The TDD circle is:
"Write failing Test" -> "Write Code to fit a Test" -> "Refactor"
At "Coding" step we are assumed to write code as simple as possible, just to fix a failing test. We should not write complex code until it is really required.
The next step is Refactor. Should we refactor just written code? I think there is no real sence, as we should be happy with code as far as tests are passing.
Probably Refactoring activity should be forced by some thing, like Code writing is foced by failing Tests. Here some possible factors:
The next test to be written requires some changes in system (refactoring)
Performance is bad. We need to improve it without breaking the functionality
Code review reveals that the code written is hard to understand.
What other reasons you see to start the Refactoring?
Also, is this scheme correct:
"Write failing Test" -> "Code" -> "Refactor" -> "Write failing Test"
or may be it should be seen as
"Write failing Test" -> "Code/Refactor" -> "Write failing Test"
+
"External factor (like bad performance)" -> "Refactor".
You can write some pretty ugly code while getting a test to pass; refactor now not because it doesn't work, but it's not very maintainable. That's the point case.
After writing code to fit several tests, you can start taking a bigger picture look -- are there overlaps between those pieces of code, where you can factor out some duplication?
TDD is a great tool to keep you on track/on task. The problem with:
"Write failing Test" -> "Code/Refactor" -> "Write failing Test"
you propose, is it can easily become:
"Write failing Test" -> "Refactor" -> "Code" -> "Write failing Test"
or then
"Write failing Test" -> "Refactor" -> "Refactor" -> "Refactor" -> "Code" -> "Write failing Test"
which is what you want to avoid. By refactoring at the beginning of implementation, you are indulging in speculative development, and, not achieving the goal of the coding session. It's easy to head off on tangents and build things you don't necessarily need. If you have the feature working and tests passing, it's much easier to decide when's the right time to stop refactoring. And you can stop at any time because your tests are passing.
Additionally, you don't want to refactor when your tests aren't green.
A couple other small pts:
I think most of the literature has a slightly different definition what refactoring is. It's not "some changes to the system" or performance enhancements, but specific changes that don't change the behavior but improve the design. If you accept the definition, then performance improvements don't really qualify: they are normal development tasks that need their own acceptance tests. I usually try to frame these as end-user facing stories, where the benefit of doing them is clear. Make sense?
I think you're right that the TDD practice doesn't specifically address the design problems revealed during code reviews. (See reflections and pair programming for other solutions to this.) These tend to be bigger, cross-story issues built up as "code debt", and have to take some time to clean it up periodically. This could be a separate project, but I, personally, always like to do this as part of another "real" story. Last time I did this, we identified we had a problem, but ended up waiting a few weeks until we had a related story to work on it. We followed the TDD practice of implementing the new feature first-- even though we knew it was way wrong. But then we really understood what was going on and why it was messy, and then spent longer than usual on the refactor phase. Worked well.
Hmm... I usually consider these sort of "external" refactorings separate from the TDD cycle itself. After finishing up feature XYZ using TDD, one ought to have a healthy set of tests to prevent from introducing bugs via refactoring (assuming code coverage is optimal, etc.). Performance bottlenecks and hard-to-understand code usually crop up after the fact anyway, so doing refactoring at that point would be ideal, I think. You can improve performance, make the code easier to understand, and do whatever else that needs doing while using the tests to make sure that you're not introducing bugs into the system.
So to answer your question, I do not believe that external refactoring fits into the TDD pattern, although the identifiers (code smells if you will) are definitely items to keep track of as you are developing the code.
Other than refactorings triggered by the need to improve performance, I usually will notice while I am writing the code to pass a test a place where I could share code or a better, more elegant way to accomplish something. At this point, you finish the code to pass the test, make sure all tests are passing, then go back and do your refactorings. This may or may not involve the exact code that you wrote, most likely it will, but it may be something that you just noticed while writing the new code.
Sometimes I'll note something that could be refactored, but decide that what I'm working on right now is more important. At that point I'll note the issue for a later refactoring. Eventually, that refactoring will become as or more important than the next feature and it will get done in the refactor phase of a red/green/refactor cycle. In this case, it probably is unrelated to what I was just working on.
Is there a bad time to refactor other than a week or two before a major release?
Rule of Three:
When you are doing something for the first time, just get it done.
When you are doing something similar for the second time, cringe at having to repeat but do the same thing anyway.
