Classes and objects in Java.

 

In Java, Classes and Objects are the fundamental building blocks of Object-Oriented Programming (OOP). Think of it like the relationship between a blueprint and a house.

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1. What is a Class?

A Class is a template or a "blueprint" for creating objects. It defines what data the object will hold and what it can do. It doesn't occupy memory space itself; it just describes the structure.

A class consists of two main components:

• Fields (Attributes): Variables that represent the state/properties (e.g., color, model).

• Methods (Behaviors): Functions that represent what the object can do (e.g., accelerate, brake).

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2. What is an Object?

An Object is an "instance" of a class. When you create an object, you are bringing that blueprint to life. Unlike a class, an object occupies memory.

You can create multiple objects from a single class. For example, if the class is Car, the objects could be myTesla and yourFord.

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3. The Code Example

Here is how you define a class and then create an object from it in Java:

Java

// The Blueprint (Class)

class Dog {

String breed;

int age;

void bark() {

System.out.println("Woof! Woof!");

}

}

public class Main {

public static void main(String[] args) {

// Creating an Object (Instance)

Dog myDog = new Dog();

// Accessing attributes

myDog.breed = "Golden Retriever";

myDog.age = 3;

// Calling a method

myDog.bark();

}

}

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4. Key Differences

Feature	Class	Object
Definition	A template for creating objects.	An instance of a class.
Memory	Does not allocate memory when defined.	Allocates memory when created.
Existence	Logical entity.	Physical entity.
How many?	Declared only once.	Multiple objects can be created from one class.

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5. The new Keyword

To create an object in Java, we use the new keyword. It performs three vital steps:

1. Declaration: Dog myDog associates a variable name with an object type.

2. Instantiation: The new keyword creates the object.

3. Initialization: The new keyword is followed by a call to a Constructor, which sets up the initial state of the object.