2.1 Defining User-Defined Classes

Introduction

In the real world, we deal with "objects" every day: a car, a dog, a smartphone, a book. Each of these objects has characteristics (such as color, size, or model) and can perform actions (such as moving, barking, or displaying information). In Object-Oriented Programming (OOP), we model these real-world objects in software.

A class in Java serves as a blueprint or template for creating objects. It does not represent a specific object but defines the common properties (data/attributes) and behaviors (methods/functions) that all objects of that type possess. Once a class is defined, you can create multiple instances (objects) from that blueprint, each with its own unique attribute values.

Explanation

A class in Java is a user-defined data type that encapsulates fields (variables) and methods (functions) together as a single unit.

·         Blueprint: Just as an architectural blueprint defines the design of a house (number of rooms, structure, etc.) without being an actual house, a class defines the structure of an object.

·         Object: The actual house built from that blueprint is an object. Multiple houses (objects) can be built from one blueprint (class), each with unique details.

Each object has its own copy of the attributes defined in the class (unless declared static), and they share the methods to perform actions on their specific data.

 

Key Components of a Class

·         Fields (Attributes/Instance Variables): Variables declared within a class that define the characteristics or state of an object.

·         Methods: Functions or procedures that define the behaviors or actions an object can perform.

 

Syntax for Defining a Class

 // Syntax for Defining a Class

[access_modifier] class ClassName {

    // Fields (Variables or Attributes)

    dataType fieldName1;

    dataType fieldName2;

    // ... add more fields as needed

    // Methods (Functions or Behaviors)

    [access_modifier] returnType methodName1([parameters]) {

        // Code to perform a task

    }

    [access_modifier] returnType methodName2([parameters]) {

        // Code to perform another task

    }

    // You can add more methods as required

}

·         access_modifier: Controls the visibility (e.g., public, private, protected, or default).

·         class: Keyword to declare a class.

·         ClassName: By convention, class names start with an uppercase letter (e.g., MyClass, BankAccount).

 

Creating Objects

Once a class is defined, create objects using the new keyword:

ClassName objectName = new ClassName();

·         ClassName: The class to instantiate.

·         objectName: Reference variable holding the object.

·         new: Allocates memory for the object.

·         ClassName(): Calls the constructor.

 

Accessing Class Members

objectName.fieldName;      // Access a field

objectName.methodName();   // Call a method

 

 // Dog.java

public class Dog {

    // Fields

    String name;

    String breed;

    // Methods

    // Makes the dog bark

    public void bark() {

        System.out.println(name + " says: Woof! Woof!");

    }

    // Simulates the dog eating

    public void eat() {

        System.out.println(name + " is eating its food.");

    }

    // Displays the dog's information

    public void displayInfo() {

        System.out.println("Name: " + name + ", Breed: " + breed);

    }

    // Main method to run the program

    public static void main(String[] args) {

        // Create first Dog object

        Dog myDog = new Dog();

        myDog.name = "Buddy";

        myDog.breed = "Golden Retriever";

        // Create second Dog object

        Dog anotherDog = new Dog();

        anotherDog.name = "Lucy";

        anotherDog.breed = "Labrador";

        // Display information for myDog

        System.out.println("--- Information for myDog ---");

        myDog.displayInfo();

        myDog.bark();

        myDog.eat();

        // Display information for anotherDog

        System.out.println("\n--- Information for anotherDog ---");

        anotherDog.displayInfo();

        anotherDog.bark();

        anotherDog.eat();

    }

}

Explanation

·         The Dog class has two fields: name and breed.

·         It defines three methods: bark(), eat(), and displayInfo().

·         Two objects (myDog and anotherDog) are created, each with its own state.

 

Example 2: Defining a Car Class

 // Car.java

public class Car {

    // Fields

    String make;

    String model;

    int year;

    boolean isEngineOn;

    // Methods

    // Starts the car engine

    public void start() {

        if (!isEngineOn) {

            isEngineOn = true;

            System.out.println(make + " " + model + " engine started.");

        } else {

            System.out.println(make + " " + model + " engine is already running.");

        }

    }

    // Stops the car engine

    public void stop() {

        if (isEngineOn) {

            isEngineOn = false;

            System.out.println(make + " " + model + " engine stopped.");

        } else {

            System.out.println(make + " " + model + " engine is already off.");

        }

    }

    // Drives the car for a given distance if engine is on

    public void drive(int distance) {

        if (isEngineOn) {

            System.out.println(make + " " + model + " is driving " + distance + " miles.");

        } else {

            System.out.println(make + " " + model + " cannot drive, engine is off.");

        }

    }

    // Displays car information

    public void displayCarInfo() {

        System.out.println("Make: " + make + ", Model: " + model + ", Year: " + year + ", Engine On: " + isEngineOn);

    }

    // Main method to test the Car class

    public static void main(String[] args) {

        // Creating first Car object

        Car myCar = new Car();

        myCar.make = "Toyota";

        myCar.model = "Camry";

        myCar.year = 2022;

        // Creating second Car object

        Car friendsCar = new Car();

        friendsCar.make = "Honda";

        friendsCar.model = "Civic";

        friendsCar.year = 2020;

        // Actions for myCar

        System.out.println("--- My Car Actions ---");

        myCar.displayCarInfo();

        myCar.start();

        myCar.drive(50);

        myCar.stop();

        // Actions for friend’s car

        System.out.println("\n— Friend’s Car Actions —");

        friendsCar.displayCarInfo();

        friendsCar.drive(30);   // Engine is off

        friendsCar.start();

        friendsCar.drive(30);   // Engine is on

        friendsCar.stop();

    }

}