When you are doing something for the third time, start refactoring.
When adding a feature:
Refactoring helps you understand other people's code. If you have to deal with someone else's dirty code, try to refactor it first. Clean code is much easier to grasp. You will improve it not only for yourself but also for those who use it after you.
Refactoring makes it easier to add new features. It's much easier to make changes in clean code.
When fixing a bug:
Bugs in code behave just like those in real life: they live in the darkest, dirtiest places in the code. Clean your code and the errors will practically discover themselves.
Managers appreciate proactive refactoring as it eliminates the need for special refactoring tasks later. Happy bosses make happy programmers!
During a code review:
The code review may be the last chance to tidy up the code before it becomes available to the public.
It's best to perform such reviews in a pair with an author. This way you could fix simple problems quickly and gauge the time for fixing the more difficult ones.
Related
I need some advice on how to work with legacy code.
A while ago, I was given the task to add a few reports to a reporting app. written in Struts 1, back in 2005. No big deal, but the code is quite messy. No usage of Action forms, and basically the code is one huge action, and a lot of if-else statements inside. Also, no one here has functional knowledge on this. We just happened to have it in our contract.
I'm quite unhappy about this, and not sure how to proceed. This application is invisible: Few people (but all very important) use it, so they don't care whether my eyes bleed while reading the code, standards, etc.
However, I feel that a technical debt is to be paid. How should I proceed on this? Continue down the if-else road, or try to do this requirement the right way, ignoring the rest of the project? Starting a huge refactor, risking my deadline?
Legacy code is a big issue, and I'm sure people will not agree!
I would say that starting a big re-factor could be a mistake.
A big re-factor means doing a lot of work to make it function exactly the way that it does now. If you choose to take this on on your own, there won't be a lot of visibility of what you are doing. If it works, no one will know the hours of work you put it. If it does NOT work, and you end up with tidy code, but add some bugs (and who has ever written code without adding some bugs) then you will get 'why did this change' type questions.
I have currently nearly completed a project working on a 10 year old code base. We have done quite a few bits of re-factoring along the way. But for each re-factor we have made we can justify 'this specific change will make the actual task we are doing now easier'. Rather than 'this is now cleaner for future work'. We have found that as we worked on the code, fixing the issues that we actually come up against one at a time, we have cleaned up a lot of it, without breaking it (much).
And I would say before you can re-factor much, you will need automated tests, so you can be fairly happy that you have put it back together right!
Most re-factoring is done to 'make maintenance and future development easier'. Your project sounds like there is not a lot of future development coming. That limits the advantage a re-factor will give the company.
Rule #1: If it ain't broke, don't fix it.
Rule #2: When in doubt, reread rule #1.
Unfortunately, legacy code can very rarely be described as "it ain't broke." Therefor we must tweak the existing code to correct a newly found bug, tweak the existing code to modify behavior that was previously acceptable, or tweak the existing code to add new functionality.
My experience has taught me that any refactoring must be done in 'infinitesimally' small increments. If you must break rule #2, I suggest that you start your search with the inner-most nested loop or IF structure and expand outward until you find a clean, logical separation point and create a new function/method/subroutine that contains only the guts of that loop or structure. This won't make anything more efficient but it should give you a clearer view of the underlying logic and structure. Once you have several new, smaller functions/methods/subroutines you can refactor and consolidate those into something more manageable.
Rule #3: Ignore my previous paragraph and reread the first two rules.
I agree with other comments. If you don't have to, then don't do it. It usually cost far more then it's worth if the code base is more or less dead any way.
On the other hand, if you feel that you cannot get your head around the code then a refactor is probably unavoidable. If this is the case then, since it's a web application, can you create a solid suite of functional tests using selenium? If so this is the fastest and most rewarding test approach for such code and will catch most bugs for the buck.
Second, start with the extract method refactoring to create compose methods of the big difficult methods. Every time you think to your self "This should have a comment to explain what it does" you should extract it to a method with a name that replaces the comment.
Once this has been done, if you still can't add the functionality required, you can go for more advanced refactorings, and perhaps even adding some unit tests. But I usually find that I can add what is required/fix the bug in legacy code by just creating self documenting code.
In a few words: before make any modifications to legacy code its good idea to start from automated unit tests.
This will give developer understanding about key things: dependencies this piece of code has, input data, output results, boundary conditions so on.
When it’s done most likely you will better understand what this code does and how it works.
