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Hi Kishan

A method is attached to a specific instance of an Object class.  A Classmethod can be called without having to instantiate the object.

So, we could have a Method on a Person object to update address

do person.UpdateAddress("New address")

Whereas, for a ClassMethod, we could define a Classmethod to give us as object instance to then work on

set person = ##class(User.Person).CreatePerson("FirstName","LastName",Age)


Hope that helps?

Thank you Alex can you tell why we using Method and Class Method can't we just use a class method itself ?

Not sure if I understood what you meant, but I'll try to answer:

When using class methods you cannot reference your THIS instance. Have you every used Java, C# or whatever programming language that supports object oriented-programming paradigms? If so then class methods are the same as static methods.

Common (non-static) methods have the advantage of providing you the instance context. Which means that all properties are accessible inside this kind of method, even the private ones.

Class Sample.Person Extends %Persistent

Property Name As %String;

ClassMethod GetPersonName(instance As Sample.Person)
   return instance.Name // This works fine, GetPersonName is receiving an external Sample.Person.

// 'My' is used to empathize that this method refers to it's own instance name.
Method GetMyName() As %String 
   return ..Name // This also works fine.  Notice that this method doesn't receives the instance, it's because you can only call it from a instance already.

ClassMethod GetMyInstanceName() As %String
  return ...Name // This won't work. Since there's no context (instance). The compiler will even warn about it.

A class method is what other languages usually call a static method. One that can be called even without an instance of that class existing.

Let's imagine you have a class Circle. It may have its own radius in a property

set myCircle = ##class(Circle).%New(25)

write myCircle.radius // 25

write myCircle.getArea() // the calculated area based on its own radius

But now you want to quickly calculate the area of a circle with radius 5, but you don't want to instantiate a new Circle object just for that. So you could have a ClassMethod inside the Circle Class that does that for you

write Circle.calcRadius(5)

You could also smash lots of utility functions as ClassMethods into a Utils class.

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When you have just a Method you need to instantiate the class to run this method.
When you use ClassMethod  is a static method so you can call directly it's not necessary to 

Technically speaking class method is a function from class whereas method is a function from arguments defined inside the class