Custom Assertions in FEST Assert

I used to write the following code quite often in my unit tests:

assertEquals(2, list.size());
assertTrue(list.contains(element1));
assertTrue(list.contains(element2));

In order to reduce duplication these three lines of code can be extracted to a custom assertion:

assertContainsOnlyTheseElements(list, element1, element2)

Instead of writing your own assertions you can use the FEST Assert library. This is how you can express it in FEST:

assertThat(list).hasSize(2).contains(element1, element2);

Or even better:

assertThat(list).containsExactly(element1, element2);

Isn’t it beautiful? It reads almost like a natural language.

It’s also very easy to use because it requires only a single static import from org.fest.assertions.Assertions. The assertThat method is overloaded for multiple different types, each with specific assertions. The error messages in case of assertion failure are also quite user-friendly. For these reasons I prefer to use FEST Assert over Hamcrest for custom assertions.