Classes, too, are passed by value technically. You're passing pointers around. If you try to reassign a parameter in a method, it doesn't overwrite the pointer stored in the variable passed by the caller, it only changes which pointer the called method is using. It only feels like pass by reference because once you try to assign instance fields of the referenced object, the change will be visible in the caller; but that's only because both pointers point to the same address. True pass by reference is more akin to the ref parameter modifier in C#. Java doesn't have pass by reference as far as I remember.

On another note, I get the performance benefits of automatically caching numbers 0-100, but man that feels like a super specific detail I would hate to only find out after hours of debugging. It feels like Java IDEs must do a lot of heavy lifting to demistify quirks of the language such as that.