What is refactoring and what is only modifying code?
I know that refactoring is "changing the structure of a program so that the functionality is not changed". I was talking with some of the guys I'm working with on my final year project at university and I was surprised that they have a much more expansive (for want of a better word) view of refactoring.
I consider refactoring to be things like extracting methods and renaming classes. They also suggested things like changing data structures (like a Java LinkedList to an ArrayList), changing algorithms (using merge sort instead of bubble sort), and even rewriting large chunks of code as refactoring.
I was quite sure that they were wrong, but I wasn't able to give a good reason why because what they were suggesting did change the program (and presumably make it better) without changing its behaviour. Am I right, and more importantly, why?
Answer
Martin Fowler's "Refactoring: Improving the Design of Existing Code" is perhaps THE reference:
Refactoring is a controlled technique for improving the design of an existing code base. Its essence is applying a series of small behavior-preserving transformations, each of which "too small to be worth doing". However the cumulative effect of each of these transformations is quite significant. By doing them in small steps you reduce the risk of introducing errors. You also avoid having the system broken while you are carrying out the restructuring - which allows you to gradually refactor a system over an extended period of time.
Refactoring goes hand-in-hand with unit testing. Write tests before you refactor and then you have a confidence level in the refactoring (proportional to the coverage of the tests).
A good reference is: Information about Refactoring