Refactoring: This class is too large

An example of refactoring from a real (flawed) code base.

In this article I walk through a set of refactorings from a real code base. This is not intended to demonstrate perfection, but it does represent reality.

14 April 2020



This is a story about refactoring. It's the third item in the TDD red-green-refactor cycle[1] and it's the thing we do all the time, right? Except when we don't.

I have an unruly code base which has suffered from refactoring neglect, so I've been bringing it back into line. In this article I will take a class that is too large, and make it smaller.

Outline of the problem

The story begins with a boring domestic chore. I've written some personal accounting software - Reconciliate. It works on the command line and performs the following actions:

  1. Loads my own comma-separated data:
    • My record of recent bank and credit card transactions.
    • My predicted monthly and annual transactions (based on data in a central spreadsheet).
    • Any unreconciled previously-recorded data.
  2. Loads third-party comma-separated data (from bank and credit card companies).
  3. Reconciles the third-party data against my own data.
  4. Writes everything back to a central spreadsheet.

I have some bugs that need fixing and some features to add. But my typical workflow is to arrive at the laptop late at night after putting my youngest to bed, with only a small amount of time before it will be my bedtime, and often a few weeks after the last time I saw the code. In these circumstances it's horribly easy to think things like, "Well I know this code is a mess, but I don't have time to get my head round it and fix it now..."

Clearly this is not ideal.

There's one class in particular - the ReconciliationIntro class - which is giving me a headache whenever I look at it. It's bloated and convoluted and impossible to fit in my head [2]. This has created a nasty feedback loop: "Because this code is so hard to reason about, refactoring will take more time and energy than I have, so I'll put up with it even longer - even though it means I can't even make small changes any more because it takes so long for me to understand the code's current state and decide where the change should go."

For instance, I want to add the ability to handle another credit card. In a lot of places I've used polymorphism and the strategy pattern to keep the unique behaviour of each credit card neatly encapsulated. But the ReconciliationIntro class is one place where, because of the poor design of the code and the lack of clear context separation, if I add another credit card I'll be making an already bloated class even worse. It contains four duplicated code paths for each of four types of data (Bank In, Bank Out, Credit Card 1 and Credit Card 2), and if I add Credit Card 3 right now I'll end up following the same anti-pattern.

My overall aim is to replace this code with more generic code, via the strategy pattern [3]. But there are four problems standing in my way:

  1. This one large class, (ReconciliationIntro), is taking too much responsibility.
  2. There's a lot of private nested code, which is hard to unit-test because it has no public interface.
  3. Within ReconciliationIntro, there's one large method that's doing too much.
  4. There are several methods in ReconciliationIntro which use the same repeated pattern but with different details.

I plan to fix all of these problems, in the order listed above. This preparatory refactoring will get me to the point where I can easily encapsulate the behaviour of each credit card / account. As Kent Beck says, "Make the change easy, then make the easy change."

This article is about tackling the first problem in the above list: This class is too large.

Why refactor?

I can't easily reason about the ReconciliationIntro class because it has too many responsibilities. It was originally designed as the entrance hall to the software, and all it did was display a few messages and then instantiate the classes that do the main work. But over time a lot of other code has snuck in. I want to make it easier to add another credit card, so I'm going to start by breaking this large class into smaller classes.

The benefits will be several:

  • By moving groups of methods into separate classes I'll create clear contexts and minimise tight coupling. This means that:
    • I'll know where to go when I want to make changes or fix issues.
    • When I amend the code, I'll only have to work with small isolated sections that fit in my head [2] - I won't have to understand the whole system.
    • Any manipulation of state will only happen locally in a small context - so I won't have to worry about one area of the system changing state in another.

How do I do it?

To my mind, the most important principle when refactoring (and when writing new code) is to move in tiny steps. I have several connected aims:

  • At every step, I want the code to compile and the tests to run.
  • If a change causes any tests to fail, I want to be able to fix them instantly. By making small changes and small commits, I can see which change caused the tests to fail and I only need to rewind / examine a small amount of code to find the problem.
  • I can keep track of where I am in in my head. It's horribly easy to embark on a relatively simple refactor, only to find it has repercussions beyond your original intention. At this point, if you're not in the habit of keeping the code compiling and the tests passing at every step, you can find yourself lost in a rabbit hole that's difficult to exit: Your code isn't compiling and your tests aren't even running, let alone passing.

For this to work, I need the code being refactored to be covered by tests. This has already been done before I start refactoring, although it's worth noting that one of my aims in a future refactoring will be to make the code even more testable.

