Because I’m in my very early 20s I missed out on the huge Java craze. Everything was Python when I started getting a more formal education and before then all my work was in C++. Knowing more languages would obviously look better on a CV but I mean if I would benefit in a practical sense? I have two friends who are long time Java devs. And recently another friend who generally works with legacy C++ based systems from the early 2000s late 90s period had to work on a bunch of stuff in Java. Java is clearly still in large scale use among older systems. So would it be likely that eventually I would need to work on Java systems myself when my job is mostly JavaScript currently?
I would say yes, Java can be useful. Look into basics of Spring Boot. My previous job was greenfield development of cloud microservices in Spring Boot and Kafka. We used Azul JVM (the paid version with some cloud goodies) but GraalVM can compile your code AOT so it starts fast (not as fast as C++ but no sloth either). Java might not be only for old legacy systems. Java craze is over but Java didn’t go away.
However Java as a language might not be everyone’s coup of tea. I like it but you might not.