define class

ex:

class Rabbit

    def  initialize( name)

        @name = name

    end

    def run 

        puts “run”

    end

end

initialize:

this method is called when the object is created

this is a private method. Hence, when using new to create object, the new method will call initialize 

instance variable:

the variable that starts with @ is instance variable

naming convention:

class name:  CuteRabbit

file name : cute_rabbit.rb

private:

the method is private

define methods after private

ex:

private  

def  test

end

protected:

the method is protected

modify existing class

ex:  the Array is built-in class.

class Array

    def test

    end

end

now the Array has new method test

class variable:

the variable that starts with @@ is class variable

class method:

ex:

def self.test

end

inheritance:

ex:

class Circle  < Shape

end

if class Square is defined inside class Shape, you must access Square with

Shape::Square outside class Shape

ex:

class Test  < Shape::Square

end

constant in the class

if TEST is the constant of class Rabbit,

we can access Test using Rabbit::Test