Refactoring, by Martin Fowler is a recommended further read and lays out some basic refactoring principles. He also emphasises the value of moving in tiny steps and building the code / running your tests after every small commit.

Beyond these basic principles, I'll aim to follow a logical series of steps which are outlined below.

1. Rearrange methods

I'll start by using regions to rearrange all the methods in the ReconciliationIntro class into sensible groupings (commit f2d9932) [4]:

Figure 1: ReconciliationIntro after methods rearranged into regions

Now that I've grouped my methods into what feels like a reasonable set of contexts, I'd like to pull these out into separate classes. But where to start? The file loading code contains the most duplication, and is causing the most pain. Ultimately I want to make this code more generic, but first I want to pull it out so I can see it without distraction. I'll extract a new FileLoader class. The other groupings will also become separate classes but they'll be a lot simpler, so I'll focus most of this article on the file-loading code.

2. Analyse relationships between file-loading methods

So far all I've done is move some code around in the same class. I rebuilt the code and ran all the tests before I committed, but unless I was clumsy I'd expect my previous commit to be trivial. I need to be a little more thoughtful about what I do next.

I've used one of my regions to identify methods to pull out into a new FileLoader class, but how can I be sure this will work? Are there any hidden dependencies? I'll discover this by drawing out the relationships between the methods I want to move. The ones to be moved are marked in blue.

Figure 3: File-loading methods to be moved (abbreviations)

I can see straight away that it's not a strictly self-contained context: Some of the blue methods are calling back out to methods (shown in black and green) that I want to keep in a separate class. There are a couple of ways I could handle this, which I discuss below. But now I'm at a place where I can define a plan of action.

Plan of action

By the time I'm done, the original large ReconciliationIntro class will have been broken down into the pared-down version plus five new classes shown in the diagram below. Note that there isn't a one-to-one mapping between regions and classes, because later on when I reach step 7, I'll end up refining my first groupings into some finer-grained boundaries.

Figure 4: Plan of action

My overall aim is to break this large class down into smaller classes. In steps 1 and 2 above, I used regions and a diagram to identify distinct areas of code and then analyse relationships between some of the methods. That gave me enough information to start thinking about how I can tackle this job in tiny steps.

The resulting plan is summarised below and then described in detail further down. I reached this plan by thinking about how I could proceed using steps that were as small as possible. Often what I'm doing is setting things up so that the next change will be as small and simple as possible. I'm making small changes to facilitate more small changes - yet again following Kent Beck's dictum to "Make the change easy, then make the easy change."

Depending on your circumstances you might not follow the same plan, but it's not a bad template if you're not sure how to proceed. Your priority is to set yourself up for safe incremental changes:

  1. Rearrange methods

    Group all the methods in the class into sensible groupings, as described above.

  2. Analyse relationships between file-loading methods

    Identify how the file-loading code is connected to the rest of the code in the ReconciliationIntro class, as described above.

  3. Modify the methods that are staying behind

    There are methods which will stay in the parent class but are currently called by those that are moving. Some modification will be required to make this work.

  4. Create covering tests for the new FileLoader class

    The new FileLoader class needs to be covered by tests. The tests for the code to be moved already exist, but they'll be moved into a new FileLoaderTests class.

  5. Create new FileLoader class and move two methods

    Following the principle of tiny steps, I'll move just two methods (a public method and a private method called by it) before I move the rest.

  6. Move the other methods to the new FileLoader class

    This is where it gets exciting. All that file-loading code I'm interested in will finally get a new home!

  7. Extract more new classes

    Once I've dealt with the file-loading code, I'll create some more new classes for the other code regions.

The new FileLoader class will be responsible for loading various sources of comma-separated data and merging them ready for reconciliation. This file-loading code is where most of the pain currently lies, so that's the bit I'll describe in the most detail. You can see the original code before refactoring here, but if you follow that link all you'll learn is that it's too big to fit in your head! The slimmed down post-refactor version is here.

So, now I can continue with the plan:

3. Modify the methods that are staying behind

I've identified two methods - Set_path and Recursively_ask_for_budgeting_months - which are called by file-loading methods:

Figure 5: Methods called by file-loading methods (abbreviations)

As long as those methods are not too tightly coupled with the file-loading class, I can either

  • Make them public so I can call back to them from the new FileLoader class.
  • Move the calls out of the file-loading code and into other native ReconciliationIntro methods instead.