After that its make sense (but not must) clean code a bit giving more accurate names to local variables, moving some functionality (repetitive code, if any) in to functions with clear human friendly names.
A simple clean up could make code more readable and at the same time save developer from regression issues with unit tests help.
Refactoring – make small changes, step by step, when you have a time and understanding of the requirements and functionality, regularly unit testing the code.
But do not start from refactoring
What do you do when you're assigned to work on code that's
atrocious and antiquated to the point where it's almost incomprehensible?
For example: hardware interface code, mixed with logic, AND user interface code, ALL in the same functions?
We see bad code all the time, but what do you actually do about it?
Do you try to refactor it?
Try to make it OO if it's not?
Or do you try to make some sense of it, make the necessary changes and move on?
Depends on a few factors for me:
Will I be maintaining this code in the future, or is it a one-off fix?
How long until this system is replaced entirely?
How busy am I at the moment?
Ideally, I'd refactor all bad code I had to maintain, but the reality is there are only so many hours in the day.
As is frequently the case, "It Depends".
I tend to ask myself some of the following questions:
Are there unit tests for the existing code?
Is refactoring the code an acceptable risk for my project?
Is the author still available to clarify any questions I might have about the code?
Will my employer consider the time spent on changing existing, functioning code to be an acceptable use of my time?
And so on...
But assuming that I have the capacity to do so, refactoring is preferential as the up front cost of fixing the code now will likely save me a lot of time and effort later in maintenance and development time.
There are other benefits as well, including the fact that the more clean and well maintained you keep your code base, the more likely other developers are to keep it that way. The Pragmatic Programmer calls this the Broken Window Theory.
Developers have an instinct to assume that code is always ugly because of other, inferior developers. Sometimes, code is ugly because the problem space is ugly. All that ugliness isn't just ugliness - it is sometimes institutional memory. Each line of ugly in your code probably represents a bug fix. So think very carefully before you rip it all out.
Basically, I would say that you shouldn't touch code like this unless you actually have to. If there's a real bug that you can solve, refactoring is reasonable, if you can be sure you're maintaining the same amount of functionality. But refactoring for the sake of refactoring (eg, "make the code OO") is what I would generally classify as a classic newbie mistake.
The book Working Effectively with Legacy Code discusses the options you can do. In general the rule is not to change code until you have to (to fix a bug or add a feature). The book describes how to make changes when you can't add testing and how to add testing to complex code (which allows more substantial changes).
You try to refactor it, in the strict sense on the word, where you're not changing the behaviour.
The first target is usually to break up giant methods.
Given the strength of some of the adjectives you use, i.e. atrocious, antiquated and incomprehensible, I'd bin it!
If it is in such a state, like the example you give, it's probably not got any test code for it either. Refactoring is mentioned in many of the other answers but, sometimes, it is not appropriate. I always find that, when refactoring, you generally need a clear path through which the old code can be gradually morphed into the new in a number of well defined steps.
When the old code is so far removed from how you want it to look, such as the extreme cases you seem to be suggesting, you could probably redesign, rewrite and test the new code in a shorter time than it would to take to refactor it.
Scrap it and start over, using the compiled legacy application as a business requirements document.
And spending time in analysis with the users to see what they want changed.
Post it to www.worsethanfailure.com!!!
If no modifications are needed, I don't touch it.
If at all possible, I write automated unit tests first, especially focused on the areas that need modification.
If automated unit tests are not possible, I do what I can to document manual unit tests.
I am just using the tests to document "current" behavior at this point.
If possible, I always keep a version of the code and executable environment that runs things the "original" way (before I touched it) so I can always add new "behavior documentation" tests and better detect regressions I may have caused later.
Once I start changing things, I want to be very careful not to introduce regressions. I do this by continually rerunning (and or adding new tests) to the tests I wrote before I started writing code.
When possible, I leave bugs as-is if there is no business need for them to be fixed. Those bugs may be "features" to some users and may have unclear side effects that wouldn't be clear until the code was redeployed to production.
As far as refactoring, I do that as aggressively as possible, but only in the code that I need to change otherwise anyway. I may refactor more aggressively in my own personal copy of the code that will never be checked in, just to improve the readability of the code for me personally. It's often times difficult to properly test changes that are only made for readability reasons, so for safety reasons, I generally don't check those changes in / deploy them unless I can confidently test that the code changes are completely safe (it's really bad to introduce bugs when you are making changes that are unnecessary for anything but readability).
