Zsolt Kacsandi

Hello One

The desire for better code usually leads people to discover the term “clean code”. It was most probably coined by Robert C. “Uncle Bob” Martin, who wrote a book with the same title. You might want to give it a read, although, I find it to be very wordy. The book covers a few underlying principles that should help you write modular code in such a way that you can later reuse those modules. In this series we are going to cover his principles and ideas, as well as those of some other authors of the clean code movement.

You might have noticed, I used the name “module” and not “class” or “object”. That’s because clean code is not specific to object-oriented programming. You can use clean code principles with any programming paradigm you like, for example, classic procedural programming.

So let’s get started with the first topic on our agenda: when is a good time to split up your modules?


When writing your code, you have to segment it somehow. The unit of organization can be classes or modules, depending on your programming paradigm. The general idea is that one piece of code should only deal with a single responsibility. In other words, do one thing and do it well.

But what is a responsibility? Let’s take a class that, for example, deals with students of a school. This class will hold the data for exactly one student. Just like this:

class Student {
  private string name;
  public void setName(string name) {
    this.name = name;
  public string getName() {
    return this.name;

Obviously, you need to save the data somewhere, for example, in a database. The question presents itself: is it a good idea to implement a save() method that stores this student record? After all, this would be terribly convenient:

Student joe = new Student();

Easy, right? Well, not so fast. Imagine the following situation. You implement this class with MySQL in mind, which is a fairly standard database engine in the web world. One faithful day, your boss comes to you and tells you that the system administrators have been complaining, the servers are overloaded. After a brief hunt you discover that your students table is insanely large and slow, so you now decide to implement caching for your student data. The data is read from MySQL and written, for example, into Memcache.

So now your Student class needs to know about both MySQL and Memcache. By now your simple Student class that was only supposed to give you easy access to the student data has grown to a considerable size and now presents a maintenance problem. There’s a lot of code which you can’t even test. But hey, such is life, right?

The following week, déjà vu, your boss is at your desk again. The sysadmins are complaining again. (Can’t they just buy more hardware? Come on.) Now it’s your courses table that’s causing problems. You decide to go the same route and copy over the code for Memcache to your Courses class.

Yes, yes, I can hear you scream that you would never do that. You would always decide to refactor your code to avoid duplication. But believe me, others won’t. Unless you work alone, you will have to deal with people who have a higher tolerance for duplicated code.

So how can we avoid this situation? How can we make sure this doesn’t happen, even if it isn’t you editing the code? The answer lies in the word responsibility. With our save() method we have made a mistake. We have put more than one responsibility into one class. The Student class is responsible for both holding the student record and saving it to the database.

In the words of the great Uncle Bob, a class has a single responsibility if it has one and only one reason for change. The previous example is clear: there are two responsibilities, storing/retrieving the data and the data structure itself. These should be decoupled so we can change them independently.

So how do we fix this? Remember, we said that we wanted to have one class to have only one responsibility. So let’s split it in two. Let’s keep the original Student class as it is and create a second class for storing and retrieving the Student object.

class MySQLStudentStorage {
  public Student getById(int id) {
  public void store(Student Student) {

Of course, this is easy when you have practically no code yet. But what do you do if you already have a ton of code that relies on your save() function? Well, you’re in a tough situation, and there is no perfect solution.

You could, for example, proxy through calls from the Student class like this:

class Student {
  public void save() {
    storage = new MySQLStudentStorage();

Using this proxy solution will help you greatly because you can rewrite your code one module at a time and you don’t need to push a huge change to your production environment. The proxy strategy can be used in almost all situations when you need to split up a class.


Let’s look at a different example. Imagine a system that handles financial data. You know, the boring stuff. You will most probably have some sort of database, and you will have to provide some reports to the people working with it. These reports will be either Excel tables or CSV files of some description.

One such report could be the monthly income/expense sheet. Initially you built this sheet for the finance people so they can run their fancy reporting software on the data you provide. One day, much to your surprise, the CEO comes into your room and asks you to change one column of the report. She wants you to format the numbers nicely so it’s easier for her to read. Naturally you want to keep your CEO happy, so you oblige and change that one column.

All’s well, until at the end of the month the finance people try to use the report again and import it into their software. All hell breaks loose. They want the report fixed. I don’t know if you have met finance people, but if they say they want their report fixed, it means they want it fixed now. After all, it’s the end of the month and they need to deliver their own reports and nobody will give a damn if they are late because of you.

What do you do? Obviously, you revert the change. Then you write an apologetic letter to the CEO, which definitely won’t look good on your score card the next time you want a raise. Or you could just take the time and refactor your code to provide two separate reports instead of one and sell it to the CEO as a feature. Needless to say, you have to spend some time on it, but having a dedicated CEO report is much better than having a black mark on your record. Also, the next time the CEO asks you to change something, you will have the freedom to do so because you have made sure that one module is only responsible for one thing, only has one reason for change.


As you can see, the single responsibility principle outlined in this article very much aligns with business interests. It is in the long term interest of a company to have you writing code that follows this idea. Squeezing in the change into a class where it doesn’t belong will result in larger and larger classes and cause huge maintenance headaches. Having convoluted code also means that any change you need to make will take months instead of days, because you have to untangle the mess that has organically evolved in your code.

Up next

Awesome! So we have our responsibilities neatly split up, every class or module has only one thing to do. But what happens if you need to replace a module? Replacing class names gets old pretty fast, so we need a better solution. But that’s a topic for another day.