The benefits of stateless objects

For my first technical post, I'm gonna talk about a too common bad practice in programming: stateful objects. This is not a simple error only novice programmers do, it's a very common error even amongst professional programmers.

Consider the following example:

class Stateful
   def calculate(x)
       @result = x * 2
   end

   def display
       puts "The value is: " + @result.to_s
   end
end

s = Stateful.new
s.calculate(10)
s.display

In this case, the user of the class needs to call the calculate method before the display method. This implies a knowledge of the inner working of the class by its client. This is bad. If you make the API of your class impossible to misuse, then nobody is going to misuse it. This includes you!

This is a simple example and one might think that the usage of the class is pretty obvious. But in medium size software, there are often hundreds or thousands of these nasty stateful classes. Most of them have no need for a state. This increases the complexity of the code and makes collaboration between developers harder.

A better implementation would be:

class Stateless
   def calculate(x)
       x * 2
   end

   def display(x)
       puts "The value is: " + x.to_s
   end
end

s = Stateless.new
x = s.calculate 10
s.display x

There is now no possibility of misusing this class.

Of course, stateful object have their use too! But you should only add a state to your application when you have to.