Here we going to figure out few tips how to make your test failures more informative, clear and valuable with AssertJ custome messages.

AssertJ Maven dependency

<dependency>
  <groupId>org.assertj</groupId>
  <artifactId>assertj-core</artifactId>
  <!-- use 2.9.0 for Java 7 projects -->
  <version>3.9.0</version>
  <scope>test</scope>
</dependency>

Check the current available version - AssertJ

By default AssertJ assertion failures

int multiplicand1 = 2
int multiplicand2 = 3
int expectedResult = 5;

assertions.assertThat(multiplicand1 * multiplicand2)
        .isEqualTo(expectedResult);

give fail mesagges like this:

1) expected:<[5]> but was:<[6]>

We can’t see what exactly the failure is about. Let’s tune it a little with as(...) method

...
assertions.assertThat(multiplicand1 * multiplicand2)
        .as("2 * 3")
        .isEqualTo(expectedResult);
...

Failure output:
1) [2 * 3] expected:<[5]> but was:<[6]>

or even

...
assertions.assertThat(multiplicand1 * multiplicand2)
        .as("Multiplication %s * %s", multiplicand1, multiplicand2)
        .isEqualTo(expectedResult);
...

Failure output:
1) [Multiplication 2 * 3] expected:<[5]> but was:<[6]>

Better?

We also can completely replace the failure message with our custome message using withFailMessage(...):

...
assertions.assertThat(multiplicand1 * multiplicand2)
        .withFailMessage("Multiplication error <%s * %s> should be equal <%s> but was <%s>", 
                multiplicand1, multiplicand2, expectedResult, multiplicand1 * multiplicand2)
        .isEqualTo(expectedResult);
...

Failure output:
1) Multiplication error <2 * 3> should be equal <5> but was <6>

And the last thing - don’t forget to give clear and informative names to test classes and test methods.

You may also find these posts interesting: