Refactoring

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Refactoring

For refactoring Wikipedia content, please see Refactoring other text (below)

Refactoring is the process of rewriting a computer program or other material to improve its structure or readability, while explicitly preserving its meaning or behavior. Neil Roodyn recommends that "if you refactor, you should also have test fixtures in place: They can validate that your refactorings don't change the behavior of your software."

See also: test-first programming and unit testing.

Refactoring code

In software engineering, the term refactoring means modifying source code without changing its external behavior, and is sometimes informally referred to as "cleaning it up". In extreme programming and other agile methodologies refactoring is an integral part of the software development cycle: developers alternate between adding new tests and functionality and refactoring the code to improve its internal consistency and clarity. Automated unit testing ensures that refactoring does not make the code stop working.

Refactoring does not fix bugs or add new functionality. Rather it is designed to improve the understandability of the code or change its structure and design, and remove dead code, to make it easier for human maintenance in the future. In particular, adding new behavior to a program might be difficult with the program's given structure, so a developer might refactor it first to make it easy, and then add the new behavior.

An example of a trivial refactoring is to change a variable name into something more meaningful, such as from a single letter 'i' to 'interestRate'. A more complex refactoring is to turn the code within an if block into a subroutine. An even more complex refactoring is to replace an if conditional with polymorphism. While "cleaning up" code has happened for decades, the key insight in refactoring is to intentionally "clean up" code separately from adding new functionality, using a known catalogue of common useful refactoring methods, and then separately testing the code (knowing that any behavioral changes indicate a bug). The new aspect is explicitly wanting to improve an existing design without altering its intent or behavior.

The term is by analogy with the factorization of numbers and polynomials. For example, x² − 1 can be factored as (x + 1)(x − 1), revealing an internal structure that was previously not visible (such as the two roots at −1 and +1). Similarly, in software refactoring, the change in visible structure can often reveal the "hidden" internal structure of the original code.

Refactoring is done as a separate step, to simplify testing. At the end of the refactoring, any change in behavior is clearly a bug and can be fixed separately from the problem of debugging the new behavior.

Martin Fowler's book Refactoring is the classic reference. Although refactoring code has been done informally for years, William Opdyke's 1993 PhD dissertation ^* is the first known paper to specifically examine refactoring. All of these resources provide a catalog of common methods for refactoring; a refactoring method has a description of how to apply the method and indicators for when you should (or should not) apply the method.

Refactoring is thought by many to be an important concept. It has been named as one of the most important software innovations by author and commentator David A. Wheeler.

Common refactorings

Change Method Signature
Convert Local Variable to Field
Encapsulate Field
Extract Class
Extract Interface
Extract Local Variable
Extract Method (to turn part of a larger method into a new method. By breaking down code in smaller pieces, it is more easily understandable)
Generalize Type
Inline
Introduce Factory
Introduce Parameter
Pull Up
Push Down
Rename Method (changing the name of a method into a new one that better reveals its purpose).
Split Large Function Functions that grow in use also grow in size. After a certain point, it becomes clear that functions can benefit from using functions themselves. Your eyes probe the function looking for a likely candidate to relocate elsewhere. This candidate is chosen for being the largest stretch of code that really has nothing to do with the enclosing function. This lets us easily think up a name that explains what the new function does without having to resort to something contrived just to distinguish it from the original function. (This section is originally from Perl Design Patterns Book).

Forth

The term "factoring" has been used in the Forth community since at least the early 1980s. Chapter Six of Leo Brodie's book Thinking Forth (1984) is dedicated to the subject.

In Forth, factoring has essentially the same meaning that refactoring does in extreme programming—to break down a function (a "word" in Forth) into smaller, more easily maintained functions.

Refactoring other text

The term refactoring originated in computer programming, but the concept has also been applied when modifying any text.

On Wiki websites, refactoring refers to the process of rewriting and reorganizing text to shorten it while preserving content. This particularly applies to discussions, which thus can be made accessible to people who are interested in the arguments made in the discussion, and the information to be gleaned from it, rather than the history of the discussion itself. Refactoring can be difficult to do in such a way that it makes all participants of a discussion happy.

Most recently, biological engineers have begun exploring the idea of refactoring the genomes of natural biological systems in order to make living organisms easier to understand and interact with.

Etymology

The first known use of the term "refactoring" in the published literature was in the article, Refactoring: An Aid in Designing Application Frameworks and Evolving Object-Oriented Systems, Proceedings of the Symposium on Object Oriented Programming Emphasizing Practical Applications (SOOPPA) September, 1990, ACM by William F. Opdyke and Ralph E. Johnson William Opdyke's Ph.D. thesis on "Refactoring Object-Oriented Framework", University of Illinois, was published in 1992 [http://martinfowler.com/bliki/EtymologyOfRefactoring.html. The term "refactoring" was almost certainly used before then. Refactoring as a technique was definitely used before then.

As a neologism, it is clearly a reference to mathematical factoring.

References

External links

What Is Refactoring? (c2.com article)
Martin Fowler's homepage about refactoring
1993 PhD dissertation by William F. Opdyke
Aspect-Oriented Refactoring by Ramnivas Laddad
A Survey of Software Refactoring by Tom Mens and Tom Tourwé
How to Write Unmaintainable Code by Roedy Green
Refactoring Tools
Refactorings - alphabetical list
Refactoring To Patterns Catalog
Citations from CiteSeer
Refactoring Java Code
General Refactoring links
Refactoring for Microcontroller Software
Refuctoring - The Opposite Of Refactoring (from spoof Waterfall2006 conference)

Extreme Programming | Software engineering

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