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)

Cookie Settings

This website uses cookies to personalize content and ads, provide social media features, and analyze website traffic. In addition, information about your use of the website is shared with social media, advertising, and analytics partners. These partners may merge the information with other data that you have provided to them or that they have collected from you using the services.

For more information, please refer to our privacy policy. There you can also change your cookie settings later on.

contact icon

Contact us now