Lecture 62:-  Class and Objects in Java

In Java, a class is a blueprint or a template that defines the properties and behaviors of objects. An object, on the other hand, is an instance of a class, representing a specific entity with its own unique characteristics. Let's dive deeper into classes and objects in Java:

1. Class Declaration: To create a class in Java, you use the class keyword followed by the class name. The class can contain fields (attributes) and methods (behaviors). Here's the general syntax of a class declaration:

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   public class ClassName {    // Fields (attributes)    dataType fieldName1;    dataType fieldName2;    // ...    // Constructor(s)    public ClassName(parameters) {        // Constructor code    }    // Methods (behaviors)    returnType methodName1(parameters) {        // Method code    }    returnType methodName2(parameters) {        // Method code    }    // ... }

2. Creating Objects: Once you have defined a class, you can create objects (instances) of that class using the new keyword. The new keyword allocates memory for the object and calls the constructor to initialize its state. Here's how you create an object:

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   ClassName objectName = new ClassName(arguments);

3. Accessing Members: You can access the members (fields and methods) of an object using the dot (.) operator. For example, to access a field or call a method, you use the following syntax:

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   objectName.fieldName; objectName.methodName(arguments);

4. Constructor: A constructor is a special method that is used to initialize the state of an object when it is created. It has the same name as the class and does not have a return type. If you don't explicitly define a constructor, Java provides a default constructor with no arguments.
5. Instance Variables (Fields): Instance variables are fields declared within the class but outside of any method. They represent the state of the object and have unique values for each object instance.
6. Methods (Behaviors): Methods are functions declared within the class. They represent the behaviors or actions that objects of the class can perform.

Here's a simple example to illustrate classes and objects in Java:

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public class Car {    // Instance variables    String make;    String model;    int year;    // Constructor    public Car(String make, String model, int year) {        this.make = make;        this.model = model;        this.year = year;    }    // Method    public void start() {        System.out.println("Starting the " + make + " " + model);    }    public static void main(String[] args) {        // Creating objects        Car car1 = new Car("Toyota", "Camry", 2022);        Car car2 = new Car("Honda", "Accord", 2021);        // Accessing members        System.out.println(car1.make); // Output: Toyota        car2.start(); // Output: Starting the Honda Accord    } }

In this example, we have a Car class with three instance variables (make, model, and year), a constructor that takes arguments to initialize these variables, and a start method representing the behavior of starting the car.

In the main method, we create two Car objects (car1 and car2) and access their members using the dot operator. We also call the start method on car2, resulting in the output "Starting the Honda Accord".