Really, it's a risk management problem. Proceed with caution. The users do not care if the code is atrocious, they just care that it gets better without getting worse. Your need for beautiful code is not important in this scenario, get past it.
Just like any other code, you leave it slightly better when you leave it than it was when you entered it. You do not ever, ever rewrite the whole code. If that is the work it takes for some reason, then you start a project (small or large) for it.
I am assuming we are talking about a substantial amount of code here.
Not every day is a great day at work you know :)
The first question to ask is: does it work?
If the answer is yes, that would be a huge disincentive to simply ditch it and start over. There may be thousands of man-hours in that code which address edge cases and nasty bugs. Worse yet, there may be other modules in the system that depend on the current incorrect (but known and possibly documented) behavior. Don't mess with it if it isn't broken.
If you are keen on cleaning it up, start by writing test cases for the current behavior. When you run across an instance where the behavior differs from the specification, you must decide whether to accept the behavior as "correct" or go with what the spec say it ought to do.
Only once you have written test cases that all pass should you begin to refactor. The tests will tell you whether your efforts are breaking anything.
I'd talk to my manager and describe the code. Most managers would not want a program held together by banding wire and duct tape per se. If the code is really that bad there are sure to be some business logic errors, hardcoding etc. stuffed in there that will eventually just destroy productivity.
I've come across some pretty bad code before (single letter variable names, no comments, everything crammed onto one line, etc.) and once I mentioned/showed it to my manager they almost always said "go ahead and re-write it", because not only are you taking the hit for reading and changing the code but future co-workers will have to go through the same pain. Better that you take a longer period of time just once to rewrite it rather than having each person who touches the code in the future have to go through and comprehend and decipher it first.
There is an old saying. If is isn't broke, do not fix it. If you have to maintain it then reverse engineer it and document it so the next time you come across it you will know what it does.
You do not know the situation the developer was in when he or she wrote the code. He or she may have been under a time crunch when it was written, (management was all over the developer, etc)
There are also situations where he or she wrote the code per the spec, The spec then changed several times, the developer had to patch the code, as rewrite is out of the question due to time constraints. This happens all of the time.
If the code impacts the performance of robustness of the application and is modular then you can re factor or re-write. Document the situation to assist future programmers in understanding.
Also many programmers consider reverse engineering other developers code as beneath them.
they would rather rewrite without considering the ramifications of doing so.
If you have never done so, try it sometime, it will make you a better developer.
Thanks
Joe
Kill it with fire.
Depends on your time frame and how important that code is to you. If you have to "just make it work" then do that and rewrite the module when time allows.
If its an important or integral part of what you do then refactor refactor refactor.
Then find the guy/girl who wrote it and send them a rude postcard!
The worst offender (in my experience) of really AWFUL code is the ease with which people can do cut & paste these days. Cut & paste should be used rarely. If you think that's the right solution, it's generally better to step back and generalize the problem a little.
Anytime you see code that is "nearly incomprehensible", PROCEED WITH CAUTION. You need to assume that any major re-factoring will result in new bugs being introduced that you'll need to find and correct.
Additionally, I've seen this scenario many times (even fell victim to it myself once or twice): Programmer inherits legacy code, decides code is ancient & unmaintainable and decides to refactor it, ends up deleting key "fixes" or "business rules" subtly patched in over the years, ends up spending a lot of time tracking down and re-introducing similar code when users complain about "a problem fixed years ago is happening again".
Re-factoring (and debugging) almost always takes longer than expected and should never be considered as a "freebie" that comes along with whatever task you're supposed to be doing.
"If it ain't broke, don't 'fix' it" still has a lot of truth.
Im my company we always Refactor Mercilessly. so we still come across atrocious code but LESS and Less and less ...
We write a lot of in-house code and the company is run for about 100 years by the same family. Management usually tells us we have to maintain the code base (evolve) for another 50 years or so. In this setting having code you don't dare to touch is considered a bigger risk to the long term survival of the company then the prospect of downtime because some under-tested code broke because of refactoring.
I run copy-paste detector and findbugs on all legacy code that comes my way.
I then plan my initial refactoring:
remove unused code, unused variable and unused methods
refactor duplicated code
set up a single step build
build a basic functional test
By that point the code meets the basic minimum for maintainability. It can be easily built and basic errors can be found via an automated test.
