### Relevant articles:

- [Collect a Java Stream to an Immutable Collection](https://www.baeldung.com/java-stream-immutable-collection)
- [Guide to mapMulti in Stream API](https://www.baeldung.com/java-mapmulti)
- [Collecting Stream Elements into a List in Java](https://www.baeldung.com/java-stream-to-list-collecting)