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Test Driven Development: By Example

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Test Driven Development: By Example


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Test Driven Development (TDD) is Kent Beck's latest focus; the approach is proven to reduce defects and produce more robust software.

° Write clean code that works with the help of this groundbreaking software method

° Begin to write automated tests that allow you to "test on the fly," and learn to optimize the practice of refactoring

° Example-driven teaching; Kent Beck's step-by-step instruction will have you using TDD to further your projects


  • Copyright 2003
  • Dimensions: 7-3/8" x 9-1/4"
  • Pages: 240
  • Edition: 1st
  • Book
  • ISBN-10: 0-321-14653-0
  • ISBN-13: 978-0-321-14653-3

Clean code that works--now. This is the seeming contradiction that lies behind much of the pain of programming. Test-driven development replies to this contradiction with a paradox--test the program before you write it.

A new idea? Not at all. Since the dawn of computing, programmers have been specifying the inputs and outputs before programming precisely. Test-driven development takes this age-old idea, mixes it with modern languages and programming environments, and cooks up a tasty stew guaranteed to satisfy your appetite for clean code that works--now.

Developers face complex programming challenges every day, yet they are not always readily prepared to determine the best solution. More often than not, such difficult projects generate a great deal of stress and bad code. To garner the strength and courage needed to surmount seemingly Herculean tasks, programmers should look to test-driven development (TDD), a proven set of techniques that encourage simple designs and test suites that inspire confidence.

By driving development with automated tests and then eliminating duplication, any developer can write reliable, bug-free code no matter what its level of complexity. Moreover, TDD encourages programmers to learn quickly, communicate more clearly, and seek out constructive feedback.

Readers will learn to:

  • Solve complicated tasks, beginning with the simple and proceeding to the more complex.
  • Write automated tests before coding.
  • Grow a design organically by refactoring to add design decisions one at a time.
  • Create tests for more complicated logic, including reflection and exceptions.
  • Use patterns to decide what tests to write.
  • Create tests using xUnit, the architecture at the heart of many programmer-oriented testing tools.
  • This book follows two TDD projects from start to finish, illustrating techniques programmers can use to easily and dramatically increase the quality of their work. The examples are followed by references to the featured TDD patterns and refactorings. With its emphasis on agile methods and fast development strategies, Test-Driven Development is sure to inspire readers to embrace these under-utilized but powerful techniques.


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    Table of Contents





     1. Multi-Currency Money.
     2. Degenerate Objects.
     3. Equality for All.
     4. Privacy.
     5. Franc-ly Speaking.
     6. Equality for All, Redux.
     7. Apples and Oranges.
     8. Makin' Objects.
     9. Times We're Livin' In.
    10. Interesting Times.
    11. The Root of All Evil.
    12. Addition, Finally.
    13. Make It.
    14. Change.
    15. Mixed Currencies.
    16. Abstraction, Finally.
    17. Money Retrospective.

    II. The xUnit Example.

    18. First Steps to xUnit.
    19. Set the Table.
    20. Cleaning Up After.
    21. Counting.
    22. Dealing with Failure.
    23. How Suite It Is.
    24. xUnit Retrospective.

    III. Patterns for Test-Driven Development.

    25. Test-Driven Development Patterns.
    26. Red Bar Patterns.
    27. Testing Patterns.
    28. Green Bar Patterns.
    29. xUnit Patterns.
    30. Design Patterns.
    31. Refactoring.
    32. Mastering TDD.
    Appendix I: Influence Diagrams.
    Appendix II: Fibonacci.
    Index. 0321146530T10172002


    “Clean code that works” is Ron Jeffries’ pithy phrase. The goal is clean code that works, and for a whole bunch of reasons:

  • Clean code that works is a predictable way to develop. You know when you are finished, without having to worry about a long bug trail.
  • Clean code that works gives you a chance to learn all the lessons that the code has to teach you. If you only ever slap together the first thing you think of, you never have time to think of a second, better, thing.
  • Clean code that works improves the lives of users of our software.
  • Clean code that works lets your teammates count on you, and you on them.
  • Writing clean code that works feels good.But how do you get to clean code that works? Many forces drive you away from clean code, and even code that works. Without taking too much counsel of our fears, here’s what we do—drive development with automated tests, a style of development called “Test-Driven Development” (TDD for short).

    In Test-Driven Development, you:

  • Write new code only if you first have a failing automated test.
  • Eliminate duplication.

    Two simple rules, but they generate complex individual and group behavior. Some of the technical implications are:

  • You must design organically, with running code providing feedback between decisions
  • You must write your own tests, since you can’t wait twenty times a day for someone else to write a test
  • Your development environment must provide rapid response to small changes
  • Your designs must consist of many highly cohesive, loosely coupled components, just to make testing easy

    The two rules imply an order to the tasks of programming:
    1. Red—write a little test that doesn’t work, perhaps doesn’t even compile at first
    2. Green—make the test work quickly, committing whatever sins necessary in the process
    3. Refactor—eliminate all the duplication created in just getting the test to work

    Red/green/refactor. The TDD’s mantra.

