Mocking Concrete Classes in Java

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One approach to interaction testing is to mock concrete classes rather than interfaces The technique is to inherit from the class you want to mock and override the methods that will be called within the test, either manually or with any of
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the mocking frameworks We think this is a technique that should be used only when you really have no other options Here s an example of mocking by hand The test veri es that the music centre starts the CD player at the requested time Assume that setting the schedule on a CdPlayer object involves triggering some behavior we don t want in the test, so we override scheduleToStartAt() and verify afterwards that we ve called it with the right argument
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public class MusicCentreTest { @Test public void startsCdPlayerAtTimeRequested() { final MutableTime scheduledTime = new MutableTime(); CdPlayer player = new CdPlayer() { @Override public void scheduleToStartAt(Time startTime) { scheduledTimeset(startTime); } } MusicCentre centre = new MusicCentre(player); centrestartMediaAt(LATER); assertEquals(LATER, scheduledTimeget()); } }
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The problem with this approach is that it leaves the relationship between the and MusicCentre implicit We hope we ve made clear by now that our intention in test-driven development is to use mock objects to bring out relationships between objects If we subclass, there s nothing in the domain code to make such a relationship visible just methods on an object This makes it harder to see if the service that supports this relationship might be relevant elsewhere, and we ll have to do the analysis again next time we work with the class To make the point, here s a possible implementation of CdPlayer:
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public class CdPlayer { public void scheduleToStartAt(Time startTime) { [ ] public void stop() { [ ] public void gotoTrack(int trackNumber) { [ ] public void spinUpDisk() { [ ] public void eject() { [ ] }
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It turns out that our MusicCentre only uses the starting and stopping methods on the CdPlayer; the rest are used by some other part of the system We would be overspecifying the MusicCentre by requiring it to talk to a CdPlayer; what it actually needs is a ScheduledDevice Robert Martin made the point (back in 1996) in his Interface Segregation Principle that Clients should not be forced to depend upon interfaces that they do not use, but that s exactly what we do when we mock a concrete class
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Don t Mock Values There s a more subtle but powerful reason for not mocking concrete classes When we extract an interface as part of our test-driven development process, we have to think up a name to describe the relationship we ve just discovered in this example, the ScheduledDevice We nd that this makes us think harder about the domain and teases out concepts that we might otherwise miss Once something has a name, we can talk about it
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There are a few occasions when we have to put up with this smell The least unacceptable situation is where we re working with legacy code that we control but can t change all at once Alternatively, we might be working with third-party code that we can t change at all (see 8) We nd that it s almost always better to write a veneer over an external library rather than mock it directly but occasionally, it s just not worth it We broke the rule with Logger in 19 but apologized a lot and felt bad about it In any case, these are unfortunate but necessary compromises that we would try to work our way out of when possible The longer we leave them in the code, the more likely it is that some brittleness in the design will cause us grief Above all, do not override a class internal features this just locks down your test to the quirks of the current implementation Only override visible methods This rule also prohibits exposing internal methods just to override them in a test If you can t get to the structure you need, then the tests are telling you that it s time to break up the class into smaller, composable features
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