Let me quote the question/answer so that you better understand:
Which statements about sealed classes are correct? (Choose all that apply.)
A sealed interface restricts which subinterfaces may extend it.
B sealed class cannot be indirectly extended by a class that is not listed in its permits clause.
C sealed class can be extended by an abstract class.
D sealed class can be extended by a subclass that uses the non-sealed modifier.
E sealed interface restricts which subclasses may implement it.
F sealed class cannot contain any nested subclasses.
G None of the above
Answers to review questions:
A, C, E. A sealed interface restricts which interfaces may extend it, or which classes may implement it, making options A and E correct. Option B is incorrect. For example, a non-sealed subclass allows classes not listed in the permits clause to indirectly extend the sealed class. Option C is correct. While a sealed class is commonly extended by a subclass marked final, it can also be extended by a sealed or non-sealed subclass marked abstract. Option D is incorrect, as the modifier is non-sealed, not nonsealed. Finally, option F is incorrect, as sealed classes can contain nested subclasses.
There is clearly a confusion with the expected answer, so there are two alternatives to correct the erratum:
- update option D with nonsealed so that the answer makes sense, or
- update the answer by adding option D as correct, also the phrase "Option D is incorrect, as the modifier is non-sealed, not nonsealed." should be changed to "Option D is correct, as the modifier non-sealed is valid."