both suggestions here are for books that cover java 8. how is it possible that java 8 is modern relative to java 21?

Modern Java in Action (the 2nd edition of Java 9 in Action) covers Java 11 and contains a final chapter that talks about "future Java" features -- many of which are now implemented.

It's a great book for Java programmers who want to make the jump to functional-style programming. If that's the transition one is hoping to make, then I would definitely recommend the book.

And I would recommend "The Well-Grounded Java Developer" (2nd edition) to those who have mastered the functional-style Java of "Modern Java in Action". It covers JDK 17 (and beyond) and also has a a polyglot JVM focus with coverage of Kotlin and Clojure.

https://www.manning.com/books/the-well-grounded-java-develop...

Core Java for the Impatient (3rd) is less than a year old and updated for Java 17.

excellent - thanks

... and I agree that there is a need for a Java 21+ book which includes discussion of algebraic data types, pattern matching etc.