Code Smells

Code Smell is a term coined by Kent Beck and introduced in Martin Fowler's book, Refactoring. Code Smells are patterns of code that suggest there might be a problem, that there might be a better way of writing the code or that more design perhaps should go into it. They were originally intended to be used as a guide for when to refactor code. More recently though, I have found them to be very useful in code reviews as a succinct language for when and how to clean up certain chunks of code. I wanted to document and standardize this language so as to make code reviews more valuable.

The book Anti Patterns by William J. Brown et al is another book that describes the same kinds of phenomena. While the Anti Patterns book came out over a year prior to Refactoring, I think the term "code smell" is better recognized. So that is what I will use as the overarching term for these degenerate code templates.

One thing to note about this initial list of code smells is that it is very much focused on object-oriented code. The book, Refactoring, was written specifically with Java and its purely object-oriented approach in mind. Over time, I would like to include code smells from a wider variety of languages and approaches, whether imperative or functional languages, object-oriented or procedure-based, or even further afield like SQL. But for now, we're going to start here with object-oriented code smells.

Notes

This list is specifically written with a header for each smell to enable linking. One can simply craft a link with an anchor target of the rule name in lower case with hyphens instead of spaces to link to a specific rule. For example: https://github.com/lee-dohm/code-smells#duplicate-code

I use the terms "function" and "method" interchangeably.

All page numbers are from Refactoring unless otherwise noted.

Why Ruby for the Code Examples?

I chose Ruby for the code examples mostly because its ability to compactly express ideas will be able to quickly convey the point of the examples with a minimum of confusion. As we gather code smells from different languages and styles of writing code, examples from other languages will be added.

List of Code Smells

Alternative Classes with Different Interfaces

p85

Classes that do similar things but have different interfaces should probably be refactored to have similar interfaces.

Comments

p87

It's surprising how often you look at thickly commented code and notice that the comments are there because the code is bad.
-- Refactoring

Many comments aren't bad in and of themselves. But comments are often used as a crutch to allow poorly written code to remain so. If at all possible, code should be written so that the comments are not necessary.

Comments within functions should bear extra scrutiny. Comments within the body of functions are, almost without exception, unnecessary in well-written code. Well-written code consists of short, declarative functions that have good identifier names. It should be obvious what the code is doing from the code itself. What the code often cannot describe though is why the code is doing what it is doing. Here is an example from one of my projects:

# Indicates whether color is a valid SVG color string.
#
# SVG color descriptions are one of the following:
#
# * RGB values
#     * #rgb
#     * #rrggbb
#     * rgb(255, 0, 0) - not currently supported
#     * rgb(100%, 0%, 0%) - not currently supported
# * SVG Color names
def valid_color?(color)
  return color =~ /^#[0-9A-Fa-f]{3}([0-9A-Fa-f]{3})?$/ if color[0] == '#'

  COLOR_NAMES.include?(color)
end

The code by itself is quite clear in what it is doing, so long as one understands regular expressions. But one wouldn't have the extra understanding of the why the code is written this way, that it is validating SVG color codes, or how exactly the code could or should be modified without this extra information. One could easily see someone coming along and adding or subtracting functionality from this method and introducing bugs if they did not know that this is intended specifically to conform to the SVG definition of a color.

Documentation Comments

Documentation comments are somewhat contentious. Some think they are important and some consider them to be a code smell. I personally believe that documentation comments are quite useful so long as they are well written and are kept up-to-date with the code. Well written documentation comments that are used to generate developer documentation can allow one to understand a code base much more quickly than rifling through the code itself. Consider this, when you are working with your pet language; would you want to dive deep into the code for the language itself to understand how opening a file really works? Or would you rather have some documentation that tells you how to open a file for writing?

