Introduction to Software Engineering/Testing/Refactoring

Code refactoring is "a disciplined way to restructure code",[1] undertaken in order to improve some of the nonfunctional attributes of the software. Typically, this is done by applying series of "refactorings", each of which is a (usually) tiny change in a computer program's source code that does not modify its functional requirements. Advantages include improved code readability and reduced complexity to improve the maintainability of the source code, as well as a more expressive internal architecture or object model to improve extensibility.

By continuously improving the design of code, we make it easier and easier to work with. This is in sharp contrast to what typically happens: little refactoring and a great deal of attention paid to expediently adding new features. If you get into the hygienic habit of refactoring continuously, you'll find that it is easier to extend and maintain code.

—- Joshua Kerievsky, Refactoring to Patterns [2]

Refactoring does not take place in a vacuum, but typically the refactoring process takes place in a context of adding features to software:

  • "... refactoring and adding new functionality are two different but complementary tasks" -- Scott Ambler



Refactoring is usually motivated by noticing a code smell.[3] For example the method at hand may be very long, or it may be a near duplicate of another nearby method. Once recognized, such problems can be addressed by refactoring the source code, or transforming it into a new form that behaves the same as before but that no longer "smells". For a long routine, extract one or more smaller subroutines. Or for duplicate routines, remove the duplication and utilize one shared function in their place. Failure to perform refactoring can result in accumulating technical debt.

There are two general categories of benefits to the activity of refactoring.

  1. Maintainability. It is easier to fix bugs because the source code is easy to read and the intent of its author is easy to grasp.[4] This might be achieved by reducing large monolithic routines into a set of individually concise, well-named, single-purpose methods. It might be achieved by moving a method to a more appropriate class, or by removing misleading comments.
  2. Extensibility. It is easier to extend the capabilities of the application if it uses recognizable design patterns, and it provides some flexibility where none before may have existed.[2]

Before refactoring a section of code, a solid set of automatic unit tests is needed. The tests should demonstrate in a few seconds [citation needed] that the behavior of the module is correct. The process is then an iterative cycle of making a small program transformation, testing it to ensure correctness, and making another small transformation. If at any point a test fails, you undo your last small change and try again in a different way. Through many small steps the program moves from where it was to where you want it to be. Proponents of extreme programming and other agile methodologies describe this activity as an integral part of the software development cycle.

List of refactoring techniques


Here are some examples of code refactorings; some of these may only apply to certain languages or language types. A longer list can be found in Fowler's Refactoring book[3] and on Fowler's Refactoring Website.[5]

  • Techniques that allow for more abstraction
    • Encapsulate Field – force code to access the field with getter and setter methods
    • Generalize Type – create more general types to allow for more code sharing
    • Replace type-checking code with State/Strategy[6]
    • Replace conditional with polymorphism[7]
  • Techniques for breaking code apart into more logical pieces
    • 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. This is also applicable to functions.
    • Extract Class moves part of the code from an existing class into a new class.
  • Techniques for improving names and location of code
    • Move Method or Move Field – move to a more appropriate Class or source file
    • Rename Method or Rename Field – changing the name into a new one that better reveals its purpose
    • Pull Up – in OOP, move to a superclass
    • Push Down – in OOP, move to a subclass

Hardware refactoring


While the term refactoring originally referred exclusively to refactoring of software code, in recent years code written in hardware description languages (HDLs) has also been refactored. The term hardware refactoring is used as a shorthand term for refactoring of code in hardware description languages. Since HDLs are not considered to be programming languages by most hardware engineers,[8] hardware refactoring is to be considered a separate field from traditional code refactoring.

Automated refactoring of analog hardware descriptions (in VHDL-AMS) has been proposed by Zeng and Huss.[9] In their approach, refactoring preserves the simulated behavior of a hardware design. The non-functional measurement that improves is that refactored code can be processed by standard synthesis tools, while the original code cannot. Refactoring of digital HDLs, albeit manual refactoring, has also been investigated by Synopsys fellow Mike Keating.[10][11] His target is to make complex systems easier to understand, which increases the designers' productivity.

