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What does it really mean to test an interface not the implementation? (lemm.ee)

submitted 1 year ago by pizzahoe@lemm.ee to c/programming@programming.dev

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let's say there is a Shape interface.

interface IShape { double Area(); }

A Rectangle class and a Triangle class implement it. Now should i write tests for:

IShape interface and test both implementations in a single test file?
Write tests for Rectangle and Triangle class separately, testing their implementation of Area() ?
Do something else?

From what I see I am testing implementations either ways. How do you even test an interface without testing the implementation? Can someone please help clarify my doubts? Thanks!

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[–] hoodlem@hoodlem.me 1 points 1 year ago

There is no functionality in the interface so there is nothing to test. You test the logic of the implemented methods.

With that said, with reflection you can at least test that an interface has the right method names and annotations present, but I’ve extremely rarely found it useful to do so.