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In Java, HelloWorld is a one-liner -- except that you are trapped in the Kingdom of Nouns, so there is boilerplate to manage.
Now you can implement HelloWorld in a perfectly natural way, and test it -- System.setOut allows you to replace the stream, so the write happens to a buffer that is under the control of the test.It's not entirely clear to me what happens, however, if you have multiple tests concurrently writing to that stream. The synchronization primitives ensure that each write is atomic, but there is a lot of time for the stream to be corrupted with other writes by the time the test harness gets to inspect the result. This is why we normally design our tests so that they are isolated from shared mutable state; we want predictable results. So in HelloWorld, this means we need to be able to ensure that the write happens to an isolated, rather than a shared stream. So instead of testing HelloWorld::main, we end up testing HelloWorld.writeTo, or some improved spelling of the same idea. Another pressure that shows up quickly is duplication - the byte sequence we want to test needs to be written into both the test and the implementation. Again, we've learned patterns for dealing with that -- the data should move toward the test, so we have a function that accepts a message/prompt as an argument (in addition to passing along the target stream). As an added bonus, we get a more general implementation for free. Did we really need a more general implementation of HelloWorld? Another practice that I associate with TDD is using the test as an example of how the subject may be used -- if the test is clumsy, then that's a hint that maybe the API needs some work. The test needs a mutable buffer, and a PrintStream around it, and then needs to either unpack the contents of the buffer or express the specification as a byte array, when the natural primitive to use is a String literal. You can, indeed, simplify the API, replacing the buffer with a useful object that serves a similar role. At which point you either have two parallel code paths in your app (duplication of idea), or you introduce a bunch of additional composition so that the main logic always sees the same interface. Our "testable" code turns more and more into spaghetti. Now, it's possible that I simply lack imagination, and that once all of these tests are in place, you'll be able to refactor your way to an elegant implementation. But to me, it looks like a trash fire.
There's a lesson here, and I think it is: left-pad.Which is to say, not only is HelloWorld "so simple that there are obviously no deficiencies", but also that it is too simple to share; which is to say, the integration cost required to share the element exceeds the costs of writing it from scratch each time you need it. Expressed a different way: there is virtually no chance that the duplication is going to burn you, because once written the implementation will not require any kind of coordinated future change (short of a massive incompatibility being introduced in the language runtime itself, in which case you are going to have bigger fires to fight).