Of course they can! They can implement interfaces, which are sealed. In fact, that's one of their most common usages: sealed interface Option { re

Of course they can! They can implement interfaces, which are sealed. In fact, that's one of their most common usages: sealed interface Option { record InputFile(Path path) implements Option {} record OutputFile(Path path) implements Option {} record MaxLines(int maxLines) implements Option {} record PrintLineNumbers() implements Option {} } (see https://www.infoq.com/articles/data-oriented-programming-java/ https://www.infoq.com/articles/data-oriented-programming-jav...) The problem of extending classes is that it can introduce hidden state and also result in non-reflexive equality. But interfaces are fine.