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25 Java OOPs Concepts Interview Questions

Updated: Dec 17, 2023


Java OOPs Concepts Interview Questions

Welcome to our blog post on Java OOPs Concepts Interview Questions. If you're looking for for Java interview questions with answers, you're in the right place. In this post, we've compiled a wide range of object oriented programming, common Java interview questions that cover essential concepts, including inheritance, polymorphism, abstraction, encapsulation, and composition. Whether you're a beginner looking to solidify your understanding or an experienced developer preparing for a technical interview, these Java OOPs interview questions and answers will help you sharpen your skills and ace your next interview.


Inheritance:

1.) Question: What is inheritance in Java? Answer: Inheritance is a mechanism in Java where a class (subclass or derived class) can inherit properties and behaviors (fields and methods) from another class (superclass or base class).

2.) Question: How is inheritance implemented in Java? Answer: In Java, inheritance is implemented using the extends keyword. Subclasses inherit the members (fields and methods) of their superclass.

3.) Question: What is method overriding in Java? Answer: Method overriding is a concept in Java where a subclass provides its own implementation of a method that is already defined in its superclass. It is achieved by using the @Override annotation.

4.) Question: What is the difference between super and this keywords in Java? Answer: The super keyword is used to refer to the superclass's members (fields or methods), while the this keyword is used to refer to the current instance of the class.

5.) Question: Can a subclass access the private members of its superclass? Answer: No, a subclass cannot directly access the private members of its superclass. They are not visible to the subclass.


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Polymorphism:

6.) Question: What is polymorphism in Java? Answer: Polymorphism is a concept in Java where objects of different classes can be treated as objects of a common superclass. It allows a single interface to represent different data types.

7.) Question: Explain the difference between compile-time (static) and runtime (dynamic) polymorphism. Answer: Compile-time polymorphism is resolved at compile time and is achieved through method overloading. Runtime polymorphism is resolved at runtime and is achieved through method overriding.

8.) Question: What is method overloading in Java? Answer: Method overloading is a feature in Java that allows a class to have multiple methods with the same name but different parameters.

9.) Question: How does Java achieve polymorphism through interfaces? Answer: Java achieves polymorphism through interfaces by allowing multiple classes to implement the same interface, and objects of these classes can be treated interchangeably when they implement the same interface.

10.) Question: What is the significance of the instanceof operator in Java polymorphism? Answer: The instanceof operator is used to determine if an object is an instance of a particular class or interface. It is often used to ensure type safety before performing typecasting.


Abstraction:

11.) Question: What is abstraction in Java? Answer: Abstraction is a concept in Java that allows you to represent the essential features of an object while hiding the unnecessary details. It is achieved through abstract classes and interfaces.

12.) Question: How do you create an abstract class in Java? Answer: An abstract class is created by using the abstract keyword in the class declaration. It may have abstract methods (methods without a body) that must be implemented by its subclasses.

13.) Question: What is the difference between an abstract class and an interface? Answer: Abstract classes can have both abstract and concrete methods, while interfaces can only have abstract methods. A class can extend only one abstract class, but it can implement multiple interfaces.

14.) Question: How does encapsulation relate to abstraction? Answer: Encapsulation is one of the principles of OOP that supports abstraction. It involves bundling data and methods into a single unit (class) and controlling access to the data through access modifiers.

15.) Question: What is the purpose of access modifiers (public, private, protected) in abstraction? Answer: Access modifiers control the visibility and access level of class members. They help in enforcing encapsulation and abstraction by specifying who can access and modify class members.


Encapsulation:

16.) Question: What is encapsulation in Java? Answer: Encapsulation is the OOP concept of bundling data (attributes) and methods (behaviors) that operate on that data into a single unit (class) and controlling access to the data through access modifiers.

17.) Question: How do getter and setter methods contribute to encapsulation in Java? Answer: Getter methods allow controlled access to the class's private data, and setter methods allow controlled modification of that data. They help ensure data integrity and provide encapsulation.

18.) Question: Explain the principle of data hiding in encapsulation. Answer: Data hiding means that the internal representation of an object is hidden from the outside world. It ensures that the data can only be accessed through well-defined methods, preventing direct manipulation.

19.) Question: What is the benefit of making class members private in encapsulation? Answer: Making class members private restricts direct access to them, ensuring that data can only be modified and accessed through controlled methods (getters and setters). This enhances data security and integrity.

20.) Question: How can you achieve immutability using encapsulation? Answer: You can achieve immutability by making the class's fields (data) final and providing no setter methods. This ensures that once an object is created, its state cannot be changed.


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Composition:

21.) Question: What is composition in Java? Answer: Composition is a design principle in Java where a class is composed of one or more objects of other classes. It is an alternative to inheritance and promotes code reuse and flexibility.

22.) Question: How does composition differ from inheritance? Answer: In composition, a class contains objects of other classes as members, whereas in inheritance, a subclass inherits properties and behaviors from a superclass.

23.) Question: What is the "HAS-A" relationship, and how does it relate to composition? Answer: The "HAS-A" relationship represents the idea that a class contains or is composed of objects of another class. It is a common indicator that composition is being used.

24.) Question: What are the advantages of using composition over inheritance? Answer: Composition allows for greater flexibility, code reusability, and the ability to change the behavior of a class at runtime. It avoids some of the issues associated with tight coupling in inheritance.

25.) Question: Can you provide an example of a class that demonstrates composition in Java? Answer: Sure, consider a Car class that contains objects of a Engine class, Wheel class, and FuelTank class as its components. This is an example of composition.


These interview questions and answers cover a wide range of topics related to object-oriented programming principles in Java, including inheritance, polymorphism, abstraction, encapsulation, and composition. Be prepared to provide detailed explanations and examples during your interviews.

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