I'll use the first approach for Recursively_ask_for_budgeting_months and the second for Set_path, which will bring me to the following situation:

Figure 6: File-loading methods after moving (abbreviations)

Note that the methods marked as FileLoader methods in the diagram will still be in the ReconciliationIntro class at the end of this step, but this will enable me to move them into the FileLoader class, which will happen in step 5 and step 6 below.

Recursively_ask_for_budgeting_months will eventually be a public method in another class, but for now I just want to make sure I can call it from elsewhere. It turns out it already is public, which was done so that it could be tested. This in itself is a code smell - it's a sign that it would be better off as part of the public interface of a separate class.

The other method called from the file-loading code is Set_path. This changes the value of an internal path variable, so I'll choose option 2: I'll call it separately and pass the resulting data into the file-loading method via a parameter.

Note that these might not normally stay as separate commits (I could use micro-commits and then squash them into larger commits), but I'm keeping the small commits intact to make the steps clear. I compile and run the tests at every step:

  1. Modify the calling method (Create_pending_csvs) so that it takes a parameter, but initially give it a default value (commit acc3519) [4] so that the code still compiles:

    private void Create_pending_csvs()
    {
      // Some code
    }
    
    private void Create_pending_csvs(string path = "")
    {
      // Some code
    }
    
  2. Call Set_path separately before calling Create_pending_csvs. Take the resulting new _path member variable (see sidebar), and pass it into Create_pending_csvs (commit c5ebc2f) [4]:

    case "1":
    {
      Create_pending_csvs();
    }
    break;
    
    case "1":
    {
      Set_path();
      Create_pending_csvs(_path);
    }
    break;
    
  3. Remove the call to Set_path from within Create_pending_csvs, and use the passed-in value instead of the member variable (commit 6df8f97) [4]:

    private void Create_pending_csvs(string path = "")
    {
      try
      {
        Set_path();
        var pending_csv_file_creator = new PendingCsvFileCreator(_path);
    
    private void Create_pending_csvs(string path = "")
    {
      try
      {
        Set_path();
        var pending_csv_file_creator = new PendingCsvFileCreator(path);
    
  4. Finally, remove the default value from the Create_pending_csvs parameter - thereby forcing all clients to pass a value in (commit 2be56ea) [4]. Note that by doing things in this order, I keep the code compiling at all times:

    private void Create_pending_csvs(string path = "")
    {
      // Some code
    }
    
    private void Create_pending_csvs(string path = "")
    {
      // Some code
    }
    

4. Create covering tests for the new FileLoader class

My first choice of method to move will be Bank_and_bank_out_ _Merge_bespoke_data_with_pending_file. And the first thing I want to do is copy any covering tests into a new test class for the about-to-be-created FileLoader class. There is already one test for this method - M_MergeBespokeDataWithPendingFile_ WillAddMostRecentCredCardDirectDebits - and its job is to make sure this method merges new direct debit data correctly with a 'pending' file (which is being built to contain all new transaction data).

Note that I'm only moving tests, not writing new ones. People can get so used to the TDD [1] concept of writing tests before writing code that they assume you need to write new tests whenever you work on code. This is often not the case when refactoring. Ideally my functionality will already be covered by tests, and when refactoring I'm not changing the functionality. So instead of writing new tests, I'm using the existing ones to verify the functionality still works as originally intended.

This is a good time to review this test: It's a while since I wrote it, so I should be able to spot quickly whether it makes sense. I want my tests to be clear and easy to read - they should act as documentation for my system behaviour. The first thing I notice is that it contains an assert method - Assert_direct_debit_details_are_correct - whose name is inadequate. What is the definition of "correct"? I rewrite the test to make it easier to read, which involves quite a lot of changes. In the interest of keeping this a digestible read I won't go into the changes made, but you can view them in commit f090f26 and commit 6a6cece. [4]

Now that I've refactored the test I'll copy it into a new test class, along with some associated private helper methods. Note that although this and other tests are destined to operate on a new FileLoader class, they'll still act on the old ReconciliationIntro class until I'm sure my new test class has everything it needs. Also note that until everything is safely moved, my test code is duplicated.

I first create the new test class in the same file as the original (commit 491c795) [4], so that it's easy to see what I'm copying. Then I can get Resharper [5] and Visual Studio to move everything into a new file for me (commit c9317c0) [4].

Figure 7: Move BBO tests

5. Create new FileLoader class and move two methods

Now that my test class is up and running, I can create the new FileLoader class.

