Ruby Science

Feature Envy

Feature envy reveals a method (or method-to-be) that would work better on a different class.

Methods suffering from feature envy contain logic that is difficult to reuse because the logic is trapped within a method on the wrong class. These methods are also often private methods, which makes them unavailable to other classes. Moving the method (or the affected portion of a method) to a more appropriate class improves readability, makes the logic easier to reuse and reduces coupling.


  • Repeated references to the same object.
  • Parameters or local variables that are used more than methods and instance variables of the class in question.
  • Methods that include a class name in their own names (such as invite_user).
  • Private methods on the same class that accept the same parameter.
  • Law of Demeter violations.
  • Tell, don’t ask violations.


# app/models/completion.rb
def score
  answers.inject(0) do |result, answer|
    question = answer.question
    result + question.score(answer.text)

The answer local variable is used twice in the block: once to get its question, and once to get its text. This tells us that we can probably extract a new method and move it to the answer class.


  • Extract method if only part of the method suffers from feature envy; then move the method.
  • Move method if the entire method suffers from feature envy.
  • Inline class if the envied class isn’t pulling its weight.


Following the law of Demeter will prevent a lot of feature envy by limiting the dependencies of each method.

Following tell, don’t ask will prevent feature envy by avoiding unnecessary inspection of another object’s state.

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