    Assuming for the moment that such a style is possible, it might be possible to dramatically reduce the defect density of code and make the subject of work crystal clear to all involved. If so, writing only code demanded by failing tests also has social implications:

  • If the defect density can be reduced enough, QA can shift from reactive to pro-active work
  • If the number of nasty surprises can be reduced enough, project managers can estimate accurately enough to involve real customers in daily development
  • If the topics of technical conversations can be made clear enough, programmers can work in minute-by-minute collaboration instead of daily or weekly collaboration
  • Again, if the defect density can be reduced enough, we can have shippable software with new functionality every day, leading to new business relationships with customers

    So, the concept is simple, but what’s my motivation? Why would a programmer take on the additional work of writing automated tests? Why would a programmer work in tiny little steps when their mind is capable of great soaring swoops of design? Courage.


    Test-driven development is a way of managing fear during programming. I don’t mean fear in a bad way, pow widdle prwogwammew needs a pacifiew, but fear in the legitimate, this-is-a-hard-problem-and-I-can’t-see-the-end-from-the-beginning sense. If pain is nature’s way of saying “Stop!”, fear is nature’s way of saying “Be careful.” Being careful is good, but fear has a host of other effects:

  • Makes you tentative
  • Makes you want to communicate less
  • Makes you shy from feedback
  • Makes you grumpy

    None of these effects are helpful when programming, especially when programming something hard. So, how can you face a difficult situation and:

  • Instead of being tentative, begin learning concretely as quickly as possible.
  • Instead of clamming up, communicate more clearly.
  • Instead of avoiding feedback, search out helpful, concrete feedback.
  • (You’ll have to work on grumpiness on your own.)

    Imagine programming as turning a crank to pull a bucket of water from a well. When the bucket is small, a free-spinning crank is fine. When the bucket is big and full of water, you’re going to get tired before the bucket is all the way up. You need a ratchet mechanism to enable you to rest between bouts of cranking. The heavier the bucket, the closer the teeth need to be on the ratchet.

    The tests in test-driven development are the teeth of the ratchet. Once you get one test working, you know it is working, now and forever. You are one step closer to having everything working than you were when the test was broken. Now get the next one working, and the next, and the next. By analogy, the tougher the programming problem, the less ground should be covered by each test.

    Readers of Extreme Programming Explained will notice a difference in tone between XP and TDD. TDD isn’t an absolute like Extreme Programming. XP says, “Here are things you must be able to do to be prepared to evolve further.” TDD is a little fuzzier. TDD is an awareness of the gap between decision and feedback during programming, and techniques to control that gap. “What if I do a paper design for a week, then test-drive the code? Is that TDD?” Sure, it’s TDD. You were aware of the gap between decision and feedback and you controlled the gap deliberately.

    That said, most people who learn TDD find their programming practice changed for good. “Test Infected” is the phrase Erich Gamma coined to describe this shift. You might find yourself writing more tests earlier, and working in smaller steps than you ever dreamed would be sensible. On the other hand, some programmers learn TDD and go back to their earlier practices, reserving TDD for special occasions when ordinary programming isn’t making progress.

    There are certainly programming tasks that can’t be driven solely by tests (or at least, not yet). Security software and concurrency, for example, are two topics where TDD is not sufficient to mechanically demonstrate that the goals of the software have been met. Security relies on essentially defect-free code, true, but also on human judgement about the methods used to secure the software. Subtle concurrency problems can’t be reliably duplicated by running the code.

    Once you are finished reading this book, you should be ready to:

  • Start simply
  • Write automated tests
  • Refactor to add design decisions one at a time

    This book is organized into three sections.

  • An example of writing typical model code using TDD. The example is one I got from Ward Cunningham years ago, and have used many times since, multi-currency arithmetic. In it you will learn to write tests before code and grow a design organically.
  • An example of testing more complicated logic, including reflection and exceptions, by developing a framework for automated testing. This example also serves to introduce you to the xUnit architecture that is at the heart of many programmer-oriented testing tools. In the second example you will learn to work in even smaller steps than in the first example, including the kind of self-referential hooha beloved of computer scientists.
  • Patterns for TDD. Included are patterns for the deciding what tests to write, how to write tests using xUnit, and a greatest hits selection of the design patterns and refactorings used in the examples.

    I wrote the examples imagining a pair programming session. If you like looking at the map before wandering around, you may want to go straight to the patterns in Section 3 and use the examples as illustrations. If you prefer just wandering around and then looking at the map to see where you’ve been, try reading the examples through and refering to the patterns when you want more detail about a technique, then using the patterns as a reference.

    Several reviewers have commented they got the most out of the examples when they started up a programming environment and entered the code and ran the tests as they read.

    A note about the examples. Both examples, multi-currency calculation and a testing framework, appear simple. There are (and I have seen) complicated, ugly, messy ways of solving the same problems. I could have chosen one of those complicated, ugly, messy solutions to give the book an air of “reality.” However, my goal, and I hope your goal, is to write clean code that works. Before teeing off on the examples as being too simple, spend 15 seconds imagining a programming world in which all code was this clear and direct, where there were no complicated solutions, only apparently complicated problems begging for careful thought. TDD is a practice that can help you lead yourself to exactly that careful thought.



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