Notes on Refactoring

I had another reading assignment before this one, but there was so much material that I have three(!) unfinished blog drafts on it. Refactoring is an even longer book, but most of the material in it is intended as reference, so it's quicker to write up my notes.

Defining refactoring

the process of changing a software system in such a way that it does not alter the external behavior of the code yet improves its internal structure

  • Also can be thought of as software decay in reverse.

  • Make code easier to understand.

  • Make code easier to modify.

  • Allows you to develop faster because it becomes easier to add new features.

  • Reduces the penalty of making changes to the overall design, which in turn means less time needed for upfront design. Also encourages simpler design even if it is less flexible, since it means design can always be changed.

General recommendations

  • Refactor before you add a new feature. Adding a new feature should not involve changing existing code (if you have correctly implemented open-closed principle or OCP).

  • Have self-checking automated tests so you can refactor safely.

    • Run tests frequently after each step in a refactoring. If you forget, go back and redo the refactoring with testing.
    • Test boundary conditions and expected exceptions.
  • Some design principles to keep in mind:

    • A method belongs with the object whose data it uses.
    • Minimize use of temporary variables and complex conditional logic.
    • Eliminate duplicate code since more lines of code are more difficult to maintain.
    • Clean code should not require you to remember anything about it to be readable and comprehensible.
  • Refactoring can be used to understand unfamiliar code or conduct a code review or understand a bug.

  • Don't set aside time to refactor. Refactoring should be done all throughout the development process in frequent, short bursts.

  • Sometimes refactoring can be as simple as renaming variables. Don't hesitate to rename.

  • Rule of Three: Refactor the third time you write code that duplicates similar behavior.


  • Software design concept that boils down to having many small modular components.

  • While it creates more pieces of code to manage, the benefits outweigh the drawbacks:

    • Easy to share logic between different parts of your code.
    • Allows (through good naming practices) explanation of the intent behind each step in your code.
    • When making modifications, keeps change isolated to one part of the system.
    • Simplify conditional logic (e.g. in object-oriented paradigm, you can use the identity of the object rather than branching to specify different behaviors)

Limitations of refactoring

  • Hard to refactor applications with tight coupling to databases

  • May involve changing published interfaces, which requires maintaining old and new versions

  • Sometimes it is better to rewrite from scratch.

  • When not to refactor:

    • Too many failing tests! Only refactor code that works.
    • Close to deadline

Performance optimization

  • Optimization often makes code harder to read and understand. But refactoring can make it easier to tune the performance.

  • Most of the time, there is a rate-limiting step that is responsible for slowing down the program. Don't optimize all parts of the code equally; identify where this step is (e.g. big-O analysis).

Code smells

(There are probably dozens of blog posts that have already regurgitated this material partially or in full, so I'm just going to include some mnemonics to help myself remember what each one is about.)

Duplicated code: Self-explanatory

Long method: Some heuristics for identification

  • "When we feel a need to comment something, we write a method instead."
  • A lot of parameters or temporary variables
  • Conditional logic
  • Loops

Large class: One sign is too many instance variables

Long parameter list: Note that this "smell" may end up being necessary in some cases to avoid unwanted dependencies.

Divergent change: Basically a single-responsibility principle (SRP) violation.

Shotgun surgery: The inverse of divergent change. Could be sort of seen as a dependency-inversion principle (DIP) violation in some cases?

Feature envy: Method shouldn't use data that doesn't belong to its object! (Except for some design patterns like Strategy or Visitor. So think about trade-offs.)

Data clumps: Data that tends to be used together need to be in their own object.

Primitive obsession: Use objects rather than primitives (e.g. Java's String).

Switch statements: Use polymorphism rather than switch-case branching.

Parallel inheritance: Duplication of class hierarchies.

Lazy class: Is this class doing anything?

Speculative generality: Functionality that isn't being used yet.

Temporary field: Instance variable that isn't being used.

Message chains: Chains of calls between objects.

Middleman: Heuristic is that if an object is delegating half its methods to the same object, those two objects should communicate directly.

Inappropriate intimacy: Boils down to lack of encapsulation and tightly coupled classes.

Alternative classes with different interfaces: Self-explanatory

Incomplete library class: Situation where you need to make a slight modification to a library.

Data class: Classes that do nothing but hold data that is being called by other classes.

Refused bequest: Not always a problem. But one example of when it is: a subclass that doesn't support the interface of its superclass.

Comments: Not inherently bad but usually pop up to mask the lack of readability or comprehensibility. Address the underlying problem directly and need for comment usually disappears.

Refactoring tools

I wonder if a refactoring tool rather resembles a compiler, since it needs to understand code syntax in much the same way.

  • Help integrate refactoring into regular development process by reducing the cost of refactoring.

  • Allows for safe refactoring without needing to rerun tests.

  • Uses parse trees to represent the internal structure of a method.

  • Needs to be safe and accurate.

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