Why is design-by-contract not so popular compared to test-driven development?

Question

You may think this question is like this question asked on StackOverflow earlier. But I am trying to look at things differently.

In TDD, we write tests that include

Answer 1

I see Design By Contract as a specification for success/failure in ALL cases, whereas Test Driven Development targets ONE specific case. If the TDD case succeeds, then it is assumed a function is doing it's job, but it doesn't take into account other cases that could cause it to fail.

Design By Contract on the other hand doesn't necessary guarantee the desired answer, only that the answer is "correct." For example, if a function returns is supposed to return a non-null string, the only thing you can assume in the ENSURE is that it will not be null.

But maybe it doesn't not return the string that was expected. There is no way for a contract to be able to determine that, only a Test can show that it was performing according to the specification.

So the two are complementary.

Greg