JUnit assertion methods should be phrased in the positive or the negative?

Question

Should I be writing
assertTrue("User logged in", user.isLoggedIn());
or
assertTrue("User is not logged in", user.isLoggedIn());

The former provides better reading inside the source files:
"I assert that the following is true: User logged in."

The error message could be read both ways:
java.lang.AssertionError: User logged in
"There is an error in asserting that the user is logged in"
"The error is that the user is logged in."

JUnit documentation doesn't provide a clear guide to which it should be, except it is
"the identifying message for the {@link AssertionError}",
And in both cases, the text identifies the test being run.

What's the common usage?

Answer 1

How about:

assertTrue("User should be logged in", user.isLoggedIn());

Works both ways.

Answer 2

Well, you could also state your assumption, and then how the assumption didn't hold. Like so:

assertTrue("Expected user to be logged it, and wasn't", user.isLoggedIn());

Makes for clearer messages, but longer to type and read.

Answer 3

You should include both cases. You have better test case when you triangulate your assertions.

Answer 4

To avoid that question, I more and more tend to use assertThat instead of "low-level" assert* methods. Indeed, like this article explains, assertThat will give you a very clear error message in case of failure.

Answer 5

You could use :

assertTrue("Test if user is logged in", user.isLoggedIn());

When you do this, you're verifying that user.isLoggedIn() is true, you can't really say that user is logged in or not, you don't know yet, you're just testing it.

Answer 6

Interesting, I would use:

assertTrue("user should be logged in", user.isLoggedIn());

which tells me what is expected state of this assertion.

I think the best choice is the one you understand.