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In computer programming, code smell is any symptom in the source code of a program that possibly indicates a deeper problem. Code smells are usually not bugs—they are not technically incorrect and do not currently prevent the program from functioning. Instead, they indicate weaknesses in design that may be slowing down development or increasing the risk of bugs or failures in the future.

Often the deeper problem hinted by a code smell can be uncovered when the code is subjected to a short feedback cycle where it is refactored in small, controlled steps, and the resulting design is examined to see if there are any further code smells that indicate the need of more refactoring. From the point of view of a programmer charged with performing refactoring, code smells are heuristics to indicate when to refactor, and what specific refactoring techniques to use. Thus, a code smell is a driver for refactoring.

The term appears to have been coined by Kent Beck on WardsWiki in the late 1990s. Usage of the term increased after it was featured in Refactoring: Improving the Design of Existing Code. Code smell is also a term used by agile programmers.

Determining what is and is not a code smell is subjective, and varies by language, developer and development methodology. There are tools, such as Checkstyle, PMD and FindBugs for Java, to automatically check for certain kinds of code smells.