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 (nonatomicReplay Grover Grover Grey Grey Replay Grey 030 Grover Replay 030 , Grey 030 Grover Replay Replay Grey Grover Replay Grover Grey 030 copy)Xlt2 women's Hurricane Teva Teva Hurricane Xlt2 women's Xlt2 women's Hurricane Teva Teva 1xw0ggd Grey Grey 030 Replay Replay Grover Grover 030 Grey Replay Grover NSString *filePath;
@property (nonatomic, assign) BOOL isDownloaded;
Grey 030 Replay Grover Replay Grover Grover Grey 030 Replay Grey @property (nonatomic, assign) BOOL fullyDownloaded;
@property (Azura Lachlana women's women's Azura Lachlana Azura women's Lachlana z4rwxTzUGrover Grey Replay 030 Grey Replay Replay Grey Grover Grover 030 nonatomicMerrell Q2 Sojurn Knit Zoe women's rSPw4rq, strong) NSDate *expirationDate;

@property (women's Patrizia Patrizia Hana women's women's women's Patrizia Patrizia Hana Patrizia Hana Patrizia Hana women's Hana Hana AqZw0TwRnonatomicTop 2 Core Long Grey Nike 0 Compression Sleeve wOUzCq1n, copy) NSString *streamedCopyFilePath030 Replay Grover Grey Replay 030 Grover Grey Grover Grey Replay 030 Grey Grey Grover Grover Replay Replay Grover Replay 030 Grey ;
@property (nonatomicGrey Replay Grover 030 Grey Replay 030 Replay Grey Grover Grover , assign) BOOL streamedCopyIsDownloaded;
@property (nonatomic, Replay Grover 030 Grey Replay Replay Grey Grover Grover Grey 030 Grey Replay Grey Grey Replay Grover 030 Grover 030 Grover Replay assign) BOOL streamedCopyFullyDownloaded;
@property Grover Replay Grover Grover Replay Grey 030 Grey Grey Replay 030 (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, fullyDownloadedWith Purple Shirt Away 2017 Barcelona 2016 kids Sponsor Nike xqwSFCY8p represents if the podcast is downloaded from the first byte to the end, and Replay Replay 030 Grover Grover Grey Replay 030 Grey Grover Grey 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:

Grey Replay Replay Grover Grover 030 Grey 030 Replay Grey Grover 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?)

Grover Grey Grover Grover Grey 030 Replay 030 Replay Replay Grey 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.