Chapter 2: Object-Oriented Programming – Classes, Objects & Encapsulation

Introduction

Object-Oriented Programming (OOP) is the foundation of modern enterprise application development.

Most enterprise applications are built by modeling real-world entities as software objects.

Examples:

Students
Employees
Customers
Companies
Placement Drives
Orders
Products

Java is primarily an Object-Oriented Programming language and provides powerful features for building modular, reusable, maintainable, and scalable software systems.

In this chapter, we will learn the core building blocks of Object-Oriented Programming and apply them to our Institute Placement Management System (IPMS).

What You Will Learn

By the end of this chapter, you will:

Understand Object-Oriented Programming
Create Classes
Create Objects
Define Fields and Methods
Understand Constructors
Use Constructor Overloading
Learn Access Modifiers
Implement Encapsulation
Create Getters and Setters
Understand Packages
Follow JavaBean Conventions
Build domain models for IPMS

What is Object-Oriented Programming?

Object-Oriented Programming is a programming paradigm that organizes software around objects rather than functions.

An object represents a real-world entity.

Examples:

Real World Entity	Software Object
Student	Student
Company	Company
Placement Drive	PlacementDrive
Interview	Interview
Administrator	Admin

Core Concepts of OOP

Every object contains:

State
Behavior
Identity

Example:

Student

State:

Name
Roll Number
CGPA

Behavior:

Register
Apply
Update Profile

Identity:

Student ID

Benefits of OOP

Code Reusability
Maintainability
Modularity
Scalability
Better Organization
Easier Testing
Enterprise Readiness

OOP Overview

Understanding Classes

A class is a blueprint used to create objects.

A class defines:

Properties
Behaviors

Example:

public class Student {

}

Creating First Class

public class Student {

    String name;

    int age;

    double cgpa;
}

Understanding Fields

Fields store object data.

Example:

String name;

int age;

double cgpa;

Field Types

Common field types:

String name;

int age;

double cgpa;

boolean eligible;

Creating Objects

Objects are instances of classes.

Example:

Student student = new Student();

Understanding Object Creation

new Student();

creates an object in memory.

Student student

creates a reference variable.

Assigning Values

Student student = new Student();

student.name = "John Doe";

student.age = 21;

student.cgpa = 8.5;

Displaying Object Data

System.out.println(student.name);

System.out.println(student.age);

System.out.println(student.cgpa);

Complete Example

public class Student {

    String name;

    int age;

    double cgpa;

    public static void main(String[] args) {

        Student student = new Student();

        student.name = "John Doe";

        student.age = 21;

        student.cgpa = 8.5;

        System.out.println(student.name);

        System.out.println(student.age);

        System.out.println(student.cgpa);
    }
}

Creating Multiple Objects

Student student1 = new Student();

Student student2 = new Student();

Example

student1.name = "John Doe";

student2.name = "Sara Elise";

Class and Object Relationship

Understanding Methods

Methods define object behavior.

Example:

public void displayInfo() {

}

Creating a Method

public void displayInfo() {

    System.out.println(name);

    System.out.println(age);

    System.out.println(cgpa);
}

Calling a Method

student.displayInfo();

Example

public class Student {

    String name;

    int age;

    double cgpa;

    public void displayInfo() {

        System.out.println(name);

        System.out.println(age);

        System.out.println(cgpa);
    }
}

Using Methods

Student student = new Student();

student.name = "John Doe";

student.age = 21;

student.cgpa = 8.5;

student.displayInfo();

Methods with Parameters

public void updateCgpa(double cgpa) {

    this.cgpa = cgpa;
}

Calling Method with Parameter

student.updateCgpa(8.75);

Methods Returning Values

public double getCgpa() {

    return cgpa;
}

Using Return Values

double result = student.getCgpa();

Understanding Constructors

A constructor initializes an object.

Constructor name must match class name.

Default Constructor

public Student() {

}

Example

public class Student {

    public Student() {

        System.out.println("Student Created");
    }
}

Creating Object

Student student = new Student();

Parameterized Constructor

public Student(
        String name,
        int age,
        double cgpa) {

    this.name = name;

    this.age = age;

    this.cgpa = cgpa;
}

Example

Student student =
        new Student(
                "John Doe",
                21,
                8.5);

Complete Example

public class Student {

    String name;

    int age;

    double cgpa;

    public Student(
            String name,
            int age,
            double cgpa) {

        this.name = name;

        this.age = age;

        this.cgpa = cgpa;
    }
}

Understanding this Keyword

this.name = name;

Constructor Overloading

public Student() {

}

public Student(String name) {

}

public Student(
        String name,
        int age) {

}

public Student(
        String name,
        int age,
        double cgpa) {

}

Access Modifiers

Java provides access control using modifiers.

