Even though we haven't covered classes in Ruby, I thought it would be good to show you the differences between class and instance methods in since it's important to see the different behavior.

For right now you can ignore the class syntax and focus on the functionality, especially the way in which both method types are called.

I'm going to create a class and add two methods in it, the first being a class method and the second an instance method.

class Invoice
  # Class method
  def self.print_out
    "Printed out invoice"
  end

  # Instance method
  def convert_to_pdf
    "Converted to PDF"
  end   
end

If you notice, the only difference in syntax is that I use the world self, for class methods, and the name by itself for instance methods.

Now, when I want to call the class method, I'm able to declare the name of the class followed by a . and then the method name, like so:

Invoice.print_out

Now, if I try to call the instance method with the same syntax, it throws an error:

If you notice, the error message says that convert_to_pdf is an undefined method. In order to run instance methods we need to create an instance of that class to run it.

I'll give you a hint of what we'll see in the OOP section and we'll instantiate a new Invoice like this:

i = Invoice.new
i.convert_to_pdf

When you run this method, it prints the message without an error.

Now, if I try to access my class method in the same manner that I called the instance method it will result an error, as shown here:

So, a class method can be called in conjunction with the name of the class whereas the instance method requires you to create an instance to call the method.

As a side note, I can shortcut my code a bit to call my instance method on the same line as the class name.

Invoice.new.convert_to_pdf

And this will work properly. However, typically it makes no sense to call a method this way because you're not going to be able to do much with it. So, the earlier way of accessing an instance method is better.

You may be wondering why you need an instance method at all when it's so much easier to call a class method.

Let's imagine that you have 15 methods in your class, you wouldn't want to call the class that many times in your program. For example:

class Invoice
  # 15 methods inside
end

Invoice.method_1
Invoice.method_2
Invoice.method_3
Invoice.method_4
Invoice.method_5
Invoice.method_6
Invoice.method_7
Invoice.method_8
Invoice.method_9
Invoice.method_10
Invoice.method_11
Invoice.method_12
Invoice.method_13
Invoice.method_14
Invoice.method_15

Calling the class every time doesn't look good and is considered bad programming practice. Essentially this is creating 15 new objects in the program.

It's a better practice to create an instance variable and call all the methods with it.

class Invoice
  # 15 methods inside
end

i = Invoice.new
i.method_1
i.method_2
i.method_3
i.method_4
i.method_5
i.method_6
i.method_7
i.method_8
i.method_9
i.method_10
i.method_11
i.method_12
i.method_13
i.method_14
i.method_15

This way you're not creating a new instance of the class every time, instead you're using the same instance for all methods.

I hope this gives you an idea of the different types of methods available in Ruby and how to call them.