In the summer of 2008, there was an intense discussion about refactoring of VHDL code on the news://comp.lang.vhdl newsgroup.[12] The discussion revolved around a specific manual refactoring performed by one engineer, and the question to whether or not automated tools for such refactoring exist.

As of late 2009, Sigasi is offering automated tool support for VHDL refactoring.[13]



In the past refactoring was avoided in development processes. One example of this is that CVS (created in 1984) does not version the moving or renaming of files and directories.

Although refactoring code has been done informally for years, William Opdyke's 1992 Ph.D. dissertation[14] is the first known paper to specifically examine refactoring,[15] although all the theory and machinery have long been available as program transformation systems. 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.

Martin Fowler's book Refactoring: Improving the Design of Existing Code[3] is the canonical reference.

The first known use of the term "refactoring" in the published literature was in a September, 1990 article by William F. Opdyke and Ralph E. Johnson.[16] Opdyke's Ph.D. thesis,[14] published in 1992, also used this term.[15]

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

In extreme programming, the Extract Method refactoring technique has essentially the same meaning as factoring in Forth; to break down a "word" (or function) into smaller, more easily maintained functions.

Automated code refactoring


Many software editors and IDEs have automated refactoring support. Here is a list of a few of these editors, or so-called refactoring browsers.

  • IntelliJ IDEA (for Java)
  • Eclipse's Java Development Toolkit (JDT)
  • NetBeans (for Java)
    • and RefactoringNG, a Netbeans module for refactoring where you can write transformations rules of the program's abstract syntax tree.
  • Embarcadero Delphi
  • Visual Studio (for .NET)
  • JustCode (addon for Visual Studio)
  • ReSharper (addon for Visual Studio)
  • Coderush (addon for Visual Studio)
  • Visual Assist (addon for Visual Studio with refactoring support for VB, VB.NET. C# and C++)
  • DMS Software Reengineering Toolkit (Implements large-scale refactoring for C, C++, C#, COBOL, Java, PHP and other languages)
  • Photran a Fortran plugin for the Eclipse IDE
  • SharpSort addin for Visual Studio 2008
  • Sigasi Studio - standalone or plugin software for VHDL and System Verilog
  • XCode
  • Smalltalk Refactoring Browser (for Smalltalk)
  • Simplifide (for Verilog, VHDL and SystemVerilog)
  • Tidier (for Erlang)


  1. Scott Ambler
  2. a b Kerievsky, Joshua (2004). Refactoring to Patterns. Addison Wesley.
  3. a b c Fowler, Martin (1999). Refactoring: Improving the design of existing code. Addison Wesley.
  4. Martin, Robert (2009). Clean Code. Prentice Hall.
  5. Refactoring techniques in Fowler's refactoring Website
  6. Replace type-checking code with State/Strategy
  7. Replace conditional with polymorphism
  8. Hardware description languages and programming languages
  9. Kaiping Zeng, Sorin A. Huss, "Architecture refinements by code refactoring of behavioral VHDL-AMS models". ISCAS 2006
  10. M. Keating :"Complexity, Abstraction, and the Challenges of Designing Complex Systems", in DAC'08 tutorial [1]"Bridging a Verification Gap: C++ to RTL for Practical Design"
  11. M. Keating, P. Bricaud: Reuse Methodology Manual for System-on-a-Chip Designs, Kluwer Academic Publishers, 1999.
  14. a b Opdyke, William F (1992). "Refactoring Object-Oriented Frameworks" (compressed Postscript). Ph.D. thesis. University of Illinois at Urbana-Champaign. Retrieved 2008-02-12. {{cite journal}}: Cite journal requires |journal= (help); Unknown parameter |month= ignored (help)
  15. a b Martin Fowler, "MF Bliki: EtymologyOfRefactoring"
  16. Opdyke, William F.; Johnson, Ralph E. (September 1990). "Refactoring: An Aid in Designing Application Frameworks and Evolving Object-Oriented Systems". Proceedings of the Symposium on Object Oriented Programming Emphasizing Practical Applications (SOOPPA). ACM. 

Further reading