Data Class

p86

These are classes that have fields, getting and setting methods for the fields, and nothing else. Such classes are dumb data holders and are almost certainly being manipulated in far too much detail by other classes.
-- Refactoring

One might argue that this is in direct conflict with Primitive Obsession. But Primitive Obsession recommends creating a class for single pieces of data that have specific boundaries or properties for which the built-in data types are not well-suited. Data Class is about creating a class that simply ties a bunch of pieces of data together and provides no other value. For example, the classic Point class is potentially an example:

class Point
  attr_accessor :x
  attr_accessor :y

  def initialize(x = 0, y = 0)
    @x = x
    @y = y
  end
end

But perhaps we calculate the distance of two points from each other (or from the origin) in a few places. We should then refactor that to:

class Point
  attr_accessor :x
  attr_accessor :y

  def initialize(x = 0, y = 0)
    @x = x
    @y = y
  end

  def distance(point = Point.new)
    Math.sqrt((@x - point.x) ** 2 + (@y - point.y) ** 2)
  end
end

So what really makes this a code smell is that, very often, a Data Class has regular operations performed on it spread around the code base that should be merged into the class itself.

Data Clumps

p81

Data items tend to be like children; they enjoy hanging around in groups together. 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.
-- Refactoring

A simple example would be a graphics library that takes x and y coordinate parameters all over the place:

def draw_line(start_x, start_y, end_x, end_y)
  # snip
end

def draw_circle(x, y, radius)
  # snip
end

# etc

when it should just have a Point class:

class Point
  # details removed
end

def draw_line(start, end)
  # snip
end

def draw_circle(center, radius)
  # snip
end

Disjointed API

Refactoring: Ruby Edition

Libraries are often written with flexibility as the number one priority. The author needs to build in this flexibility so that her library can be used by many different people in many different ways. This flexibility often presents itself as a relatively fine-grained, disjointed API, with many configuration options.
-- Refactoring: Ruby Edition

Divergent Change

p79

Divergent change occurs when one class is commonly changed in different ways for different reasons. If you look at a class and say, "Well, I will have to change these three methods every time I get a new database; I have to change these four methods every time there is a new financial instrument," you likely have a situation in which two objects are better than one. That way each object is changed only as a result of one kind of change.
-- Refactoring

See Single Responsibility Principle

Duplicate Code

p76

If you see the same code structure in more than one place, you can be sure that your program will be better if you find a way to unify them.
-- Refactoring

By far the most common code smell, Duplicate Code comes in many shapes and sizes. There are the obvious instances where chunks of code are simply copy and pasted around the code base. But there are also more subtle instances too where chunks of code are parameterized to one extent or another. Figuring out that bits of code are duplicates of each other and how to remove that duplication is what separates beginning developers from intermediate and advanced developers.

See Don't Repeat Yourself

Feature Envy

p80

A method that seems more interested in a class other than the one it actually is in. The most common focus of the envy is the data. We've lost count of the times we've seen a method that invokes half-a-dozen getting methods on another object to calculate some value.
-- Refactoring

Private methods are just another class waiting to be born.
-- Unknown

This is the function-level version of Middle Man.

Inappropriate Intimacy

p85

Sometimes classes become far too intimate and spend too much time delving in each others' private parts. We may not be prudes when it comes to people, but we think our classes should follow strict, puritan rules.
-- Refactoring

Incomplete Library Class

p86

Large Class

p78

Typically a catch-all class that all the functionality that doesn't go anywhere else is placed in.

See Single Responsibility Principle

Lazy Class

p83

Each class you create costs money to maintain and understand. A class that isn't doing enough to pay for itself should be eliminated.
-- Refactoring

While Data Class and Primitive Obsession are examples of classes not being powerful enough or simply not written, a Lazy Class is powerful enough to stand on its own. It simply isn't used much or at all. If a class just isn't used much, then perhaps its functionality needs to go someplace else.

Striking the right balance between Lazy Class and Feature Envy is sometimes challenging.

Long Method

p76

The longer a function is, the more difficult it is to understand.

Long Parameter List

p78

More than three arguments to a function is generally an issue.

Message Chains

p84

You see message chains when a client asks one object for another object, which the client then asks for yet another object, which the client then asks for yet another another object, and so on.

