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21 Interview Questions of OOPs Concepts

Updated: Dec 17, 2023


Interview Questions of OOPs Concepts

1.) What is oops in Java? - Questions on Object Oriented Programming

Answer: Object-Oriented Programming (OOP) is a paradigm in computer programming that revolves around the concept of objects, which are instances of classes. In OOP, the world is modeled as a collection of objects that interact with each other to perform tasks and solve problems. Each object encapsulates both data (attributes or properties) and the methods (functions or behaviors) that can be applied to manipulate that data. OOP promotes modularity, reusability, and a clear organization of code, making it easier to design and maintain complex systems. Key principles of OOP include encapsulation, inheritance, polymorphism, and abstraction, which provide a framework for designing software that is efficient, scalable, and easier to understand. This approach is widely used in modern software development, with languages like Java, C++, and Python being prominent examples of languages that embrace the OOP paradigm.

Interview questions on OOPs concepts in Java are crucial as they assess a candidate's ability to apply fundamental principles, ensuring they can design efficient and maintainable code, a key skill in modern software development. Dive into a curated set of interview questions of OOPs concepts. Strengthen your understanding of Java core interview questions on object oriented programming with these comprehensive answers, preparing you to excel in technical interviews.

2.) What are the four main principles of OOP? Answer: The four main principles of OOP are encapsulation, inheritance, polymorphism, and abstraction.

3.) Explain the concept of a class in Java. Answer: A class in Java is a blueprint or template that defines the structure and behavior of objects. It contains fields (variables) and methods (functions) that define the characteristics and operations of objects.

4.) What is an object in Java, and how is it different from a class? Answer: An object in Java is an instance of a class. It represents a real-world entity and has state (fields) and behavior (methods). A class defines the blueprint for creating objects, while objects are instances of that class.

5.) Describe the difference between encapsulation and abstraction in OOP. Answer: Encapsulation is the practice of hiding the internal details of an object and exposing only the necessary information through methods. Abstraction, on the other hand, is the process of simplifying complex systems by breaking them into smaller, manageable parts. In Java, encapsulation is achieved through access modifiers, while abstraction is implemented using abstract classes and interfaces.


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6.) How does inheritance work in Java, and what is its purpose? Answer: Inheritance in Java allows a class (subclass or child class) to inherit the properties and methods of another class (superclass or parent class). It promotes code reusability and establishes a relationship between classes where the child class inherits the attributes and behaviors of the parent class.

7.) What is method overriding, and how is it different from method overloading? Answer: Method overriding occurs when a subclass provides a specific implementation for a method that is already defined in its superclass. Method overloading, on the other hand, involves defining multiple methods in the same class with different parameter lists.

8.) Explain the concept of polymorphism in Java with an example. Answer: Polymorphism allows objects of different classes to be treated as objects of a common superclass. It enables a single interface to represent a general class of actions. For example, you can have a Shape superclass with various subclasses (e.g., Circle and Rectangle). You can then create an array of Shape objects and call the draw method on each object, regardless of its specific type.

9.) What is a constructor, and why is it used in Java classes? Answer: A constructor in Java is a special method that is called when an object is instantiated. It initializes the object's state. Constructors have the same name as the class and do not have a return type. They are used to set initial values and perform setup tasks for objects.

10.) Differentiate between the super keyword and the this keyword in Java. Answer: The super keyword is used to refer to the superclass or parent class of the current object. It is often used to call superclass constructors or access superclass members. The this keyword, on the other hand, is used to refer to the current instance of the class. It is used to differentiate between instance variables and parameters or local variables with the same name.

11.) What is a static method, and how is it different from an instance method? Answer: A static method in Java belongs to the class itself rather than to any specific instance of the class. It can be called using the class name and does not require an instance of the class to be created. In contrast, an instance method is associated with an instance of the class and can access instance variables and methods.

12.) Describe the concept of an abstract class in Java. Answer: An abstract class in Java is a class that cannot be instantiated on its own. It serves as a blueprint for other classes and can have abstract methods that must be implemented by its subclasses. Abstract classes are declared using the abstract keyword.

13.) What is an interface in Java, and how does it differ from an abstract class? Answer: An interface in Java is a collection of abstract methods (methods without a body) that a class can implement. Unlike abstract classes, interfaces can be implemented by multiple classes, allowing a class to inherit the behavior of multiple interfaces. Classes that implement interfaces must provide concrete implementations for all interface methods.

14.) What is method overloading, and how is it achieved in Java? Answer: Method overloading is a feature that allows a class to have multiple methods with the same name but different parameter lists (number or types of parameters). Java distinguishes between overloaded methods based on the number and types of arguments provided during a method call.


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15.) Explain the "has-a" relationship in object-oriented programming. Answer: The "has-a" relationship, also known as composition, refers to a class having an instance of another class as one of its members. It is used to model a stronger association between classes, where one class is composed of or contains objects of another class.

16.) What is the purpose of the final keyword in Java, and how is it used? Answer: In Java, the final keyword is used to indicate that a variable, method, or class cannot be changed or extended. A final variable cannot be reassigned, a final method cannot be overridden, and a final class cannot be extended.

17.) What are access modifiers in Java, and what are their different levels of visibility? Answer: Access modifiers control the visibility and accessibility of class members (fields, methods, constructors) within and outside of the class. The four levels of access modifiers in Java are private, default (package-private), protected, and public.

18.) Explain the concept of method visibility or access control in Java. Answer: Method visibility determines which classes or code can access a method. It is controlled by access modifiers like public, private, protected, and default. public methods are accessible from anywhere, private methods are only accessible within the class, protected methods are accessible within the same package and subclasses, and default methods are accessible within the same package.

19.) What is the difference between shallow copy and deep copy in Java? Answer: Shallow copy creates a new object that is a copy of the original object, but it does not create copies of the objects referenced by the original object. Deep copy, on the other hand, creates copies of both the original object and all the objects referenced by it, recursively.

20.) What is the purpose of the this reference in Java constructors? Answer: In Java constructors, the this reference is used to differentiate between instance variables and parameters with the same names. It allows you to explicitly refer to instance variables when there is a naming conflict.

21.) Explain the concept of method chaining in Java. Answer: Method chaining, also known as fluent interface design, allows consecutive method calls on an object to be chained together. Each method returns the modified object, making it possible to call multiple methods in a single line of code, improving readability and conciseness.

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