Object Modelling – Entities and Their Behaviour in Java

 

Introduction to Object Modelling in Java

    Java is an Object-Oriented Programming (OOP) language. In OOP, programs are designed by modelling real-world entities as objects.

👉 Object Modelling means representing real-life things inside a program using:

• Their properties (data)

• Their behaviour (actions)

Components of Object Modelling

Every object consists of three main components:

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1. State (Properties / Attributes)

State represents the data stored in an object.

Examples:

• Student → Name, Roll Number

• Car → Color, Speed

In Java → Variables

int roll;
String name;

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2. Behaviour (Actions / Functions)

Behaviour represents what the object can do.

Examples:

• Student → Study, Write Exam

• Car → Start, Stop

In Java → Methods

void study()
{
    System.out.println("Student is studying");
}

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3. Identity (Unique Name)

Each object has a unique identity.

Example:

Student s1;
Student s2;

Here:

• s1 and s2 are different objects.

Steps in Object Modelling 

Step 1: Identify the Entity

Example → Student

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Step 2: Identify Properties

• Name

• Roll number

• Age

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Step 3: Identify Behavior

• Study

• Write exam

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Step 4: Represent in Java

class Student
{
    int roll;
    String name;

    void study()
    {
        System.out.println("Studying...");
    }
}

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Real-Life Example of Object Modelling

Example: Car

Properties:

• Color

• Speed

Behaviour:

• Start

• Stop

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Java Representation

class Car
{
    String color;
    int speed;

    void start()
    {
        System.out.println("Car Started");
    }
}

Program: Model a Fan Object

class Fan
{
    String brand;
    int speed;

    void rotate()
    {
        System.out.println("Fan is rotating");
    }
}

class Main
{
    public static void main(String args[])
    {
        Fan f1 = new Fan();
        f1.brand = "Usha";
        f1.speed = 3;

        f1.rotate();
    }
}

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Importance of Object Modelling

Object modelling helps in:

• Understanding real-world problems
• Organizing programs properly
• Making code reusable
• Reducing complexity
• Easy debugging

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Difference Between Entity and Object

Entity	Object
Real-world thing	Program representation
Physical or logical	Software model
Example: Car	Object of class Car

What is a Method in Java?

A method is a block of code that performs a specific task when it is called.

👉 Simply:
A method is a function inside a class.

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Real-Life Example

Think of a method like a machine button:

• Press button → Machine works

• Call method → Code runs

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Why Do We Use Methods?

Methods help to:

• Reduce repetition of code
• Make program easy to understand
• Organize code properly
• Save time and effort

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Parts of a Method

• Every method has: Method Name
• Name of the function.
• Return Type
• Type of value returned.
• Parameters (Optional)
• Values passed to method.

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Example:

void greet(String name)

• void → return type

• greet → method name

• name → parameter

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Creating Objects in BlueJ (Step-by-Step)

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Step 1: Write the Class

Example:

class Student
{
    void show()
    {
        System.out.println("Welcome to BlueJ");
    }
}

Step 2: Compile the Class

Click Compile button in BlueJ.

Step 3: Create Object

Right-click class → Select new Student() → Press OK.

An object box appears on screen.

Step 4: Call Method

Right-click object → Select method → Output appears.

Program: Method and Object Creation

class Student
{
    String name;

    void input(String n)
    {
        name = n;
    }

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

In BlueJ:

Create object → Call:

input("Rahul")
display()

Output:

Name: Rahul

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Important Points to Remember

• Method must be inside a class
• Object is needed to call non-static methods
• Method name should be meaningful
• Always compile before creating object

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Common Errors Students Make

• Forgetting to compile
• Writing method outside class
• Not creating object before calling method

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ICSE Exam Important Questions

❓Very Short Questions

1. Define a method.

2. What is an object?

3. What does void mean?

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 Short Answer Questions

Q. Why do we create objects in Java?
Ans: Objects are created to access class variables and methods.

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Practical Question

Q. Write steps to create object in BlueJ.

👉 Write:

• Compile class

• Right-click class

• Select new

• Call method

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Quick Revision Trick

👉 Remember:

Method = Action
Object = User of Action

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Read also:

• JVM,JRE and BlueJ