Testen AssertJ

Better testing, better life – with AssertJ


How many variants do you know to check whether a collection is empty or not empty in JUnit? You certainly thought of assertTrue(foo.isEmpty()). But have you ever come across assertEquals(0, foo.size()) or assertFalse(foo.size() > 0)? Now think about how many ways there are to compare the contents of one collection with another. This is even more confusing when the order of the elements plays a role (or not!).

If your head isn’t about to explode yet: consider comparing objects if equals() has not been implemented or has been implemented incorrectly. All of this and more can be done easily and elegantly with an expressive framework - e.g. AssertJ.

better testing, better life with assertJ

Solution and Examples

Thanks to Fluent interfaces, AssertJ allows you to chain your expectations to check multiple properties easily:

assertThat(jambit.getMission()).isNotNull().isEqualTo("100% Begeisterung");

Assertions of typed objects directly eliminate some error classes when creating tests. For example, the size and contents of collections can be checked easily:

assertThat(Arrays.asList("Where", "innovation", "works")).hasSize(3)
    .containsExactly("Where", "innovation", "works") // Order is important
    .containsOnlyOnce("works", "Where", "innovation"); // Order is unimportant

Furthermore, checking predicates for a whole collection is trivial:

    .allMatch(JambitEmployee::isTopOfMind); // alternatively {any,none}Match

Individual fields and results of methods can be verified easily with extracting():

    .extracting(Jambit::getBosses).extracting("nickname", JambitBoss::getFullName)
    .containsOnly(tuple("Felli", "Peter F. Fellinger"), tuple("Harti", "Markus Hartinger"));

As you can see, field names can also be used instead of accessors. In addition to individual values, you can also compare objects where equals() is missing:


For exceptions, there are also some useful matchers:

assertThatThrownBy(() -> throw DeveloperException("Out of coffee"))
    .isInstanceOf(DeveloperException.class).hasMessage("Out of coffee");

Finally, here is a more complex example where a collection is compared pairwise to a second one and all assertions are evaluated for each pair:

    .zipSatisfy(fetchDivisionNames(), (JambitDivision actualDivision, String expectedDivisionName) -> {
        try (AutoCloseableSoftAssertions softly = new AutoCloseableSoftAssertions()) {

Further Aspects

As you can see, AssertJ has a lot to offer. It's best to have a look at the feature highlights and get started right away. There are also some migration guides in the documentation – but you can also use AssertJ together with other frameworks.


Author: Alexandros Panagiotidis is Senior Software Architect at jambit in Stuttgart.

Download the Toilet Paper #107: AssertJ (pdf)

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