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Clean Code definition

Consistency

The code is written in a uniform and conventional way. All the code looks similar and follows a regular pattern, even with multiple contributors at different times.

Consistent code is formatted, conventional, and identifiable.

The code presentation is systematic and regular. Non-semantic choices, such as spacing, indentation, and character placement, remain consistent throughout the codebase, maintaining uniformity across files and authors.

Example

The example below shows inconsistent indentation in Java code. It’s not about tabs versus spaces, it’s about consistency.

Non-compliant code

class Foo {

  public int a;

    public int b;

  public void doSomething() {

    if(something) {

          doSomethingElse();

  }

  }

}

Compliant code

class Foo {

  public int a;

  public int b;

  public void doSomething() {

    if(something) {

      doSomethingElse();

    }

  }

}

For more information, see the corresponding Sonar rule.

Conventional

The code performs tasks with expected instructions. Faced with equally good options, the code adheres to a single choice across all instances, preferring language conventions. This includes using the appropriate programming interfaces and language features.

Example

In C++ from version 11, type aliases can be declared via either typedef or using, however, you should prefer the latter for modern code.

Non-compliant code

typedef void (*FunctionPointerType)(int);

Compliant code

using FunctionPointerType = void (*)(int);

For more information, see the corresponding Sonar rule.

Identifiable

The names follow a regular structure based on language conventions. The casing, word separators, suffixes, and prefixes used in the identifiers have purpose, without arbitrary differences.

Example

Consider code written in C#, where PascalCase is used for all identifiers except parameter names. In this context, using underscores or other casing styles to differentiate words in an identifier is unacceptable.

Non-compliant code

class my_class {...}
class SOMEName {...}

Compliant code

class MyClass {...}
class SomeName {...}

For more information, see the corresponding Sonar rule.

Intentionality

The code is precise and purposeful. Every instruction makes sense, is adequately formed, and clearly communicates its behavior.

Intentional code is clear, logical, complete, and efficient.

Clear

The code is self-explanatory, transparently communicating its functionality. It is written in a straightforward way that minimizes ambiguity, avoiding unnecessary clever or intricate solutions.

Example

In the non-compliant example of Python code below, and you'll notice that variables message and i are defined but never used. When readers encounter such cases, they might wonder if it's a coding error that was supposed to do something else or if it's just leftover code that can be safely deleted.

Non-compliant code

def hello(name):
    message = "Hello " + name
    print(name)
for i in range(10):
    foo()

Compliant code

def hello(name):
    message = "Hello " + name
    print(message)
for _ in range(10):
    foo()

For more information, see the corresponding Sonar rule.

Logical

The code has well-formed and sound instructions that work together. It is free of explicit errors, contradictions, and commands that could be unpredictable or objectionable.

Example

In JavaScript, there's NaN, which stands for Not-a-Number. It represents a numeric data type that isn't a valid number. NaN is not equal to any value, even itself, and this behavior can lead to unexpected results.

Non-compliant code

if (a !== NaN) {

  console.log("this is always logged");

}

Compliant code

if (!isNaN(a)) {

  console.log("a is not NaN");

}

For more information, see the corresponding Sonar rule.

Complete

The code constructs are comprehensive and used adequately and thoroughly. The code is functional and achieves its implied goals. There are no obviously incomplete or lacking solutions.

Example

An example in PHP is the use of secure cookies. The method setcookie allows you to create cookies that can be transmitted via HTTP by default, making their contents readable. Since cookies often carry sensitive data, it's important to ensure they are transferred securely to fulfill their intended purpose. You need to pass a last argument to enable HTTPS only.

Non-compliant code

$value = "sensitive data";

setcookie($name, $value, $expire, $path, $domain);

Compliant code

$value = "sensitive data";

setcookie($name, $value, $expire, $path, $domain, true);

For more information, see the corresponding Sonar rule.

Efficient

The code uses resources without needless waste. It prioritizes economical options when available, avoiding unnecessary consumption of memory, processor, disk, or network resources.

Example

Most Linux package managers create a cache by default when working with Docker. Unless you remember to remove these files in your Dockerfile, they will increase the size of your image without providing any additional value.

Non-compliant code

RUN apt-get update \
  && apt-get install nginx

Compliant code

RUN apt-get update \
  && apt-get install nginx \
  && apt-get clean

Adaptability

The code is structured to be easy to evolve and develop with confidence. It makes extending or repurposing its parts easy and promotes localized changes without undesirable side-effects.

Adaptable code is focused, distinct, modular, and tested.

