Refactor Code : Separate Query from Modifier

In this post we will learn about the one of the important code refactoring algorithm "Separate Query from Modifier".When you have a method that gives you a value and has no observable side effects, you have a lot less to worry. You can move the call to other places in this method freely. A good rule to follow is to say that any method that returns a value should not have other observable side effects. 

You have a method that returns a value but also changes the state of an object . Create two method, one for the query and one for the modification.

We first take the example of the below and will find out what is wrong with this code and refactor it.


In the above , method findMiscreant(List<Person> people) signifies that its responsibility will be to find miscreant only. But if we give clear look to the method , apart from finding Miscreant , it is sending some notification as  SendAlert() to other system .

Hence we can easily see that this method is doing two job as :

1) querying to find  miscreant
2) sending notification and may change state of the some object.

First principle of Single Responsibility  from S.O.L.I.D design principle  state that "every class or method should do only one job".

We can break the findMiscreant(List<Person> people)  into two method as SendMiscreantNotification(List<Person> people)  and  findMiscreant(List<Person> people) itself
where responsibility of each one will be to send notification and find miscreant respectively.

The code after refactoring will be as below :



Reference :

https://www.csie.ntu.edu.tw/~r95004/Refactoring_improving_the_design_of_existing_code.pdf


It covers the "Separate Query from Modifier" concepts , if you have any issue or anything to add , please comment below .