Java OOPs (Object-Oriented Programming) Concepts

Java OOPs Concepts

Java OOPs (Object-Oriented Programming) Concepts

• Class
• Object
• Inheritance
• Polymorphism
• Abstraction
• Encapsulation

1. Class in Java

A class in Java is a blueprint or template for creating objects. It defines fields (variables) and methods that represent the properties and behaviors of the object. Think of a class like a house blueprint — it defines the design (rooms, doors, windows), but the actual house is the object built from that blueprint.

Syntax of a Class

class ClassName {
    // Fields (variables)
    dataType variableName;

    // Methods (functions)
    returnType methodName(parameters) {
        // method body
    }
}

Example – Creating and Using a Class

// Defining a class
class Car {
    // Fields (Properties)
    String brand;
    int speed;

    // Method (Behavior)
    void displayDetails() {
        System.out.println("Brand: " + brand);
        System.out.println("Speed: " + speed + " km/h");
    }
}

// Main class to test
public class Main {
    public static void main(String[] args) {
        // Creating an object of Car class
        Car myCar = new Car();

        // Assigning values
        myCar.brand = "Toyota";
        myCar.speed = 120;

        // Calling method
        myCar.displayDetails();
    }
}

Output:

Brand: Toyota
Speed: 120 km/h

Key Points about Classes in Java

• Keyword: A class is created using the class keyword.
• Naming Convention: Class names should start with an uppercase letter (Car, Employee).
• Members: Can have variables (fields) and methods.
• Objects: Created using the new keyword from a class.
• Access Modifiers: Can be public, private, protected, or default.
• Constructor: A special method with the same name as the class used for initialization.

Example with Constructor

class Student {
    String name;
    int age;

    // Constructor
    Student(String n, int a) {
        name = n;
        age = a;
    }

    void display() {
        System.out.println("Name: " + name);
        System.out.println("Age: " + age);
    }
}

public class Main {
    public static void main(String[] args) {
        // Creating objects with constructor
        Student s1 = new Student("Amit", 21);
        Student s2 = new Student("Priya", 22);

        s1.display();
        s2.display();
    }
}

Output:

Name: Amit
Age: 21
Name: Priya
Age: 22

Advantages of Using Classes

• Code Reusability – Same class can be used to create multiple objects.
• Data Organization – Combines related variables and methods.
• Object-Oriented Design – Forms the basis of OOP concepts like Inheritance and Polymorphism.
• Encapsulation – Data and methods are bundled together.

❓ FAQs with Answers

Q1: What is encapsulation in Java?

A: It is the process of wrapping variables and methods into a single unit. It helps in data hiding and securing code.

Q2: What is inheritance in Java?

A: Inheritance allows one class to acquire properties and methods of another class, enhancing code reusability.

Q3: What is the difference between method overloading and overriding?

• Overloading: Same method name, different parameters in the same class.
• Overriding: Redefining the parent method in a subclass.

Q4: What is abstraction and how is it implemented?

A: Abstraction hides complexity and exposes only relevant parts. It is implemented via abstract classes and interfaces.

Q5: Interface vs Abstract Class?

Feature	Abstract Class	Interface
Methods	Abstract and Concrete	Only Abstract (till Java 7), default/static (from Java 8)
Multiple Inheritance	Not Supported	Supported
Constructor	Yes	No
Access Modifiers	Any	Public Only

Q6: Why is OOP important in Java?

A: It promotes modularity, scalability, and maintainability. It models real-world problems more naturally.

More on Java Classes

• Explore: Java Classes in Depth – Complete Guide

Written by: Op Verma

Tags: Java, OOP, Inheritance, Polymorphism, Encapsulation, Abstraction

Java OOPs