Explain the terms "Class" and "Object" using examples.

What is the Class?

💓A class is an entity that determines how an object will behave and what the object will contain. In other words, it is a blueprint or a set of instruction to build a specific type of object.

Syntax

class <class_name>{
    field;
    method;
  }  

What is an Object?

💓An object is nothing but a self-contained component which consists of methods and properties to make a particular type of data useful. Object determines the behaviour of the class.

💔 When you send a message to an object, you are asking the object to invoke or execute one of its methods.

💓From a programming point of view, an object can be a data structure, a variable or a function. It has a memory location allocated. The object is designed as class hierarchies.

Syntax

ClassName ReferenceVariable = new ClassName();

 Difference Between Object & Class

💙A class is a blueprint or prototype that defines the variables and the methods (functions) common to all objects of a certain kind.

💙An object is a specimen of a class. Software objects are often used to model real-world objects you find in everyday life.

Understand the concept of Java Classes and Objects with an example.

💛Let's take an example of developing a pet management system, specially meant for dogs. You will need various information about dogs like different breeds of dogs, the age, size, etc.

💛You need to model real-life beings, i.e., dogs into software entities.



💛Moreover, the million-dollar question is, how you design such software? Here is the solution-

First, let's do an exercise.

💜You can see the picture of three different breeds of dogs below.



💜Stop here right now! List down the differences between them.

💜Some of the differences you might have listed out maybe breed, age, size, colour, etc. If you think for a minute, these differences are also some common characteristics shared by these dogs. These characteristics (breed, age, size, colour) can form data members for your object.



💜Next, list out the common behaviours of these dogs like sleep, sit, eat, etc. So these will be the actions of our software objects.


💜So far we have defined the following things,

💨Class - Dogs
💨Data members or objects- size, age, colour, breed, etc.
💨Methods- eat, sleep, sit and run.


💜Now, for different values of data members (breed size, age, and colour) in Java class, you will get different dog objects.



💜You can design any program using this OOPs approach.

💜While creating a class, one must follow the following principles.

💧Single Responsibility Principle (SRP)- A class should have only one reason to change
💧Open Closed Responsibility (OCP)- It should be able to extend any classes without modifying it
💧Liskov Substitution Responsibility (LSR)- Derived classes must be substitutable for their base classes
💧Dependency Inversion Principle (DIP)- Depend on abstraction and not on concretions
💧Interface Segregation Principle (ISP)- Prepare fine grained interfaces that are client-specific.

Example Code: Class and Object

// Class Declaration
public class Dog {
    // Instance Variables
    String breed;
    String size;
    int age;
    String color;


    // method 1
    public String getInfo() {
        return ("Breed is: "+breed+" Size is:"+size+" Age is:"+age+" color is: "+color);
    }
 

    public static void main(String[] args) {
        Dog maltese = new Dog();
        maltese.breed="Maltese";
        maltese.size="Small";
        maltese.age=2;
        maltese.color="white";
        System.out.println(maltese.getInfo());
    }
}

Output:

Breed is: Maltese Size is:Small Age is:2 color is: white
Object and Class Example: main outside class
In previous program, we are creating main() method inside the class. Now, we create classes and define main() method in another class. This is a better way than previous one.

// Class Declaration
class Dog {
    // Instance Variables
    String breed;
    String size;
    int age;
    String color;


    // method 1
    public String getInfo() {
        return ("Breed is: "+breed+" Size is:"+size+" Age is:"+age+" color is: "+color);
    }
}
public class Execute{
    public static void main(String[] args) {
        Dog maltese = new Dog();
        maltese.breed="Maltese";
        maltese.size="Small";
        maltese.age=2;
        maltese.color="white";
        System.out.println(maltese.getInfo());
    }
}

Output:

Breed is: Maltese Size is:Small Age is:2 color is: white