Difference between JUnit Theories and Parameterized Tests
What is the difference between a Theory and a Parameterized test?
I'm not interested in implementation differences when creating the test classes, just when you would choose one over the other.
Answer
From what I understand: With Parameterized tests you can supply a series of static inputs to a test case.
Theories are similar but different in concept. The idea behind them is to create test cases that test on assumptions rather than static values. So if my supplied test data is true according to some assumptions, the resulting assertion is always deterministic. One of the driving ideas behind this is that you would be able to supply an infinite number of test data and your test case would still be true; also, often you need to test an universe of possibilities within a test input data, like negative numbers. If you test that statically, that is, supply a few negative numbers, it is not guaranteed that your component will work against all negative numbers, even if it is highly probable to do so.
From what I can tell, xUnit frameworks try to apply theories' concepts by creating all possible combinations of your supplied test data.
Both should be used when approaching a scenario in a data-driven scenario (i.e only inputs change, but the test is always doing the same assertions over and over).
But, since theories seem experimental, I would use them only if I needed to test a series of combinations in my input data. For all the other cases I'd use Parameterized tests.