Programming

That advice does not literally refer to interface the programming language feature. It means to test the observable behavior of a component, not internal implementation details.

In your example, write tests for both Rectangle and Triangle that call area, and assert the result is correct. But do not test, for example, the order of mathematical operations that were run to calculate the result. The details of the math are an internal detail, not part of the "interface".

Testing interface is not about exactly interface as language’s feature. It’s about testing outer public interface (public API) of you things, looking at it as a black-box and doesn’t matter what happens inside. That's all it means.

It's a form of Black-Box Testing, essentially you want to validate expected behavior. Implementation can change, but your outcome should remain the same.

This is a big target for Test Driven Development, since your first step is to write the test with the expected outcome, then you write the most basic implementation, and when you can verify the behavior, then you are free to re-factor to improve implementation knowing your test will tell you if the behavior changes with each internal change.

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.