@sqrtminusone You can have and actually have mutable data structures in Java and most of other JVM langs.

Immutable data structures is a part of the language design. Actually, hosting it on JVM is also a part of the design.

http://lampwww.epfl.ch/papers/idealhashtrees.pdf
https://hypirion.com/musings/understanding-persistent-vector-pt-1

And there is a good reasoning/rationale behind it:
https://www.youtube.com/watch?v=-6BsiVyC1kM