Wednesday, November 26, 2008

Guide to Writing Testable Code

It is with great pleasure that I have been able to finally open-source the Guide to Writing Testable Code.

I am including the first page here for you, but do come and check it out in detail.

To keep our code at Google in the best possible shape we provided our software engineers with these constant reminders. Now, we are happy to share them with the world.

Many thanks to these folks for inspiration and hours of hard work getting this guide done:

Flaw #1: Constructor does Real Work

Warning Signs

  • new keyword in a constructor or at field declaration
  • Static method calls in a constructor or at field declaration
  • Anything more than field assignment in constructors
  • Object not fully initialized after the constructor finishes (watch out for initialize methods)
  • Control flow (conditional or looping logic) in a constructor
  • Code does complex object graph construction inside a constructor rather than using a factory or builder
  • Adding or using an initialization block

Flaw #2: Digging into Collaborators

Warning Signs

  • Objects are passed in but never used directly (only used to get access to other objects)
  • Law of Demeter violation: method call chain walks an object graph with more than one dot (.)
  • Suspicious names: context, environment, principal, container, or manager

Flaw #3: Brittle Global State & Singletons

Warning Signs

  • Adding or using singletons
  • Adding or using static fields or static methods
  • Adding or using static initialization blocks
  • Adding or using registries
  • Adding or using service locators

Flaw #4: Class Does Too Much

Warning Signs

  • Summing up what the class does includes the word “and”
  • Class would be challenging for new team members to read and quickly “get it”
  • Class has fields that are only used in some methods
  • Class has static methods that only operate on parameters


  1. Good stuff, none of the links work though :)

  2. I have constructors that take an XPathNavigator instance that contains the configuration for that class.

    In your view, is it legitmate to have this sort of XML navigation work in the constructor?

  3. This comment has been removed by the author.

  4. Thank you for your good post.
    I have a question about "Flaw #2: Digging into Collaborators" and "Dynamic type casting".
    "Dynamic type casting" means casting from one interface type to another interface type in Java or C#. Or I think QueryInterface() of COM does same thing.

    My question is that:
    Casting the interface,which is passed in, to another interface conforms to Flaw #2, Or Not.

    Hyun Chang Lee.


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