Can you add a custom message to AssertJ assertThat?

Question

We have a test suite that primarily uses JUnit assertions with Hamcrest matchers. One of our team started experimenting with AssertJ and impressed people with its syntax, fl

Answer 1

To add another option to Patrick M's answer:

Instead of using Descriptable.as, you can also use AbstractAssert.withFailMessage():

try {
  // set a bad age to Mr Frodo which is really 33 years old.
  frodo.setAge(50);
  // you can specify a test description via withFailMessage(), supports String format args
  assertThat(frodo.getAge()).
    withFailMessage("Frodo's age is wrong: %s years, difference %s years",
      frodo.getAge(), frodo.getAge()-33).
    isEqualTo(33);
} catch (AssertionError e) {
  assertThat(e).hasMessage("Frodo's age is wrong: 50 years, difference 17 years");
}

The difference to using Descriptable.as is that it gives you complete control over the custom message - there is no "expected" and "but was".

This is useful where the actual values being tested are not useful for presentation - this method allows you to show other, possibly calculated values instead, or none at all.

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Do note that, just like Descriptable.as, you must call withFailMessage() before any actual assertions - otherwise it will not work, as the assertion will fire first. This is noted in the Javadoc.