Public

public class Student {

}

Private

private String name;

Protected

protected String name;

Default

String name;

Access Modifier Comparison

Modifier	Same Class	Same Package	Subclass	Other Package
public	Yes	Yes	Yes	Yes
protected	Yes	Yes	Yes	No
default	Yes	Yes	No	No
private	Yes	No	No	No

Encapsulation

Encapsulation protects object data from unauthorized access.

Poor Design

public class Student {

    public double cgpa;
}

Better Design

public class Student {

    private double cgpa;
}

Getter Method

public double getCgpa() {

    return cgpa;
}

Setter Method

public void setCgpa(double cgpa) {

    this.cgpa = cgpa;
}

Encapsulated Class

public class Student {

    private double cgpa;

    public double getCgpa() {

        return cgpa;
    }

    public void setCgpa(double cgpa) {

        this.cgpa = cgpa;
    }
}

Using Encapsulation

Student student = new Student();

student.setCgpa(8.5);

System.out.println(student.getCgpa());

Validation Using Setters

public void setCgpa(double cgpa) {

    if (cgpa >= 0 && cgpa <= 10) {

        this.cgpa = cgpa;
    }
}

JavaBean Convention

Enterprise Java applications commonly follow JavaBean standards.

Rules

Private fields
Public getters
Public setters
No argument constructor

Example

public class Student {

    private String name;

    public Student() {

    }

    public String getName() {

        return name;
    }

    public void setName(String name) {

        this.name = name;
    }
}

Understanding Packages

Packages organize classes.

Package Example

package com.ipms.student;

Package Structure

src
│
└── com
    └── ipms
        ├── student
        ├── company
        └── drive

Student Package

package com.ipms.student;

Company Package

package com.ipms.company;

Drive Package

package com.ipms.drive;

Import Statement

import com.ipms.student.Student;

Object Lifecycle

Objects go through three stages.

Stage 1

Creation

Student student = new Student();

Stage 2

Usage

student.displayInfo();

Stage 3

Garbage Collection

Object becomes eligible for cleanup.

Understanding Memory

Objects are stored in:

Heap Memory

References are stored in:

Stack Memory

IPMS Domain Model

Student Class

package com.ipms.student;

public class Student {

    private Long studentId;

    private String name;

    private double cgpa;
}

Company Class

package com.ipms.company;

public class Company {

    private Long companyId;

    private String companyName;
}

PlacementDrive Class

package com.ipms.drive;

public class PlacementDrive {

    private Long driveId;

    private String title;
}

Complete Student Example

package com.ipms.student;

public class Student {

    private Long studentId;

    private String name;

    private double cgpa;

    public Student() {

    }

    public Student(
            Long studentId,
            String name,
            double cgpa) {

        this.studentId = studentId;

        this.name = name;

        this.cgpa = cgpa;
    }

    public Long getStudentId() {

        return studentId;
    }

    public void setStudentId(Long studentId) {

        this.studentId = studentId;
    }

    public String getName() {

        return name;
    }

    public void setName(String name) {

        this.name = name;
    }

    public double getCgpa() {

        return cgpa;
    }

    public void setCgpa(double cgpa) {

        this.cgpa = cgpa;
    }
}

Mini Project

Build Student Registration Module.

Features:

Create Student
Update Student
Display Student
Validate CGPA
Organize using Packages

Best Practices

Keep fields private
Use constructors
Use getters and setters
Follow JavaBean conventions
Organize classes using packages
Keep classes focused

Common Beginner Mistakes

Mistake	Correct Approach
Public fields	Private fields
Missing constructor	Create constructors
No package structure	Organize packages
Large classes	Small focused classes
Duplicate code	Create reusable methods

Practice Exercises

Create Employee class.
Create Department class.
Implement encapsulation.
Implement constructors.
Create JavaBeans.
Organize classes using packages.
Create Student Registration Module.

Chapter Summary

In this chapter, you learned:

Object-Oriented Programming
Classes
Objects
Fields
Methods
Constructors
Constructor Overloading
Access Modifiers
Encapsulation
Getters and Setters
JavaBean Convention
Packages
Object Lifecycle

In the next chapter, we will learn:

Inheritance
Polymorphism
Abstraction

These concepts help build reusable and extensible enterprise applications.