Is it advisable to write a test case for every class in your program?

Question

Im am just getting introduced to unit testing and test driven development. Thus far, I have only used Junit as testing framework. A question which emerged and for which I have not yet found a definite answer is: how much test cases do I need to write? Do I have to write a test case for every single class in my program? Or is this a stupid question because unit testing implies testing at the lowest (i.e. class) level?

I think writing a test case for every class is probably the more safe way to go (after all the more you test the lower the number of unforeseen bugs). But I was wondering if there are any widely agreed upon strategies regarding the amount of test cases to write?

Answer 1

If you're trying on TDD, then you shouldn't be writing any code at all without having a failing test that tells you to do so. By implication, then, you'll never have a class that doesn't have one or more tests for it. A rule of thumb that tends to get quoted is that you should end up with about 2.5 times as much test source as main source.