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

Cannot instantiate a subclass of Foo (attempted subclass: Bar).
Use the overload of #withRedefinedSubclass() to specify a subclass.

Cannot instantiate the superclass of Bar (attempted superclass: Foo
Use the overload of #withRedefinedSuperclass() to specify a superclass.

EqualsVerifier needs instantiated objects to work and uses reflection to get them. This also applies to subclasses and superclasses when using the #withRedefinedSubclass() and #withRedefinedSuperclass() methods.

In Java 26 and above, JEP 500 makes it so that warnings will be issued when such reflection happens, and in later versions, an error will be thrown.

In situations when an error would be thrown, EqualsVerifier produces the above error message instead. You can solve it by using the overloads of #withRedefinedSubclass() and #withRedefinedSuperclass(), like this:

EqualsVerifier.forClass(Foo.class)
    .withFactory(values -> new Foo(values.get("bar")))
    .withRedefinedSubclass(values -> new Baz(values.get("bar"), values.get("quux")))
    .verify();

EqualsVerifier.forClass(Baz.class)
    .withFactory(values -> new Baz(values.get("bar"), values.get("quux")))
    .withRedefinedSuperclass(values -> new Foo(values.get("bar")))
    .verify();

For more details about this, read the chapter in the manual about “final means final”.

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