EqualsVerifier can be used in Java unit tests to verify whether the contract for the equals and hashCode methods is met.

WARNING: Final field equalsVerifierCheckIfReflectionIsAllowed in class nl.jqno.equalsverifier.internal.instantiators.InstantiatorFactory$1C has been mutated reflectively by class nl.jqno.equalsverifier.internal.instantiators.InstantiatorFactory in unnamed module @3cdf2c61 (file:/home/jqno/w/equalsverifier/equalsverifier/equalsverifier-core/target/classes/)
WARNING: Use --enable-final-field-mutation=ALL-UNNAMED to avoid a warning
WARNING: Mutating final fields will be blocked in a future release unless final field mutation is enabled

You are using Java 26 or higher. With JEP 500: “Prepare to Make Final Mean Final”, Java issues warnings when final fields are mutated. In some later version of Java (it is not yet known which version), these warnings will turn into exceptions. If you use --illegal-final-field-mutation=deny, these warnings will also turn into exceptions.

The warning is EqualsVerifier checking if final fields can be mutated, by mutating a final field named equalsVerifierCheckIfReflectionIsAllowed. If that throws an exception, EqualsVerifier knows to take a different path and ask the user for factories.

If EqualsVerifier didn’t do this check, a similar warning on a different field would occur later in the test, when EqualsVerifier mutates a final field on the class that you’re testing.

See the manual entry on “final means final” for more details.

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