Assertion Failure Messages JUnit 5 | Java Fresher

Direct Answer

JUnit already reports expected and actual values, so add a custom message when it supplies missing context such as the scenario or business rule: assertEquals(expected, actual, "premium discount"). In Jupiter assertion overloads the message or Supplier<String> comes last; a supplier is evaluated only on failure. AssertJ uses as(...) or describedAs(...) before the assertion.

Deep Dive & Architecture

JUnit already prints expected and actual values, so a custom message should add the missing context: the scenario or business rule that failed. In JUnit 5 the message is the final argument of an assertion.

For example, assertEquals(90, total, "premium order should receive 10% discount") is more useful in CI than assertEquals(90, total), because it identifies the rule without opening the test. Avoid messages that merely repeat “values differ.” When building the message is expensive, pass a Supplier<String> such as () -> render(order); JUnit evaluates it only after a failure. AssertJ provides the same descriptive idea through as(...) or describedAs(...) before the final assertion.

Common Mistakes & Pitfalls

• JUnit 4 parameter order. In Jupiter assertion overloads, the message follows the values or executable.
• Forcing a message onto every assertion. A descriptive test name and JUnit's expected/actual output may already be enough.
• Eager expensive messages. Use a Supplier when constructing the message has meaningful cost.
• Restating the assertion. Add the scenario or rule rather than saying only "values should be equal".