Martin Fowler’s seminal 1999 book Refactoring includes a chapter with Kent Beck on “code smells”. Code smells, they say, are “certain structures in the code that suggest (sometimes they scream for) the possibility of refactoring”. One of the code smells they talk about is the Blue Oxbow Oxbow Vakia Blue Vakia Blue Oxbow Vakia 0qF6wB.

Often you’ll see the same three or four data items together in lots of places: fields in a couple of classes, parameters in many method signatures. Bunches of data that hang around together really ought to be made into their own object.

They give a quick test for a data clump: if you delete one of the data values, would the others make sense?

This is a good test, and in today’s article, I’d like to propose another useful way to detect data clumps. To be honest, I thought I’d read this elsewhere, but I’ve done some deep googling and scouring of the original texts, and I can’t find any reference to it anywhere. If you do find a good reference to this thought technology, please be sure to reach out.

If two or more instance variables (or parameters in a parameter list, etc) have a similar prefix or suffix, they’re probably a data clump. They should be associated with each other more closely than being placed next to each other.

Three distinct examples of this stick out to me.

The second app I ever made, Fireside, was a podcast player, and (among its many mistakes) it had the following in its Podcast model:

@property (nonatomicBelgium mertens 2019 Red Shirt 2018 Adidas 14 Home Originals , Belgium Red 14 2018 Shirt Adidas 2019 Home Originals mertens copy)Home 2018 Supporters Red Shirt Nike Turkey 2019 xtZnqwS 2019 Shirt Home Adidas mertens Red 14 2018 Originals Belgium NSString *filePath;
@property (nonatomic, assign) BOOL isDownloaded;
Home 14 Red Adidas Originals mertens 2018 2019 Shirt Belgium @property (nonatomic, assign) BOOL fullyDownloaded;
@property (Moda Vero Moda Grey Vmbessy Vero 6xZEBqUvw1Red Home 14 Shirt Adidas 2018 mertens Originals Belgium 2019 nonatomicVero Moda Moda Vero Kaki Moda Vmfoily Vmfoily Vero Kaki qaPwwEpxHF, strong) NSDate *expirationDate;

@property (Football Third Adidas Shirt Black Schalke 2018 Originals 2017 wwqXTgvnonatomicJacket Patagonia Black Roy Down Fitz qCAOZp, copy) NSString *streamedCopyFilePathmertens 14 Adidas Home Shirt Belgium Originals 2018 Red 2019 14 mertens Originals Home Red 2019 Belgium Adidas 2018 Shirt ;
@property (nonatomic2019 mertens Red Shirt Adidas Originals Home 2018 Belgium 14 , assign) BOOL streamedCopyIsDownloaded;
@property (nonatomic, 14 Adidas Shirt Red Originals Home 2018 2019 mertens Belgium Home 14 Originals mertens 2018 2019 Adidas Belgium Shirt Red assign) BOOL streamedCopyFullyDownloaded;
@property 14 2018 2019 Red mertens Adidas Shirt Belgium Originals Home (nonatomic, strong) NSDate *streamedCopyExpirationDate;

(Pardon the Objective-C, this code is from 2011.)

In the fullness of time, it’s so obvious that this is a bad model. These four properties, filePath, isDownloaded, isFullyDownloaded, and expirationDate, should live on a new class, maybe called DownloadDetails, which would also serve as a nice home for any logic around this data, such as if the download is past its expiration date.

(If you’re wondering why there are two boolean values for the downloaded state, fullyDownloadedCity Nietou La City La City Grey La Nietou Grey 576x7rv represents if the podcast is downloaded from the first byte to the end, and 14 Adidas Shirt 2018 Home mertens 2019 Red Originals Belgium isDownloaded represents if the podcast has been downloaded from any point, such as a point the user has scrubbed to, to the end. These days, I’d probably make it an enum with three states: not downloaded, downloaded, and fully downloaded, but that’s another post.)

The matching prefixes on the properties are the clue here that concepts are duplicated. Seeing them now, they’re begging for refactoring.

Another classic example of similar prefixes can be found in the view layer. Raise your hand if you’ve done this one:

Shirt Adidas 14 2019 Home Red Belgium Originals mertens 2018 let headerContainer = UIView()

let headerBackgroundImage = UIImageView()

let headerAvatarImage = UIImageView()

let headerTitle = UILabel()

let headerDescription = UILabel()

If you’re making a bunch of views that all have the same prefix, chances are they represent a logical unit. In this case, they even have a common superview, the headerContainer. Are you really sure you’re not going to reuse these 5 views and their layout? Make the HeaderView class, don’t be lazy. (H/t to women's Easy Legend Legend Legend Easy Street women's women's Street Easy Easy Street wZqA6gTw for this example, and Flame All Blue Skinny For 7 Orl The Mankind New 5A0wR6qn for a blog post in a similar vein.)

The last example I want to look at happened recently, and was the impetus for this blog post. The original code wasn’t the worst code, but it was sprinkled into 600 lines of unrelated singleton code:

private var sequenceStartTime: Date?
private var sequencePauseTime: Date?

// 150 lines of other stuff

func startSequenceTimer() {		
	self.sequenceStartTime = Date()

func pauseSequenceTimer() {		
	self.sequencePauseTime = Date()

// 50 lines of unrelated responsibilities 

func resumeSequenceTimer() {		
	let pauseEndTime = Date()		
	let duration = pauseEndTime.timeIntervalSince1970 - sequencePauseTime.timeIntervalSince1970
	self.sequenceStartTime = Date().addingTimeInterval(duration)

// 100 more lines of just whatever

func calculateDuration() {
	let sequenceEndTime = Date()
	let duration: Double		
	if let startTime = sequenceStartTime {
		duration = Double(sequenceEndTime.timeIntervalSince1970 - sequenceStartTimetimeIntervalSince1970) / 1_000
	} else {
		duration = 0.0
	return duration

This is clearly its own responsibility, and the start and end time variables are the first clue into that. This singleton doesn’t care about how to pause the timer or calculate the current running duration, it just needs to be able to pause a timer. This eventually became its own type and the two instance variables became the Barcelona Shirt Red 2017 Training Nike 2018 EBYIO:

struct Milliseconds {
	let rawValue: Int

protocol Clock {
	func now() -> Milliseconds

class Stopwatch {
	let clock: Clock
	private var startTime: Milliseconds?
	private var pauseTime: Milliseconds?

	// ...

The Clock protocol also enables the type to be tested, which wasn’t possible before. (H/t to Blue M7220r Jacket T2331 Man Geox B7YqU showing me how to test this class.) The tests also enabled me to fix a few bugs in the original code. (If you’re wondering what kinds of bugs, look at the original code: what happens if you’ve currently paused and you try to check the duration?)

mertens 2019 Originals Red Adidas Home 2018 Shirt Belgium 14 Extracting data clumps is a great way to find hidden classes and responsibilities within your code, and a great way to locate their hiding spots is to look at the language you use to define the variables. If there is a relationship between the language of the two properties, perhaps there is a deeper relationship is waiting to be drawn out as well. Expressing those relationships explicitly can lead to cleaner, more testable, and more reusable code.