Given IAEx being declared as thrown by, perhaps NPEx could also be added for style consistency?
Or IAEx declaration removed?

Both have @throws clauses. Or did you mean something else?

IllegalArgumentException is RTEx (as NPEx is). And is declared as being thrown by this method. Thus - let's add NullPointerException to the throws list.
Or remove throws.
Here, and in the second modified method.

But that's already done in this PR. Are you looking at the left side (old code) instead of the right side (new code)?

I see this:

protected static String extractJavadocVersion(String output) throws IllegalArgumentException {

I'd like to see this instead:

protected static String extractJavadocVersion(String output) throws IllegalArgumentException, NullPointerException {

or this:

protected static String extractJavadocVersion(String output) {

Good spot. That should go the other way per guidelines. Removed both.