I often add code like this:
log.debug("is foo null? " + (foo == null));
log.debug("is discount < raw price ? " + (foo.getDiscount() < foo.getRawPrice()));
Some of that code will be recovered for unit tests when I can refactor to it.
I've worked places where we ship that kind of code.
I try to make sense of it, make the necessary changes, and move on.
Of course, making sense of it usually involves some changes; at the very least, I move around the whitespace and line up corresponding braces in the same column like so:
if(condition){
doSomething(); }
// becomes...
if(condition)
{
doSomething();
}
I'll also often change variable names.
And very often, "the necessary changes" require refactoring. :)
Get the idea of what they're doing and the deadline to finish. A larger deadline, typically rebuild much of the code from the ground up, as I find it a very worthwhile experience to not only decipher terrible code and make it legible and document, but somewhere in your brain those neurons are pressed to avoid similar mistakes in the future.
In doing test driven development I have been in the habit of writing the first unit test for a new piece of functionality first, then writing the code for that functionality. If I have additional tests to write to cover all scenarios, I usually write them after the code is written. Is this considered bad form? Should I try and write every conceivable test for a piece of functionality first, before ever writing that code?
In order to do TDD properly, you always write the test first, and then the functionality second.
To add to that, I would take one scenario at a time, don't write 20 tests and then write the code for those 20 tests. Write one test, red/green flag it, then move on to your next test. This makes sure you're also doing one of the core tenets of TDD, which is to do the simplest implementation possible that meets all of your requirements/scenarios.
actually no, I often discover functionality "on-the-go". Let me explain the "no" a bit further:
I usually start out writing a test case for a high level feature, defining its Interface. After that, I usually set this test to ignore and continue writing tests for each of the Interfaces functionality. My cycle goes like:
Integration Test for Story A (high level API)
Write Unit Test for method xyz called in Integration Test
Implement method (red/green/refactor)
Repeat 2+3 till Integration Test passes.
While doing so, I often realize I have forgotten some small functionality in my main test. I then usually take time to look back at my customers requirements. If its a fit, I go back and add a test for it, set to ignored as I first want to finish what I started.
Sometimes I see the chance to do a refactoring. I usually finish an implementation till I reach a commit point and do refactoring then, however sometimes I stash my changes, go back and do the refactoring (including new tests if nescessary) first. This workflow is powererd by Mercurial MQ.
For most people, TDD and incremental/agile development go together. This looks something like:
Write a test for some feature
Write just enough code to make the test pass, refactoring as necessary
Repeat.
If you happen to have a detailed specification ahead of time, you could write all of the tests first, but you'd have to live with having sone tests not passing for a while.
The sooner you write the tests, the better. I usually find writing tests being harder tasks than actually implementing the functionality because you have to be aware of all the possible outcomes. So I tend to write more tests when I'm "in the zone". And when during coding I realize I might have missed a test case I just note that down on the to-do lists.
So in my opinion it's up to your leisure but I would implement tests in multiple batches.
The way I see it, test driven development isn't necessarily tests first development. Your tests drive your development and you are really writing your tests as you develop your application. You start by writing a simple test that fails because you haven't written the functionality yet. Then you write your code to implement that so that the tests pass.
Then you go back to your test, make modifications that will force you to add more functionality or refactor your code to follow better practices or reduce duplicate code, go fix your code to make the test pass...repeat, repeat, repeat.
A couple of videos that demonstrates this is below, although you can probably find a lot more by googling "TDD Video"
http://agilesoftwaredevelopment.com/videos/test-driven-development-basic-tutorial
(oops, only one video, new users can't insert more than one link)
I try to write a test at some level before each bit of functionality. Sometimes, I have to write a little more code to get through the compiler, but I try to minimise that. Writing the test first means that I've thought about what the code is supposed to achieve before writing it.
One technique I find useful is to keep an index card or notepad handy, and make a note of all the cases that I think of along the way. That allows me to focus on the current task without losing track of all the other things I'm supposed to think about. Afterwards, I can work through the list and either complete the extra cases or drop them as not necessary.
You could do that, but you wouldn't be doing TDD. The problem (well, one of them, anyway) with writing all of your tests up front is that in any case where the requirements are non-trivial, your tests will be building in a lot of assumptions about the structure of the code you're test-driving. Big steps lead to missteps.
