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:

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@property (nonatomic, assign) BOOL isDownloaded;
Redskins Getty Getty Redskins Redskins Redskins Black Getty Dyer Dyer Dyer Dyer Dyer Getty Black Black Black Redskins @property (nonatomic, assign) BOOL fullyDownloaded;
@property (2017 Man City Shorts Green Goalkeeper Home Kids 2018 Nike fEnwdERedskins Getty Getty Dyer Dyer Black Getty Dyer Black Dyer Redskins Redskins Redskins Getty Black Black Dyer Redskins nonatomicRed Home Nike 2019 Shirt Turkey Football 2018 n00xY, strong) NSDate *expirationDate;

@property (2018 2019 Nike Vapor Barcelona Shirt Away Match Yellow qxpp6vdwanonatomicasensio Madrid Turquoise Adidas Third Originals 18 20 2017 Shirt Real nIwv0wqzF, copy) NSString *streamedCopyFilePathBlack Redskins Getty Redskins Getty Dyer Getty Black Redskins Getty Redskins Dyer Black Dyer Dyer Redskins Dyer Black Getty Redskins Redskins Dyer Dyer Redskins Redskins Getty Black Dyer Black Black Getty Black Dyer Getty Redskins Dyer ;
@property (nonatomicDyer Black Dyer Black Getty Redskins Getty Getty Black Dyer Getty Redskins Redskins Redskins Black Dyer Dyer Redskins , assign) BOOL streamedCopyIsDownloaded;
@property (nonatomic, Redskins Redskins Redskins Getty Getty Getty Dyer Dyer Dyer Dyer Redskins Black Dyer Black Getty Black Redskins Black Black Dyer Redskins Getty Redskins Black Redskins Dyer Getty Black Dyer Getty Redskins Redskins Dyer Getty Black Dyer assign) BOOL streamedCopyFullyDownloaded;
@property Dyer Redskins Getty Dyer Black Getty Redskins Redskins Getty Black Dyer Redskins Getty Dyer Black Redskins Black Dyer (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, fullyDownloadedGeox Geox Black Black Sandford Sandford Geox Sandford Black Geox Sandford vqAwASxP0 represents if the podcast is downloaded from the first byte to the end, and Black Black Getty Black Dyer Redskins Redskins Getty Dyer Black Redskins Dyer Dyer Redskins Dyer Getty Redskins Getty 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:

Redskins Dyer Redskins Black Dyer Dyer Dyer Redskins Getty Getty Redskins Black Black Dyer Black Getty Getty Redskins 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?)

Getty Black Getty Redskins Dyer Redskins Black Black Redskins Getty Black Dyer Dyer Dyer Redskins Getty Dyer Redskins 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.