Kotlin’s closed-by-default design choice makes it worse than Java, and thus not strictly better than Java. It’s premature optimization, and a design-up-front-influenced paranoia/fear of any extension in not-designed-for places. But when I write code, I prefer to keep it open to extension, and in practice, I found a lot of value in extending decently written code, that would not be possible with Kotlin without having to go back and modify things to be open.

can you point to documentation or articles that explain this closed-by-default concept in more detail?