One of the keys of successful TDD involves taking small steps. Small steps mean fewer changes to back out when something goes wrong. Small steps mean you can more often get your head around the effects of the changes you're making. And because small steps are easier to take with confidence, they have the paradoxical effect of increasing your velocity.
The TDD cycle starts with requirements. Start by choosing a requirement you know how to define through tests immediately, in a few short steps. If you look at a requirement and you're not sure how to test it, or you think, "Yeah, but to do that, I'd need to [insert ill-defined steps] first", then you should either skip to another requirement that you know how to do, or you should break this requirement into smaller requirements that you know how to do.
Once you have that, you work in a short red-green-refactor cycle: Write a test that quantifies some part of the requirement ("red", because it fails, because it has no implementation to test yet), write any code that will pass the test ("green"), then rework the code to remove duplication, magic numbers, and other code smells ("refactor"). During the refactoring phase, you should continue working in small steps, frequently re-running the test to make sure you haven't broken anything. Continue this cycle until you can look your boss/client in the eye and call the requirement met.
Now that you have one simple piece of your system defined, you've opened up the list of requirements available to implement - requirements that are adjacent to or dependent on the one you just implemented can now be tested and implemented in smaller steps building on what you've already done.
So the upshot of all that is: Don't try to do all your tests at once. One (small) thing at a time.
The point of TDD is that you have to observe that test fails when feature is not yet implemented. So you have to write test before code.
When you get into the TDD rhythm you write one test at a time and make it work. Very short red-green-refactor cycles really feel the rhythm. That being said, there is nothing wrong with other approaches (and they may even make more sense for some types of problems) but typically the only thing you need to do about other tests you are thinking of is write them down (or have your pair if you are pair programming write them down) so you don't forget them. You have to do that anyway because you could forget about a test in the middle of writing a different test.
Do just enough tests to test 1 unit of code at a time.. then write the actual code until it passes the test.. rinse, wash, repeat until done.
If you find yourself needing to write many tests for one unit of code ( a method, a function etc) it might be a sign that you are trying to do too much in that unit... which in turn makes the unit dificult to test & to refactor at a later time.
Imagine you are implementing the user story containing various new features and adding complexity to the code base. The existing code is quite well covered and you have just decided upon interfaces. You are starting to implement the functionality starting with tests.
Now you have fairly complex test cases based on the requirements but the implementation is nowhere near the point when you are able to commit to the SCM fully working code and many test are failing (as they should).
There is an assumption that in continuous integration all builds should be green if possible and thus you shouldn't commit as you would break the build. But you also shouldn't "Go dark" and keep such amount of code for yourself...
What is the suggested procedure in such situation?
Do not decide on all interfaces beforehand. Develop incrementally in a typical TDD rhythm: write a test; make the test pass; refactor. That should keep everything in good shape, the bar will always be green and you can check code in without worrying that you will break the build.
It requires a different style of writing code, but you will get used to the rhythm eventually.
What about skipping those tests that you know won't pass because the functionality is currently missing?
Make it obvious that you are skipping the tests too! Really make it scream "like a stuck pig", as they say in Oz! (-:
As you add functionality, enable the associated tests and keep "your bar green!"
Here's another great article over at The Pragmatic Programmers that covers making broken windows obvious to others.
HTH
cheers,
Rob
When doing TDD, how to tell "that's enough tests for this class / feature"?
I.e. when could you tell that you completed testing all edge cases?
With Test Driven Development, you’ll write a test before you write the code it tests. Once you’re written the code and the test passes, then it’s time to write another test. If you follow TDD correctly, you’ve written enough tests once you’re code does all that is required.
As for edge cases, let's take an example such as validating a parameter in a method. Before you add the parameter to you code, you create tests which verify the code will handle each case correctly. Then you can add the parameter and associated logic, and ensure the tests pass. If you think up more edge cases, then more tests can be added.
By taking it one step at a time, you won't have to worry about edge cases when you've finished writing your code, because you'll have already written the tests for them all. Of course, there's always human error, and you may miss something... When that situation occurs, it's time to add another test and then fix the code.
Kent Beck's advice is to write tests until fear turns into boredom. That is, until you're no longer afraid that anything will break, assuming you start with an appropriate level of fear.
On some level, it's a gut feeling of
"Am I confident that the tests will catch all the problems I can think of
now?"
On another level, you've already got a set of user or system requirements that must be met, so you could stop there.
