What is a Class and Object in Java?

What is a Class and Object in Java?

A class is a blue print to create an object.

Let us try to understand this by an example:

Let us say, we have a feedback form which needs to be filled by all the workers regarding a feedback of a product or application of their organization.

The feedback form may contain the below details

 So, the admin of the organization prepares this form and takes 100 photo copies of the form and distribute to all the 100 workers present in the organization. Once the employees/workers get the form, they fill in the details and return the forms to the admin.

Now the admin have the original form (master copy) from which he/she took the other 100 photo copies and also the 100 forms which are filled by the workers.

If we relate this example with Java, we can term the master copy(blue print) as a Class.

The other 100 forms which are replicated from the master copy are called as objects.

The name, id or any field will not be filled in the master copy so as to use that to take more photo copies in future. So, the master copy won’t be bearing any value to itself. But the photo copies that were taken and filled by the workers have values assigned to them.(name, id, address, feedback of diff workers)

In the same way in Java, a class is a blue print from which the objects can be created and each object have its own value.

If we want to create 1000 employees in java, we create an Employee class and then create 1000 employee objects and each object will have its own value. The object just uses the blue print/structure of the class.

As the Name, id and other fields in the form differ from employee to employee, they are called as variables in java.

Let us say, we need the company name to be printed on the form. Instead of asking the workers to fill the company name, the master copy can be edited such that it has the company name. So, once the photo copies are taken, all the copies will have the same value.

As we want this value to be same across multiple objects, we made the change at the class level and so these variables (like company name) are called as class variables.

The way the admin created the form, let us try to create a class in Java which represents the feedback form.

Class name can be ‘FeedBackForm’

-       As Name, Address, Feedback  are  alphanumeric, they should be String variables in Java

-       As ID is  numeric field, it should be  a Int variable in Java

-       A class is defined in a curly brace with the key word class.

The class of the FeedbackForm can be :

And this should be saved in a file with the name FeedBackForm.java (should be the name of the public class)

Now, as the admin of the organization took photo copies of the master copy and made objects, we will see how to do that photo copy in java.

We create a copy of the class by the following command.

FeedBackForm f1 = new FeedBackForm();

Where FeedBackForm is the name of the class, f1 is the name of the object.If we want to create 100 objects:

The FeedBackForm() you see in the object creation is called as constructor. A constructor is used to create a object in Java. We will know more about the constructors in other sections.