IsNullImpl implements IsNull<T, R> { @Override public IsNotNull<T, R> negate() { return new IsNotNullImpl<>(expression); } IsNullImpl(ToNullable<T, R, ?> expression); @Override IsNotNull<T, R> negate(); @Override ToNullable<T, R, ?> expression();  }
@Test void negate() { assertTrue(instance.negate() instanceof IsNotNull); }