Java catch block, caught exception is not final

It's pretty much the same as method arguments:

You usually don't modify them and many people agree that they should be treated as final (whether or not to actually write final in front of them is a matter of some debate).

But since there's no technical requirement that says it must be final, the language gives you the option to choose.

Personally I know of no good reason to modify the exception reference of a catch-block.

I cannot think of a convincing use-case for modifying an exception in a classic catch clause. However, that doesn't mean it should be forbidden. Especially given that you can modify a parameter variable. If you find this worrisome, you have the option of declaring the exception variable to be final.

On the other hand, allowing modification in the multi-exception catch would introduces the possibility of truly bizarre and confusing code such as this:

  catch (IOException | NullPointerException ex) {
      ...
      ex = new IllegalArgumentException(...);
  }

I imagine that's what the designers had in mind when they added the restriction in this case.

But either way, this is how the Java language is defined, and what we have to live with. There's not a lot of point in debating the apparent inconsistencies ... unless you are intending to design and implement a new language.