The method that's furthest down the chain - the lowest leaf in my tree of methods - is Bank_and_bank_out__ Add_most_recent_credit_card_direct_debits. This is a private method with no independent tests (it's tested via the public calling method), so I'm going to move both it and its caller (Bank_and_bank_out__ Merge_bespoke_data_with_pending_file). These will be the first two methods to be moved into my new class.

Again, I'll move in baby steps to avoid my tests going red and keep my code building at all times. After each of the following steps I make sure the code builds and the tests are passing.

  1. This is the situation I begin with. A FileLoaderTests class has been created, but it is testing code that still lives in the ReconciliationIntro class:

    Figure 8: New FileLoader class part 1 (abbreviations)

  2. I start by creating a new FileLoader class. One of my file-loading methods (Bank_and_bank_out__ Add_most_recent_credit_card_direct_debits) is private, and will also be private at the end of this process. It will only ever be called by the method that's about to follow it into the new class. It's not individually covered by tests, so I can just copy it over to the new class and have it ready and waiting when its caller is moved (see commit 0341476) [4]. But I will need to make it temporarily public:

    Figure 9: New FileLoader class part 2 (abbreviations)

  3. Now I can create an instance of my new FileLoader class in the ReconciliationIntro class and call its new public method, instead of the old private method. I can also delete the old private method (see commit bde2ae2) [4]:

    Figure 10: New FileLoader class part 3 (abbreviations)

  4. Copy the original caller into the new class (see commit f0a5a59) [4]. Note that at this point it's duplicated:

    Figure 11: New FileLoader class part 4 (abbreviations)

  5. Change the object I'm testing from being a ReconciliationIntro instance to being a new FileLoader instance. Point the tests at the new copy of the original caller. Note that because the private method it calls has also been copied, my tests will pass (see commit 3d573e3) [4]:

    Figure 12: New FileLoader class part 5 (abbreviations)

  6. Now I can call the original caller directly from the ReconciliationIntro class (see commit 77c0b14) [4]:

    Figure 13: New FileLoader class part 6 (abbreviations)

  7. Make the originally-private method private again, and delete the original caller. I'll delete the old test class too, as all its tests have now been duplicated in FileLoaderTests. (see commit 27f1a59) [4]:

    Figure 14: New FileLoader class part 7 (abbreviations)

6. Move the other methods to the new FileLoader class

Now I can move all the other methods. I'll handle them one by one, starting with the simplest and keeping an eye out for dependencies (between methods, and on any member data). I'll need to consider the following:

  • Do I want to rename any methods?
  • Do I want to replace any parameter lists with new objects?
  • Are there any redundant parameters?
  • Are any of the internal nested callees altering state? What impact does this have?

I could inline those lower down in the chain before moving them, but if I did I'd break the tests which call them as public methods. So instead I move them on their own. I do it in the order explained below, acting on one at a time and starting with those at the end of the chain - ie the outermost leaves on the following tree:

Figure 15: FileLoader method tree (abbreviations)

For each one, I use the following approach:

  • Create a copy of the method in the destination class, keeping the original in place. Make the new method public.
  • Call the new method instead of the original.
  • Copy any covering tests into the new test class, and make sure they test the new code.
  • Delete the old method and the old tests.

I already moved Bank_and_bank_out__ Merge_bespoke_data_with_pending_file and Bank_and_bank_out__ Add_most_recent_credit_card_direct_debits (commit 0341476 to commit 27f1a59), so now I repeat the same actions for each of the other methods (commit 7cd53f6 to commit 7ab95f2) [4]. I don't commit every tiny step this time, but I still go through the same set of steps. After each step, I make sure the code builds and the tests are passing (apart from when I deliberately make a test fail).

I refactored some tests earlier, and I can repeat those changes for tests that follow the same pattern. For some methods the move is very simple, because they have no test coverage. This is part of why I'm doing this refactor, to make that code more easily testable.

7. Extract more new classes

When adding regions at the start, I identified some Get budgeting months functionality which creates its own clear context, so I extract these methods out into a new BudgetingMonthService class. This is very quick and simple because these methods only have one public entry point (see commit 6103f0b) [4].

ReconciliationIntro is still too big, but the methods all call one another and now that I've spent more time refamiliarising myself with the code I'm not convinced my two remaining regions of User Instructions and Input and Debug Spreadsheet Operations are the best ways of dividing the remaining code. To help myself think about it, I use a spreadsheet to quickly illustrate the call hierarchy.

