Classes and Objects in Java

 Let's explore the fundamentals of object-oriented programming — Classes and Objects — in Java, with clear explanations and code examples.

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✅ What is a Class in Java?

A class is a blueprint or template that defines the structure and behavior (data and methods) that the objects created from it will have.

🔹 Think of a class as:

• A design for creating objects.

• It contains fields (variables) and methods (functions).

🔸 Syntax:

class ClassName {
    // Fields (variables)
    // Methods (functions)
}

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✅ What is an Object in Java?

An object is an instance of a class.

• It represents a real-world entity.

• Each object has its own state (fields) and can perform actions (methods).

🔸 Example:

If Car is a class, then myCar and yourCar are objects of that class.

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✅ Java Program: Creating and Using a Class and Object

// Define a class
class Student {
    // Fields (data members)
    String name;
    int age;

    // Method (behavior)
    void displayDetails() {
        System.out.println("Name: " + name);
        System.out.println("Age: " + age);
    }
}

// Main class
public class Main {
    public static void main(String[] args) {
        // Create an object of Student class
        Student s1 = new Student();

        // Set values
        s1.name = "Anjali";
        s1.age = 20;

        // Call method
        s1.displayDetails();
    }
}

✅ Output:

Name: Anjali
Age: 20

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🔸 Key Terminologies

Term	Description
class	Keyword to define a class
new	Keyword used to create an object
Constructor	Special method used to initialize objects (explained below)
Dot operator .	Used to access members of an object (e.g., object.method())

✅ Constructor in Java

A constructor is a special method that is automatically called when an object is created.

🔹 Example:

class Student {
    String name;
    int age;

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

    void display() {
        System.out.println(name + " is " + age + " years old.");
    }
}

public class Main {
    public static void main(String[] args) {
        Student s1 = new Student("Rahul", 21);  // Object created with values
        s1.display();
    }
}

Output:

Rahul is 21 years old.

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✅ Summary

Feature	Description
Class	Blueprint that defines variables and methods
Object	Instance of a class
Constructor	Initializes object when created
new keyword	Used to create object
Dot . operator	Access fields and methods of object