SECTION 2 – Object-Oriented Programming in Java (Q21–Q40)

Q21. What is Object-Oriented Programming in Java and why is it important?

Explanation

Object-Oriented Programming (OOP) is a programming approach where software is designed around objects instead of functions.
An object represents a real-world entity and contains both data (variables) and behavior (methods).
Java is fully based on OOP principles, which makes large applications easier to design, scale, and maintain.
OOP helps reduce code duplication by promoting reuse through inheritance and composition.
In enterprise systems, OOP improves readability, testing, and teamwork because code is organized logically.

Example

class User {
    String name;

    void greet() {
        System.out.println("Hello " + name);
    }
}

public class Main {
    public static void main(String[] args) {
        User u = new User();
        u.name = "Sumit";
        u.greet();
    }
}

Tip

Interviewers expect you to explain OOP using real-world examples like “Car”, “User”, or “BankAccount”.

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Q22. What is a Class in Java?

Explanation

A class is a blueprint or template used to create objects.
It defines what data an object will store and what actions it can perform.
Classes help organize code into logical units.
Without classes, Java programs would become unmanageable for large projects.
Every object in Java is created from a class.

Example

class Car {
    String brand;

    void drive() {
        System.out.println("Car is driving");
    }
}

Tip

Remember: Class = design, Object = actual thing.

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Q23. What is an Object in Java?

Explanation

An object is a real instance of a class.
It occupies memory and represents actual data during program execution.
Objects interact with each other using methods.
Each object has its own state and behavior.
Most Java applications work by passing objects between layers.

Example

Car c = new Car();
c.brand = "BMW";
c.drive();

Tip

Say clearly: objects are created using the new keyword.

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Q24. Explain Encapsulation in Java.

Explanation

Encapsulation means hiding internal data and exposing it through controlled methods.
Variables are marked private and accessed using getters and setters.
This protects data from accidental modification.
Encapsulation improves maintainability and security.
It is one of the most asked interview topics.

Example

class Student {
    private int age;

    public void setAge(int a) {
        age = a;
    }

    public int getAge() {
        return age;
    }
}

Tip

Use this phrase: Encapsulation = data hiding + controlled access.

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Q25. What is Inheritance?

Explanation

Inheritance allows one class to acquire properties of another class.
The child class reuses code from the parent class.
It promotes code reuse and hierarchical structure.
Java supports single inheritance using extends.
It is commonly used in framework development.

Example

class Animal {
    void eat() {
        System.out.println("Eating");
    }
}

class Dog extends Animal {
    void bark() {
        System.out.println("Barking");
    }
}

Tip

Parent → Child relationship is written using extends.

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Q26. What is Polymorphism?

Explanation

Polymorphism means one method behaving differently in different situations.
It is achieved using method overriding and interfaces.
It allows writing flexible and scalable code.
Runtime polymorphism is heavily used in Spring and enterprise apps.
It follows the concept “one interface, many implementations”.

Example

class Shape {
    void draw() {
        System.out.println("Drawing shape");
    }
}

class Circle extends Shape {
    void draw() {
        System.out.println("Drawing circle");
    }
}

Tip

Explain: same method name, different behavior.

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Q27. What is Abstraction?

Explanation

Abstraction hides implementation details and shows only required features.
It focuses on what an object does, not how.
Java provides abstraction using abstract classes and interfaces.
It simplifies complex systems.
Used heavily in API design.

Example

abstract class Bank {
    abstract void interest();
}

Tip

Use real-life example: ATM hides banking logic.

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Q28. Difference between Abstract Class and Interface?

Explanation

Abstract class can have both abstract and concrete methods.
Interface supports only abstract methods (Java 8 added default methods).
A class can extend only one abstract class but implement multiple interfaces.
Interfaces are used for full abstraction.
Abstract classes are used for partial abstraction.

Example

interface Payment {
    void pay();
}

Tip

Say: interfaces support multiple inheritance.

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Q29. What is Constructor?

Explanation

Constructor initializes objects.
It has same name as class.
It runs automatically when object is created.
Used to set default values.
Constructors cannot return values.

Example

class User {
    User() {
        System.out.println("User created");
    }
}

Tip

No return type, not even void.

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Q30. What is Method Overriding?

Explanation

Method overriding occurs when child class provides its own version of parent method.
Used for runtime polymorphism.
Method signature must be same.
Access level cannot be reduced.
Common in framework callbacks.

Example

class A {
    void show() {}
}

class B extends A {
    void show() {
        System.out.println("Overridden");
    }
}

Tip

Always happens at runtime.

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Q31. What is super keyword?

Explanation

super refers to parent class object.
Used to access parent variables and methods.
Also used to call parent constructor.
Helps avoid ambiguity.
Common in inheritance chains.

Example

super.show();

Tip

Think: super = parent reference.

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Q32. What is this keyword?

Explanation

this refers to current object.
Used to differentiate instance variables.
Helps in constructor chaining.
Improves code clarity.
Frequently used in setters.

Example

this.age = age;

Tip

this always points to current class.

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Q33. What is Method Overloading?

Explanation

Same method name with different parameters.
Occurs at compile time.
Improves readability.
Return type alone cannot overload.
Used in utility classes.

Example

void add(int a);
void add(int a, int b);

Tip

Compile-time polymorphism.

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Q34. What is Final keyword?

Explanation

Final prevents modification.
Final variable cannot change.
Final method cannot override.
Final class cannot inherit.
Used for security and constants.

Example

final int x = 10;

Tip

Final = restriction.

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Q35. What is Static keyword?

Explanation

Static belongs to class, not object.
Shared across all instances.
Used for utility methods.
Reduces memory usage.
Main method is static.

Example

static void show() {}

Tip

Static members accessed without object.

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Q36. What is Composition?

Explanation

Composition means HAS-A relationship.
One class contains another class object.
Preferred over inheritance.
Provides flexibility.
Used in microservices.

Example

class Engine {}
class Car {
    Engine e = new Engine();
}

Tip

Composition > inheritance in real projects.

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Q37. What is Access Modifier?

Explanation

Controls visibility of classes and members.
Public: everywhere.
Private: same class only.
Protected: same package or subclass.
Default: package scope.

Example

private int age;

Tip

Private is most restrictive.

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Q38. What is instanceof?

Explanation

Checks object type at runtime.
Used before casting.
Prevents ClassCastException.
Common in polymorphism.
Returns boolean.

Example

if(obj instanceof Dog)

Tip

Used for safe downcasting.

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Q39. What is Object class?

Explanation

Root class of Java hierarchy.
Every class inherits it.
Provides methods like toString().
Used internally by JVM.
Important for equality checks.

Example

Object o = new String("Hi");

Tip

All classes extend Object automatically.

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Q40. What is SOLID in OOP?

Explanation

SOLID is a design principle set.
Helps build scalable systems.
Each letter represents one rule.
Used in enterprise architecture.
Improves maintainability.

Example

// Single Responsibility Principle

Tip

Mention SRP, OCP, LSP, ISP, DIP in interviews.