the most popular programming paradigm and widely used in the software industry today. It is an extension of procedural programming.

3. What is the difference between procedural programming and object-oriented programming?

Ans: Procedural programming means writing code without objects.

Object-oriented programming means writing code with objects which contain data in the form of fields (often known as properties in java) and functionality code in the form of methods (often known as behavior in java).

4. What is Object-Oriented Programming System (OOPs) in Java?

Ans: Writing a program using features like encapsulation, abstraction, inheritance, and polymorphism is called object-oriented programming system.


5. Name some programming languages that implement OOPs features.

Ans: Smalltalk, C++, Java, PHP, Python, etc implement OOPs features. Java is a pure object-oriented language. Smalltalk is the first truly object-oriented programming language.

C#, C++ is a procedural language with object-oriented extension.

6. What is the basic difference between Object Oriented Programming language and Object Based Programming language?

Ans: Object Oriented Programming languages like Java, Python, C++ follow concepts of OOPS like Encapsulation, Abstraction, Polymorphism, and Inheritance, etc.

Object-Based Programming languages like JavaScript, VBScript, etc follow some features of OOPS but they do not support Polymorphism and Inheritance.

7. What are some important features of OOPs concept in Java?

Ans: Some of the important striking features of OOPs concept are as follows:

  • Higher priority is focused on data rather than functions.
  • Programs are divided into the number of entities known as objects.
  • Objects communicate with each other through functions (methods).
  • Methods that operate on data of an object are closely bound together in the data structure.
  • Data is hidden in the OOP and cannot be accessed by external methods. Hence, it is safe from accidental modification.
  • Data cannot move freely out of the object.
  • New data and methods can be easily added whenever needs.

8. What are the advantages/benefits of OOPs in Java?

Ans: OOPs concept in Java offers several advantages that are not available in procedural programming like C, Pascal, etc. Some of the major and important benefits of object-oriented programming are as follows:

  • Security
  • Reusability
  • Effective communication
  • Developing complex software
  • Easily upgraded
  • Easy partition of work
  • Maintenance
  • Efficiency

9. What are applications of OOPs concepts in Java?

Ans: Due to its reusability feature, it is widely used in many areas. Some of the important application areas of OOPs are as follows:

  • Real-time systems
  • Object-oriented database
  • Graphical user interface design in the Windows operating system.
  • Artificial intelligence and expert systems
  • Parallel programming
  • CAD/CAM software and in many areas.

10. What are the key points for developing a clean Object Oriented program?

Ans: There are several key points for developing a clean object-oriented code that must be kept in mind. They are:

  1. Program to an interface (or the super type) not the implementation.
  2. Interacting of Classes should be loosely coupled among themselves.
  3. Code should implement tightly encapsulation. Avoid the use of public, static variables, and singleton design pattern wherever possible.
  4. Always reuse the code using inheritance, composition, and utility methods.
  5. Has-A relationship is better than Is-A relationship because it provides more flexibility.
  6. In the case of multi-threading applications, use immutable objects to represent the state.
  7. Make proper use of design patterns wherever possible.
  8. Use up-to-date software dependencies and make the best use of the latest technology available to us.

11. What is Encapsulation in Java?

Ans: Encapsulation is the programming technique of making fields in a class private and providing access to those fields via public methods.
In other words, encapsulation is the technique of hiding implementation details and providing methods for data access.

12. What is Data hiding?

Ans: Data hiding is the process in which the field is declared private within the class to hide so that no one can access it from outside the class.

13. How to implement Encapsulation in Java?

Ans: There are two key points thereby we can achieve or implement encapsulation in Java program.

  • Declaring the instance variable of the class as private so that it cannot be accessed directly by anyone from outside the class.
  • Provide the public setter and getter methods in the class to set/modify the value of the variable.

14. How to achieve data hiding programmatically?

Ans: By declaring data members (variables) as private, we can achieve data hiding. If variables are declared as private in the class, nobody can access them from outside the class.

The biggest advantage of data hiding in java is we can achieve security in the application.