Focused

The code has a single, narrow, and specific scope. Each unit should have only one concise purpose, without an overwhelming accumulation of instructions or excessive amounts of complexity.

Example

In Swift, it's best practice to keep types, such as classes, in separate files. This helps prevent an excessive accumulation of instructions or an overwhelming amount of complexity within a single file.

Non-compliant code

class MyViewController: UIViewController {
  // …
}
extension MyViewController: UIScrollViewDelegate {
  // …
}
class UnrelatedController: UIViewController {
  // …
}

Compliant code

class MyViewController: UIViewController {
  // …
}
extension MyViewController: UIScrollViewDelegate {
  // …
}

For more information, see the corresponding Sonar rule.

Distinct

The code procedures and data are unique and distinctive, without undue duplication. The codebase has no significant repetition where it could be decomposed into smaller shared segments.

Example

Duplicating string literals raises the risk of errors when making updates since each occurrence must be changed separately. A better approach is to use constants that can be referenced from multiple places, allowing updates to be made in a single location. Here’s an example using Ruby.

Non-compliant code

def foo()
  prepare('action random1')
  execute('action random1')
  release('action random1')
end

Compliant code

def foo()
  action1 = 'action random1'
  prepare(action1)
  execute(action1)
  release(action1)
end

For more information, see the corresponding Sonar rule.

Modular

The code has been organized and distributed to emphasize the separation between its parts. The relationships within the code are carefully managed, ensuring they are minimal and clearly defined.

Example

A key aspect of this is encapsulation. In Object-Oriented languages, encapsulation often involves making fields private. This way, the class retains control over the details of its internal representation and prevents other parts of the code from having too much knowledge about its inner workings.

However, there are multiple levels of encapsulation, and even minor improvements can make a difference. For example, if you're working with VB.Net, which allows publicly accessible fields, it's better to avoid using them and instead use properties. Properties work similarly to fields but are part of the interface and can be overridden by getters and setters.

Non-compliant code

Class Foo
    Public Bar = 42
End Class

Compliant code

Class Foo
    Public Property Bar = 42
End Class

For more information, see the corresponding Sonar rule.

Tested

The code has automated checks that provide confidence in the functionality. It has enough test coverage which enables changes in implementation without the risk of functional regressions.

Examples

There are odd examples, where you have a test folder or test files, without actual test cases inside, which can mislead other developers. See the corresponding Sonar rule.

There are also cases where tests are skipped and accidentally committed like that, which might go unnoticed if not tracked in any way. See the corresponding Sonar rule.

Responsibility

The code takes into account its ethical obligations on data, as well as societal norms.

Responsible code is lawful, trustworthy, and respectful.

Lawful

The code respects licensing and copyright regulation. It exercises the creator’s rights and honors other’s rights to license their own code.

Example

One common example is companies enforcing copyright headers in their code files:

/*
 * SonarQube, open source software for clean code.
 * Copyright (C) 2008-2024 SonarSource
 * mailto:contact AT sonarsource DOT com
 *
 * SonarQube is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; either
 * version 3 of the License, or (at your option) any later version.
 *
 * SonarQube is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public License
 * along with this program; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
 */

For more information, see the corresponding Sonar rule.

Trustworthy

The code abstains from revealing or hard-coding private information. It preserves sensitive private information such as credentials and personally identifying information.

Example

Below is a simplified example using Go.

Non-compliant code

func connect()  {
  user := "root"
  password:= "supersecret"

  url := "login=" + user + "&passwd=" + password
}

Compliant code

func connect()  {
  user := getEncryptedUser()
  password:= getEncryptedPass()

  url := "login=" + user + "&passwd=" + password
}

For more information, see the corresponding Sonar rule.

Respectful

The code refrains from using discriminatory and offensive language. It chooses to prioritize inclusive terminology whenever an alternative exists that conveys the same meaning.

Example

Non-compliant code

Master / Slave
Blacklist / Whitelist

Compliant code

Primary / Secondary
Denylist / Allowlist

Learn more

Check the Clean Code benefits page to better understand software qualities. On the Code analysis based on Clean Code page you can learn how Clean Code attributes and software qualities are impacted by your code issue.

If you want to see how the Clean Code attributes look in SonarLint for Eclipse, check out The Rule Descriptions view article for a screenshot.

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Clean Code definition

On this page

Consistency

Formatted

Conventional

Identifiable

Intentionality

Clear

Logical

Complete

Efficient

Adaptability

Focused

Distinct

Modular

Tested

Responsibility

Lawful

Trustworthy

Respectful

Learn more

Clean Code definition

On this page

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Consistency