Last updated: 14 February 2021 by

We have listed here the most sensitive questions from the OOP interviews in Java with the best possible answers.

These real-time OOP interview questions in Java can be asked in any technical interview, from beginners to experienced children aged 3 to 5.

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

We have listed all the questions and answers asked during the PLO interview on a sensitive topic, so you will have no problem understanding them.

Frequently asked questions about OOP interviews in Java

1. What are the basic concepts of OOP in Java?
Or what are the basic principles of the PPO concept in Java?
Or, what are the basic principles (main pillars) of OOP concepts in Java?

Ens: There are four basic principles that make a language object-oriented. They are as follows:

Encapsulation : This is a mechanism to hide the implementation of data by restricting access to public methods.

Abstraction: It is a method by which data that the user does not need can be hidden.

Heredity: It is the process by which a class of children acquires all the characteristics and behaviors of the parent class.

Polymorphism: Polymorphism means multiple forms. It is a technique that allows us to perform a task in different ways. There are two types: static and dynamic.

Static polymorphism is obtained by the overload method, dynamic polymorphism by the redefinition method.

2. What is object-oriented programming in Java?

Ens: Object-oriented programming (OOP) in Java is a methodology or programming paradigm (model) for designing a computer program that uses classes and objects.

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

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

Ens: Procedural programming consists of writing code without objects.

Object-oriented programming consists of writing code with objects that contain data in the form of fields (often called properties in Java) and code functionality in the form of methods (often called behaviors in Java).

4. What is object-oriented programming (OOP) in Java?

Ens: Writing a program using features such as encapsulation, abstraction, inheritance, and polymorphism is called object-oriented programming.

5. Name some programming languages that implement OOP functions.

Ens: Smalltalk, C++, Java, PHP, Python, etc. implement OOP functions. Java is a purely object-oriented language. Smalltalk is the first true object-oriented programming language.

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

6. What is the main difference between an object-oriented programming language and an object-oriented programming language?

Ens: Object-oriented programming languages such as Java, Python, C++ follow OOPS concepts such as encapsulation, abstraction, polymorphism, inheritance, etc.

Object-oriented programming languages such as JavaScript, VBScript, etc. follow some of the features of OOPS, but do not support polymorphism and inheritance.

7. What are the important features of the OOP concept in Java?

Ens: These are some of the key features of the OOP concept.

  • Data has a higher priority than functions.
  • Programs are divided into a number of units called objects.
  • Objects interact with each other through functions (methods).
  • The methods that operate on the object data are closely linked in the data structure.
  • The data is hidden in the OOP and is not accessible to external methods. It is therefore safe from accidental changes.
  • The data cannot move freely from the object.
  • New data and methods can easily be added at any time.

8. What are the advantages and disadvantages of OOP in Java?

Ens: The concept of OOP in Java offers several advantages that are not present in procedural programming such as C, Pascal, etc. Here are some of the major and important benefits of object-oriented programming:

  • Security
  • Reuse
  • Effective communication
  • Integrated software development
  • expandable
  • A simple division of labour
  • Maintenance
  • Efficiency

9. How are OOP concepts applied in Java?

Ens: Because of its reusable function, it is used in many areas. Here are some important applications of the OOP :

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

10. What are the key points for developing a pure object-oriented program?

Ens: There are a few important points to consider when developing purely object-oriented code. That’s it:

  1. The program is per interface (or supertype), not per implementation.
  2. Interactions between classes must be freely coupled.
  3. The code must be tightly encapsulated. Where possible, avoid using public and static variables and single-button design models.
  4. Always use code that uses inheritance, composition and auxiliary methods.
  5. The if-a relationship is better than the is-a relationship because it offers more flexibility.
  6. For multi-threaded applications, use immobile objects to display the report.
  7. Use design templates whenever possible.
  8. Use modern software dependencies and take full advantage of the latest technology available.

11. What is Java encapsulation?

Ens: Encapsulation is a programming method that makes the fields of a class private and allows access to them via public methods.
In other words: Encapsulation is a method of hiding implementation details and providing methods of accessing data.

12. What’s behind the data?

Ens: Data masking is declaring a private field within a class, so no one can access it from the outside.

13. How to implement encapsulation in Java?

Ens: In this way we can realize or implement the encapsulation in a Java program.

  • Declare an instance variable as private, so that no one outside the class has direct access to it.
  • Provide public setter and getter methods in the class to set/change variable values.

14. How can programmatic data be hidden?

Ens: By declaring the members of the data (variables) as private, we can achieve data masking. When variables in a class are declared private, no one can access them from the outside.

The main advantage of hiding data in Java is that we can ensure the security of the application.

15. In the following indicator, the department is declared private in the class Test and t is an object of the class Test. Any problems with the code? If so, can you explain why?

public final class Test {
private double radius = 1; public double getArea() {
return radius * radius * Math.PI;
public static void main(String[] args)
Test t = new Test();
System.out.println(Circle radius: +t.radius);
System.out.println(Circle area: +t.getArea());

Ens: Yes, the above code was executed successfully. That won’t be a problem. On your way out.