While I do use code coverage to tell me if I didn't follow my TDD process and to find code that can be removed, I would not count code coverage as a useful way to know when to stop. Your code coverage could be 100%, but if you forgot to include a requirement, well, then you're not really done, are you.
Perhaps a misconception about TDD is that you have to know everything up front to test. This is misguided because the tests that result from the TDD process are like a breadcrumb trail. You know what has been tested in the past, and can guide you to an extent, but it won't tell you what to do next.
I think TDD could be thought of as an evolutionary process. That is, you start with your initial design and it's set of tests. As your code gets battered in production, you add more tests, and code that makes those tests pass. Each time you add a test here, and a test there, you're also doing TDD, and it doesn't cost all that much. You didn't know those cases existed when you wrote your first set of tests, but you gained the knowledge now, and can check for those problems at the touch of a button. This is the great power of TDD, and one reason why I advocate for it so much.
Well, when you can't think of any more failure cases that doesn't work as intended.
Part of TDD is to keep a list of things you want to implement, and problems with your current implementation... so when that list runs out, you are essentially done....
And remember, you can always go back and add tests when you discover bugs or new issues with the implementation.
that common sense, there no perfect answer. TDD goal is to remove fear, if you feel confident you tested it well enough go on...
Just don't forget that if you find a bug later on, write a test first to reproduce the bug, then correct it, so you will prevent future change to break it again!
Some people complain when they don't have X percent of coverage.... some test are useless, and 100% coverage does not mean you test everything that can make your code break, only the fact it wont break for the way you used it!
A test is a way of precisely describing something you want. Adding a test expands the scope of what you want, or adds details of what you want.
If you can't think of anything more that you want, or any refinements to what you want, then move on to something else. You can always come back later.
Tests in TDD are about covering the specification, in fact they can be a substitute for a specification. In TDD, tests are not about covering the code. They ensure the code covers the specification, because the code will fail a test if it doesn't cover the specification. Any extra code you have doesn't matter.
So you have enough tests when the tests look like they describe all the expectations that you or the stakeholders have.
maybe i missed something somewhere in the Agile/XP world, but my understanding of the process was that the developer and the customer specify the tests as part of the Feature. This allows the test cases to substitute for more formal requirements documentation, helps identify the use-cases for the feature, etc. So you're done testing and coding when all of these tests pass...plus any more edge cases that you think of along the way
Alberto Savoia says that "if all your tests pass, chances are that your test are not good enough". I think that it is a good way to think about tests: ask if you are doing edge cases, pass some unexpected parameter and so on. A good way to improve the quality of your tests is work with a pair - specially a tester - and get help about more test cases. Pair with testers is good because they have a different point of view.
Of course, you could use some tool to do mutation tests and get more confidence from your tests. I have used Jester and it improve both my tests and the way that I wrote them. Consider to use something like it.
Kind Regards
Theoretically you should cover all possible input combinations and test that the output is correct but sometimes it's just not worth it.
Many of the other comments have hit the nail on the head. Do you feel confident about the code you have written given your test coverage? As your code evolves do your tests still adequately cover it? Do your tests capture the intended behaviour and functionality for the component under test?
There must be a happy medium. As you add more and more test cases your tests may become brittle as what is considered an edge case continuously changes. Following many of the earlier suggestions it can be very helpful to get everything you can think of up front and then adding new tests as the software grows. This kind of organic grow can help your tests grow without all the effort up front.
I am not going to lie but I often get lazy when going back to write additional tests. I might miss that property that contains 0 code or the default constructor that I do not care about. Sometimes not being completely anal about the process can save you time n areas that are less then critical (the 100% code coverage myth).
You have to remember that the end goal is to get a top notch product out the door and not kill yourself testing. If you have that gut feeling like you are missing something then chances are you are have and that you need to add more tests.
Good luck and happy coding.
You could always use a test coverage tool like EMMA (http://emma.sourceforge.net/) or its Eclipse plugin EclEmma (http://www.eclemma.org/) or the like. Some developers believe that 100% test coverage is a worthy goal; others disagree.
Just try to come up with every way within reason that you could cause something to fail. Null values, values out of range, etc. Once you can't easily come up with anything, just continue on to something else.
If down the road you ever find a new bug or come up with a way, add the test.
It is not about code coverage. That is a dangerous metric, because code is "covered" long before it is "tested well".