What is the difference between classes and object instances?

Question

A class is a binding of methods and variables in a single unit.
An object is an instance of a class.

These are definitions for classes and objects in programming books. My friend said that a class is a blueprint of an object. An object is a real "thing" of a blueprint. He has given an example: if Company is a class, INFOSYS, CTS, and TCS etc are objects. Whenever I think about classes and objects, these definitions confuse me. If channel is a class, what would be objects for the class? If water is a class, what would be objects of class? Can you clarify?

Answer 1

I'll give you a classic explanation, you can find different versions of this all over the place.

A class is like a blueprint. Say you want to build a car, the first thing you would need is a plan, This is the class. The plan will describe 'methods' such as brake and hoot. It will also describe the various components of the car. These are variables.

A car object is an instantiation of a car class. You can have lots of these for one car class.

For example:

class Car:
    def __init__(self,color):
        self.color = color

    def hoot(self):
        "do stuff"


red_car = Car('red')
red_car.hoot()
blue_car = Car('blue')
blue_car.hoot()

Now, depending on the language you are using classes themselves can be objects (this is the case in Python). Think of it this way: All blueprints have some stuff in common. That common stuff is described in the blueprint's class (which is in itself a blueprint).

Then on the point of 'water' as a class you can approach it in a few ways depending on what you want to do:

way 1:

rather have a class called Liquid with variables describing stuff like viscosity, smell, density, volume, etc. Water would be an instance of this. So would orange juice

way 2:

say you were putting together a game with a bunch of blocks that would be made up of different terrain. You could then have classes such as Grass, Water, Rock, etc (think Minecraft). Then you can have a water class instance (a water object) occupy a specific position on the map.

Answer 2

If channel is a class, Start Sports, BBC, and ESPN are its objects. If water is a Class, "Fresh Lime Water" and "Soup" are its objects.

Although you might find this explanation vague, this is the answer that I could think of.

Here is how you can learn about and distinguish classes:

Say you have a class "CAR"

Its objects are Hyundai, Ford, Suzuki. It will have the same methods but different designs -> this is how you can relate objects and classes with the real world.

Answer 3

A class specifies the behavior of its instances.

A class is an instance of a class too (class for a class is named a "metaclass").

A class is an abstraction. You find a class by finding generic properties applying to a group of objects.

Then a class is a template which defines the methods (behavior) and variables (state) to be included in a particular kind of object

Recognition of classes (outside classroom) requires experience.

Read anything from Alan Kay, he is the inventor of Object Technology, and one of the inventors of Smalltalk, the only pure objects environment as of today.