Java OOPs Interview Questions and Answers for Beginners

Object-Oriented Programming (OOP) is a foundational concept in Java programming, widely used to design software that models real-world entities with objects. For beginners preparing for Java interviews, understanding OOP principles is essential. This knowledgebase covers frequently asked OOPs interview questions along with clear answers to help novices confidently approach interviews.

What is Object-Oriented Programming (OOP)?
OOP is a programming paradigm based on the concept of "objects," which are instances of classes. It provides a structured way to organize code by bundling data (attributes) and methods (functions) that operate on the data into single units called classes. This approach models real-world entities and promotes code reuse, scalability, and maintainability.

Why is OOP Important in Java?
Java is inherently an object-oriented language, and using OOP principles enhances modularity and flexibility. It allows developers to create reusable code through inheritance, encapsulate data to protect it, abstract complexities, and implement polymorphic behaviors, thereby reducing code duplication and improving system design.

What are Classes and Objects?
A class is a blueprint or template for creating objects. It defines variables (properties) and methods (behaviors). An object is a concrete instance of a class with actual values. For example, a class “Car” may define properties like color and speed, and an object “myCar” is an instance with specific values assigned.

What are the Four Main Principles of OOP?

Encapsulation: Wrapping data and methods into a single unit (class) and restricting direct access using access modifiers like private or protected.

Abstraction: Hiding implementation details and exposing only essential features via abstract classes or interfaces.

Inheritance: Enabling one class to inherit properties and methods from another, promoting code reuse.

Polymorphism: Allowing methods or operators to take multiple forms, primarily through method overloading (compile-time polymorphism) and overriding (runtime polymorphism).

How is Abstraction Different from Encapsulation?
Abstraction focuses on hiding unnecessary details from users and showing only relevant functionalities. It is typically implemented with abstract classes and interfaces. Encapsulation, on the other hand, involves bundling data with methods and controlling access to data fields using access modifiers to ensure data integrity and security.

What is Method Overloading and Method Overriding?

Method Overloading occurs when multiple methods have the same name but different parameter lists within the same class. For example, a method “add” could add integers or doubles depending on the parameters.

Method Overriding happens when a subclass provides its own implementation for a method already defined in its superclass, enabling dynamic polymorphism.

Why Does Java Not Support Multiple Inheritance with Classes?
Java avoids multiple inheritance with classes to prevent ambiguity issues known as the "diamond problem," where multiple parent classes have the same method signature. Instead, Java supports multiple inheritance via interfaces, allowing a class to implement multiple interfaces unambiguously.

What is the ‘this’ Keyword Used For?
In Java, the ‘this’ keyword refers to the current instance of a class. It is used to differentiate between instance variables and parameters with the same name, invoke another constructor in the same class, and pass the current object as a parameter.

What is the Purpose of the ‘super’ Keyword?
The ‘super’ keyword accesses members of the immediate parent class. It is used to call superclass methods, access superclass variables, and invoke parent class constructors from a subclass.

How Does Java Implement Polymorphism?
Java supports polymorphism primarily through method overloading (compile-time) and method overriding (runtime). Runtime polymorphism happens when the overridden method of a subclass is called through a parent class reference, deciding the method implementation to execute during program execution.

What are Constructors and How Can They Be Overloaded?
A constructor is a special method that initializes new objects. Java allows constructor overloading, which means defining multiple constructors with different parameter lists to initialize objects in various ways.

What are Access Modifiers in Java?
Access modifiers control the visibility of classes, methods, and variables:

public: accessible from anywhere

private: accessible only within the class

protected: accessible within the package and subclasses

default (package-private): accessible within the package only

What is the Difference Between Shallow Copy and Deep Copy?

Shallow Copy duplicates the object but not the objects referenced by it, so changes in referenced objects affect both copies.

Deep Copy duplicates the object and the objects referenced by it recursively, creating independent copies.

How Does Java Manage Memory Related to Objects?
Java uses automatic memory management via Garbage Collection. Objects are stored in the heap, and when objects are no longer referenced, the Garbage Collector frees up memory to prevent memory leaks.

What are Interfaces and How Do They Promote Abstraction?
Interfaces define a contract with abstract methods that implementing classes must fulfill. They allow abstraction by specifying what methods a class should have without detailing the implementation, enabling multiple inheritance of type.

Why is Encapsulation Considered Important?
Encapsulation secures the internal state of an object by restricting direct access to data and exposing only necessary parts via public methods. This prevents unintended interference and misuse of data.

What is Inheritance and Its Types in Java?
Inheritance allows a class (child) to inherit fields and methods from another class (parent). Types include:

Single Inheritance: one child, one parent

Multilevel Inheritance: a chain of inheritance

Hierarchical Inheritance: multiple subclasses from one parent
(Note: Multiple inheritance is achieved using interfaces.)

How is Exception Handling Related to OOP?
Exception handling in Java uses OOP concepts via classes like Throwable, Exception, and RuntimeException to represent errors as objects. This allows structured handling of runtime errors with try-catch blocks.

What is Coupling in OOP?
Coupling refers to the degree of dependency between classes. Low coupling is preferred as it enhances modularity and eases maintenance, while high coupling can make systems more complex.

These questions represent the foundational knowledge every beginner Java programmer should master for OOPs interviews. Understanding these concepts, along with writing simple code examples to demonstrate them, will greatly aid in confidently answering interview questions and excelling in Java programming roles.