Figure 16: Call hierarchy

This allows me to see that there are THREE self-contained areas of code, not two: User instructions, Gathering file / path info and Debug mode switching code.

I remove the original regions (commit 4c57927) and replace them with four new regions (commit 3446a54) [4]. I rearrange the methods to fit in the new regions, and these will now translate into three new classes: Communicator, PathSetter and DebugModeSwitcher (FileLoader and BudgetingMonthService are not shown on this diagram because they have already been extracted):

Figure 17: Identifying final ReconciliationIntro classes

I create these new classes gradually and safely using the same principles as for BudgetingMonthService, removing the new regions once they've served their purpose (see commit 7f464a4 to commit 7e118c1).

It's worth noting that when extracting new classes from a larger class, I want them to be independent. For this reason I'm deliberately avoiding the anti-pattern which can sometimes arise, where extracted classes are automatically injected as a dependency into the constructor of the original class.

The PathSetter class turns out to be non-trivial - see commit 2921220 to commit 398539a[4]. This is due to the currently-slightly-tortuous nature of the path-setting code, something I noticed at the end of step 3. By extracting this code into a separate class and giving it its own clear context, I'm already making this code a bit better - but it will still need some attention.

Finally, my ReconciliationIntro class is cleaner and simpler, and the original 41 methods have been reduced to three: Start, Reconciliate and Do_matching:

Figure 18: Final ReconciliationIntro class

Recap

My class was too large. I had developed undesirable coding habits and I wanted to stop and make things better before adding any new functionality.

To fix my too-large class, I took the following actions:

I moved in tiny steps, and I compiled and ran the tests at every step. Now I have a much smaller class which calls functionality from several other also-smaller classes, and each one of them is easier to fit in my head.

What's next?

Note that this article ends in the middle of the refactor, so if you want to see the relevant state of the code before moving on to the next steps, you need to checkout the commit 6103f0b. Note also that I completed most of step 7 in this article after that commit (explanation here).

At commit 6103f0b, I've pulled the file-loading code out into the FileLoader class and have a clearer view of the main challenge. The following four methods (visible in situ here) are clearly problematic:

  • Load_bank_and_bank_in
  • Load_bank_and_bank_out
  • Load_cred_card1_and_cred_card1_in_out
  • Load_cred_card2_and_cred_card2_in_out

They have the following problems:

  • There is a lot of duplication - at a glance, they look identical.
  • They are way too long.
  • They create objects internally and pass them into one another in a closely inter-dependent way which is not testable.

These are all problems I want to solve, but before I do any more refactoring I really need some tests around those methods. That's what I'll do next. My desire is to write about it in a follow-up article, but I've learnt better than to make promises about that kind of thing, so for now it's an exercise for the reader.


Acknowledgements

Many thanks to the following people, who very kindly read early drafts of this article and contributed invaluable feedback and suggestions: Paula Paul, Martin Fowler, Priti Biyani, Riccardo Novaglia, Dan Terhorst-North, Kevlin Henney, Steve Freeman, Jon Skeet, Sal Freudenberg, Joe Ray, Chris Shepherd, Luke Morton, Scott Giminiani, Richard Foster, Marcos Bezerra, Sam Carrington.

Footnotes

1: TDD stands for Test-Driven Development, a technique to ensure that all units of code are tested, and that the tests describe the behaviour of the system. This is done by writing the tests before you write the code to make those tests pass. There's a lot more that can be said about it, but it's not the focus of this article. You can read more about it here.

2: "Fits In My Head" is one of the patterns in Dan Terhorst-North's Software, Faster book (a work still in progress). He talks about the importance of being able to comprehend any chunk of code by "fitting it in your head." The name came from James Lewis, and Dan describes it in this talk, but Dan tells me the concept may have originated with Alistair Jones.

3: Refactoring to the strategy pattern: A large part of the aim of this refactoring is to enable use of the strategy pattern. The business of "refactoring to patterns" has a whole book devoted to it, by Joshua Kerievsky - and is worth a read if you want to know more. Indeed as Martin Fowler says, "Many people have said they find a refactoring approach to be a better way of learning about patterns, because you see in gradual stages the interplay of problem and solution."

4: Commit links: Don't feel obligated to follow the commit links! More on that in the How to read this article sidebar.

5: Resharper is a commonly-used Visual Studio extension used for things such as code editing. However it is not free, and is being increasingly eclipsed by native Visual Studio tooling.

Significant Revisions

14 April 2020: first published