# Message Chain
salary = database.get_company(company_name).
                  get_manager(manager_name).
                  get_team_member(employee_name).
                  salary

# Better
salary = database.get_employee_by_name(employee_name).salary

In the above example, the function calling the message chain has to understand that it needs to go to the database, get the company object, get the manager object from that, get the individual employee's object from there and finally get the salary. If the structure of how employees are stored changes, this code will break. It would be better to simply ask the database for the employee by name and then ask that for its salary, then the structure of how employees are stored can change and the code will continue to work.

Message Chains are distinct from fluent interfaces in that with fluent interfaces all of the functions in the chain are essentially called on the first object.

# Fluent Interface
canvas.draw_line(from, to).
       draw_circle(center, radius).
       draw_text(upperLeft, someText)

Metaprogramming Madness

Refactoring: Ruby Edition

While in most cases Ruby's dynamic nature provides great benefits, it can be misused. Some metaprogramming techniques can result in obfuscated code. The method_missing hook, for example, often results in code that is difficult to understand.
-- Refactoring: Ruby Edition

Middle Man

p85

You look at a class' interface and find half the methods are delegating to this other class.
-- Refactoring

This is the class-level version of Feature Envy.

Parallel Inheritance Hierarchies

p83

Parallel inheritance hierarchies is really a special case of shotgun surgery. In this case, every time you make a subclass of one class, you also have to make a subclass of another.
-- Refactoring

Primitive Obsession

p81

People new to objects usually are reluctant to use small objects for small tasks, such as money classes that combine number and currency, ranges with an upper and a lower, and special strings such as telephone numbers and ZIP codes.
-- Refactoring

Primitive Obsession is an over-reliance on the built-in simple data types in a language such as integers, floating-point numbers, strings and the like. For example, a string can hold a hexadecimal color code such as C0C0C0. But even a simple Color class can help reduce bugs:

class Color
  attr_reader :code

  def initialize(code)
    @code = validate(code)
  end

  def to_s
    @code.to_s
  end

  private

  def validate(code)
    raise InvalidColorError unless code =~ /[0-9A-Fa-f]{6}/

    code
  end
end

Refused Bequest

p87

A class that inherits from another, but hides or removes a lot of the functionality of the parent class.

Repetitive Boilerplate

Refactoring: Ruby Edition

One of the easiest ways to remove duplication is [the Extract Method refactoring]. Extract the method and call it from multiple places. Some kinds of methods become so commonplace that we can go even further. Take for example attr_reader in Ruby. Implementing attribute readers is so common in object-oriented languages that the author of Ruby decided to provide a succinct way to declare them.
-- Refactoring: Ruby Edition

Shotgun Surgery

p80

[Shotgun Surgery] is similar to [the Divergent Change code smell] but is the opposite. You whiff this when every time you make a kind of change, you have to make a lot of little changes to a lot of different classes. When the changes are all over the place, they are hard to find, and it's easy to miss an important change.
-- Refactoring

Speculative Generality

p83

You get it when people say, "Oh, I think we need the ability to do this kind of thing someday" and thus want all sorts of hooks and special cases to handle things that aren't required. The result often is harder to understand and maintain. If all this machinery were being used, it would be worth it. But if it isn't, it isn't. The machinery just gets in the way, so get rid of it.
-- Refactoring

See You Ain't Gonna Need It

Switch Statements

p82

Often you find the same switch statement scattered about a program in different places. If you add a new clause to the switch, you have to find all these switch statements and change them.
-- Refactoring

Temporary Field

p84

Sometimes you see an object in which an instance variable is set only in certain circumstances. Such code is difficult to understand, because you expect an object to need all of its variables.
-- Refactoring

Bibliography

Fields, Jay et al. Refactoring: Ruby Edition. Addison-Wesley Professional, October 25, 2009.
Fowler, Martin et al. Refactoring. Addison-Wesley Professional, July 8, 1999.
Martin, Robert C. Clean Code. Prentice Hall, August 11, 2008.

License

This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License.

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