What are sealed classes in Kotlin?

Question

I\'m a beginner in Kotlin and recently read about Sealed Classes. But from the doc the only think I actually get is that they are exist.

The doc stated, that they are

Answer 1

If you've ever used an enum with an abstract method just so that you could do something like this:

public enum ResultTypes implements ResultServiceHolder {
    RESULT_TYPE_ONE {
        @Override
        public ResultOneService getService() {
            return serviceInitializer.getResultOneService();
        }
    },
    RESULT_TYPE_TWO {
        @Override
        public ResultTwoService getService() {
            return serviceInitializer.getResultTwoService();
        }
    },
    RESULT_TYPE_THREE {
        @Override
        public ResultThreeService getService() {
            return serviceInitializer.getResultThreeService();
        }
    };

When in reality what you wanted is this:

val service = when(resultType) {
    RESULT_TYPE_ONE -> resultOneService,
    RESULT_TYPE_TWO -> resultTwoService,
    RESULT_TYPE_THREE -> resultThreeService
}

And you only made it an enum abstract method to receive compile time guarantee that you always handle this assignment in case a new enum type is added; then you'll love sealed classes because sealed classes used in assignments like that when statement receive a "when should be exhaustive" compilation error which forces you to handle all cases instead of accidentally only some of them.

So now you cannot end up with something like:

switch(...) {
   case ...:
       ...
   default:
       throw new IllegalArgumentException("Unknown type: " + enum.name());
}

---

Also, enums cannot extend classes, only interfaces; while sealed classes can inherit fields from a base class. You can also create multiple instances of them (and you can technically use object if you need the subclass of the sealed class to be a singleton).