Class And Object

You must understand these basic concepts:

• Class

• Object

• Class Loading Concept

Class

A class is a user-defined data type used to create objects.

It acts as a blueprint/template for creating objects.

java

class ClassName {
    // members
}

• class → Java keyword

• ClassName → Identifier

Members of a Class:

A class can contain:

• Variables (data members)

• Methods (functions)

• Constructors

• Blocks (static / instance)

• Enums

• Inner Classes

• Interfaces

java

class Student {
    int sid;
    String name;

    void display() {
        System.out.println("Student Info");
    }
}

Here, Student defines a category/type.

Object

An object is an instance of a class.

It represents a real-world entity and holds actual data.

• Object is created using new keyword

• Memory is allocated in Heap memory

• Reference is stored in a reference variable

• Created at runtime (dynamic memory allocation)

java

class Student {
    int sid;
    String sname;
    String semail;
}

public class Main {
    public static void main(String[] args) {
        
        Student s1 = new Student();
        
        s1.sid = 101;
        s1.sname = "Anil";
        s1.semail = "anil@gmail.com";
    }
}

🔍 Explanation:

• Student → Class

• new Student() → Object created in Heap

• s1 → Reference variable (stores address of object)

Class Loading Concept

A Class Loader loads classes into JVM memory when required.

It is part of JRE (Java Runtime Environment)

• Not all classes are loaded at once

• Classes are loaded dynamically when needed

When Class is Loaded?

1. Object Creation

java

Student s1 = new Student();

JVM checks:

• If class is not loaded → load first

• Then create object

2. Running main() Method

java

public static void main(String[] args)

The class containing main() is loaded first

3. Accessing Static Members

java

ClassName.staticMethod();

If class is not loaded → JVM loads it first

4. Dynamic Class Loading

Java allows loading classes at runtime using Class.forName()

java

Class.forName("ClassName");

• It loads class using String name

• It is a static method

• Belongs to java.lang.Class

java

ClassNotFoundException

Occurs when:

• .class file is not found

java

class A {
    static {
        System.out.println("Static Block in class A");
    }

    private A() {}
}

public class Main {
    public static void main(String[] args) throws Exception {
        Class.forName("A");
    }
}

🔍 Explanation:

• Class.forName("A") → Loads class A

• Static block executes when class is loaded

• Constructor is private → object cannot be created

• Still, class loads successfully