  • Radius of the circle: 1.0
  • Circular area : 3.141592653589793

16. What is a tightly encapsulated class in Java?

Ens: In Java, when every variable within a class is declared private, it is called a tightly encapsulated class.

17. Which of the following classes are hermetically sealed?

class A
private int x = 20 ;
class B is an extension of A
int y = 50 ;
class C is an extension of A
private int z = 10 ;

Ens: Class A and Class C are hermetically sealed classes. Class B is not tightly encapsulated due to the non-private variable y. Everyone can access it outside the classrooms.

18. Are all classes P, Q, R tightly encapsulated in the following code? If so, how?

class P {
int a = 10 ;
class Q is an extension of P {
private int b = 20 ;
class R is an extension of Q {
private int z = 30 ;

Ens: Class Q is a class of children in class P, and so is not tightly encapsulated either. A non-private data member of class P is available in subclass Q by default.

Similarly, class R is a subclass of class Q. A member of class Q, which does not provide private data, is available in class R by default.

19. What is the Accessory method and what is the Mutator method?

Ens: The method used to obtain the value of a variable is called the getter or accessor method.

The method used to update or determine the value of a variable is called the update method or mutation method.

20. Are there any problems with the code in the source below? If so, why?

public class Student {
private string name;
public string getName() {
return name;
public void setName(String studentsName) {
name = studentsName;
class EncapsulatedTest {
public static void main(String[] arg) {
Student obj = new Student();
obj.nom = John;

Ens: The above code generates a compilation error because the name of the variable in the Student class is declared private. Thus, it is not possible to directly access a data member outside the class.

21. What happens if we don’t use encapsulation in a Java program?

Ens: If we do not use encapsulation in the program, the fields are not private and can be viewed by anyone outside the class.

For more questions on encapsulation, see this tutorial : QUESTIONS AND ANSWERS.

22. What is inheritance in Java?

Ens: The technique of creating a new class using the existing functionality of the class is called inheritance in Java. In other words, inheritance is the process by which a child class acquires all the traits and behaviors of the parent class.

23. Why should we use the inheritance?

Ens: Legacy is one of the main pillars of the OP concept. Some objects have certain properties and behaviors. Through inheritance, the child class acquires all the characteristics and behaviors of the parent class.

The following reasons justify the use of inheritance in Java.

  • We can reuse the code of the base class.
  • Legacy allows us to extend the capabilities of a class or method by stepping over it.
  • Inheritance is used to take advantage of the features of existing classes.
  • It is used to perform the polymorphism during execution, i.e., to bypass the method.

24. What is a Java relationship?

Ens: The Is-A relationship represents inheritance. Implemented with the keyword extends. Used to reuse the code.

25. What are superclass and underclass?

Ens: The class whose properties a subclass inherits is called a superclass. It is also called the base class or parent class.

A class that inherits all of its members (fields, methods, and nested classes) from another class is called a subclass. It is also called a derived class, child class or extended class.

26. How is inheritance implemented/realized in Java?

Ens: Heredity can be implemented or realized with two key words :

  1. extends: extends is a keyword used to develop inheritance between two classes and two interfaces.
  2. implements the keyword used to develop inheritance between a class and an interface.

For more information on best practice legacy issues, see this tutorial : The 50 most important questions and answers about Java legacy for both newcomers and experienced professionals.

27. Can a derived class be referenced for an object of the base class?

Ens: If School is a superclass and Student is a subclass, the following reference is wrong – Student st = new School() ;

28. Is it possible for an object of a child class to refer to a parent class?

Ens: Yes. This applies to Java. Example – School sc = new student() ;

29. What is the point of the super keyword in Java?

Rep: A super keyword is a reference variable that points to a direct object of the superclass. It is used for the following purposes:

  • To refer to a direct instance of the parent class.
  • To call the immediate constructor of the parent class.
  • Call a method of immediate superclassification.

30. Why do we use this keyword in Java?

Ans: This keyword is a reference variable that refers to the current class object. Contains a reference to the current object in the class or to an object in the same class.

There are six ways to use Java, that’s the key word. They are as follows:

1. This reference can be used to refer to the current variable of the instance of the class.
2. This keyword is used to call a non-static method of the current class.
3. ce() may be used to call the constructor of the current class.

4. This keyword can be used as a parameter when calling the method.
5. The keyword this can be used as a parameter when calling the constructor.
6. It can also be used to return an object of the method’s current class.

31. Can we use this() and this super() in the same constructor?

Ens: No, Java does not allow you to use both super() and this() in the same constructor. According to the Java specification, super() or this() must be the first expression in the constructor.

32. What is the difference between super and these keywords?

Ens: Take this course: The difference between super and this keyword.

We hope these Java DTP interview questions will help you understand the level of questions the interviewer is asking. These are common PLO interview questions that should be prepared by those who want to decipher the tests and technical interviews.

New questions are updated regularly. Share this information in social media for your friends.
Good luck! !!

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