Can Room of Requirement predict the future, and does it need to be empty?

Question

I was wondering why the Room of Requirement can predict the future, and whether it needs to be empty in order to change?

I clearly remember that in Harry Potter and the Order of Phoenix, Harry thinks he really needs a whistle, and immediately discovers one on the shelf beside him even though it had not been there moments before. (The room also kept adding beds and even toilets in Deathly Hallows.) So, did the room predict he would need the whistle, or did it create one when he thought of it?

In Harry Potter and the Deathly Hallows, Ginny, Tonks, and Mrs. Weasley were inside and needed to leave it so the trio can change the room into the Room of Hidden Things. But if the room can change and create whistles, why can't it create or summon Ravenclaw's Diadem while someone is inside?

So my thoughts are: Either the room can predict the future and knows that Harry will need the whistle, or it can change when someone is in the room. But in that case, there is no need for anyone to leave to retrieve the diadem.

Does anyone understand what I'm trying to say?

EDIT: This question did not provide the answer I am looking for.

Answer 1

The answers are pretty much found in The Deathly Hallows, The Lost Diadem chapter.

The room needs to be empty of a person in order to completely change. Or, rather, to allow the room to be reset.

However, the room doesn't need to be empty in order to change for the people already inside of it. They just need to be specific with what they'd like the room to be.

So, it doesn't need to predict the future, it just needs to detect a strong, specific desire in order to create something. Harry's whistle on the shelf was created the same way Neville made a door to Hog's Head, and how the DA expanded the room.

The relevant section of the book:

 ‘Where are we?’

 ‘Room of Requirement, of course!’ said Neville. ‘Surpassed itself, hasn’t it? The Carrows were chasing me, and I knew I had just one chance for a hideout: I managed to get through the door and this is what I found! Well, it wasn’t exactly like this when I arrived, it was a load smaller, there was only one hammock and just Gryffindor hangings. But it’s expanded as more and more of the DA have arrived.’

 ‘And the Carrows can’t get in?’ asked Harry, looking around for the door.

 ‘No,’ said Seamus Finnigan, whom Harry had not recognised until he spoke: Seamus’s face was bruised and puffy. ‘It’s a proper hideout, as long as one of us stays in here, they can’t get at us, the door won’t open. It’s all down to Neville. He really gets this Room.

 You’ve got to ask it for exactly what you need – like, “I don’t want any Carrow supporters to be able to get in” – and it’ll do it for you! You’ve just got to make sure you close the loopholes! Neville’s the man!’

 ‘It’s quite straightforward, really,’ said Neville modestly. ‘I’d been in here about a day and a half, and getting really hungry, and wishing I could get something to eat, and that’s when the passage to the Hog’s Head opened up. I went through it and met Aberforth. He’s been providing us with food, because for some reason, that’s the one thing the Room doesn’t really do.’

Answer 2

This is a guess, but I am going to run with it. The diadem was cursed and therefore couldn't be simply conjured. So the room of requirement needed to bring the room of hidden things to the door so that Harry could look for it manually.

And/or the room of requirement behaves similarly to Accio in that it can conjure up things that the witch or wizard clearly knows, but not things he knows general existence of.