I agree about your unspoken assumption, but many interviewees ask. And the post I was responding to pointed to the Apache commons library, so I hazard to say it

I agree about your unspoken assumption, but many interviewees ask. And the post I was responding to pointed to the Apache commons library, so I hazard to say it's not universally known. A fail is not a red flag. A senior candidate not asking clarifying questions does not disqualify them (what I would call a red flag) but it is brought up in the debrief as a potential gap. I don't expect them to state this could be implemented using a single call to Apache commons. I call those candidates out in the debrief as being aware of the library, as a bonus. But not saying it isn't a negative.