This has all sort of benefits: I can instantly see if a class has a test. I can instantly jump from the test to the testee and vice-versa. I don't have to create two parallel hierarchies of folders. It is very unlikely that I will rename the production class but not its test. Renaming of a package affects both tests and production classes. etc. There is one word that summarizes these benefits: Locality. Things that change together ought to be placed as close as possible.
There are IDE plugins out there that provide similar capabilities over a project structure that has separate source folders. However, these plugins will not help you when you access your code not through your IDE (for instance, I often explore my code repository via a browser).
But even more importantly, as I got more and more (test) infected I realized that I don't want to have this test-production separation. The tests are not something external that needs to be put away. Tests are a central piece of knowledge about my code. I want them to be near by. Think about it: Would you separate your Javadoc text from the class/method it is describing? Will you find it productive to write code in a language which dictates that fields are defined in one file and methods are defined in another file? (Actually, this is pretty much what happens in C++ ...).
Of course not.
The main difficulty is the packaging/deployment phase. I often heard the argument that "we need to have two separate folders because otherwise our deliverable (.jar/.war/...) will include both production code and testing code, and this is bad".
Is it really bad? First, In many situations (web-apps anyone?) the size of the binary does not matter much. Second, in situations where the deliverable includes the source code, the tests can be quite handy as usage samples.
If you're in a situation where these arguments do not hold, and you absolutely cannot place test code in your deliverable, then the remainder of this post is for you.
I often said that it is not hard to write a program that will delete .class files of test classes. It requires some knowledge of bytecode inspection techniques which apparently is not very common. So, as a service to the Java community and for the common good, I cleared a few hours and wrote it. The result is class-wiper (sources) a command line utility that will recursively scan the given directories and will delete all .class files therein that are related to JUnit.
Specifically, it will delete a class if either: (a) it mentions the @Test annotation; or (b) It uses directly or indirectly a class that mentions @Test. Your production code will never meet neither of these conditions. If you don't want test classes to reach your client you just need to invoke it from your build script, as follows:
java -jar class-wiper <binary-dir1> <binary-dir2> ...
This software is provided absolutely for free. Use it anyway you like. Change it. Tweak it. Hack it. Sell it. Whatever. I only ask one simple thing: please stop saying that you need to place your tests in a separate directory because you don't have a way to prevent your tests from reaching the deliverable.