1. Introduction to Classes and Objects

1.1 Overview

Classes and objects are used to structure programs in Java.

1.2 Relationship

Class → Blueprint
Object → Instance of a class

1.3 Real-World Example

Class → Car
Object → BMW, Audi

1.4 Importance

Helps organize code
Makes programs modular and reusable

2. What is a Class

2.1 Definition

A class is a blueprint used to create objects.

2.2 Structure

A class contains:

Variables (fields)
Methods (functions)

2.3 Example

class Student {
    String name;
    int age;    void display() {
        System.out.println(name + " " + age);
    }
}

2.4 Naming Convention

Use PascalCase
Example: StudentRecord

3. What is an Object

3.1 Definition

An object is an instance of a class.

3.2 Characteristics

State → variables
Behavior → methods
Identity → unique instance

3.3 Memory Allocation

Objects are created in heap memory.

3.4 Example

Student s1 = new Student();

4. Creating Classes and Objects

4.1 Creating Object

Student s1 = new Student();

4.2 Accessing Members

s1.name = "Pooja";
s1.age = 20;

4.3 Calling Method

s1.display();

4.4 Multiple Objects

Student s2 = new Student();

4.5 Complete Example

class Student {
    String name;
    int age;    void display() {
        System.out.println(name + " " + age);
    }    public static void main(String[] args) {
        Student s1 = new Student();
        s1.name = "Pooja";
        s1.age = 20;
        s1.display();
    }
}

5. Constructors

5.1 Definition

A constructor is a special method used to initialize objects.

5.2 Characteristics

Same name as class
No return type
Automatically called

5.3 Default Constructor

class Student {
    Student() {
        System.out.println("Constructor called");
    }
}

5.4 Parameterized Constructor

class Student {
    String name;    Student(String n) {
        name = n;
    }
}

5.5 Constructor Overloading

Student() {}
Student(String name) {}

5.6 this Keyword

this.name = name;

5.7 Constructor vs Method

Constructor	Method
Initializes object	Performs action
No return type	Has return type

6. Instance vs Static Members

6.1 Instance Members

Belong to object
Each object has its own copy

class Student {
    String name;
}

6.2 Static Members

Shared among all objects
Belong to class

class Student {
    static String college = "ABC";
}

6.3 Accessing Static Members

Student.college;

6.4 Differences

Instance	Static
Object-based	Class-based
Separate copy	Shared copy

6.5 When to Use Static

Common values
Utility methods

Conclusion

Classes and objects are the core of Java programming. By understanding how to create and use them, along with constructors and static members, beginners can build structured and reusable applications. These concepts are the foundation for advanced topics in Java and real-world software development.

Classes and Objects