How to flatten and sort a HashMap with ArrayList values in Java?

So, I have a hashmap which has a String as key, and an Arraylist as Value.

static HashMap<String, ArrayList<Integer>> scores = new HashMap<>();

Then I just request Input with the Scanner Class:

Scanner sc = new Scanner(System.in);
String input = sc.nextLine();
String[] inputArr = input.split(" ");

And then I just add the score to the player if he exists, and if he doesn't it just creates a new entry

for (int i = 0; i < inputArr.length; i += 2) {
    String UID = inputArr[i];
    int score = Integer.parseInt(inputArr[i + 1]);
    if (!scores.containsKey(UID)) {
        ArrayList<Integer> list = new ArrayList<>();
        list.add(score);
        scores.put(UID, list);
    } else {
        scores.get(UID).add(score);
    }
}

So for example, if I Input 1 100 2 200 3 300 1 200 into the console,
I receive {1=[100, 200], 2=[200], 3=[300]}

Now what I need is this Hashmap to be "opened up" and look like this: 3 300 1 200 2 200 1 100.
Note that the Output is sorted descending by value.

I also tried different methods, like a SortedTreeMap or a TreeBag, but none of them seem to work

You could do a flatMap on the map's entries, transforming each entry of String to List<Integer>, into a stream of multiple entries of String to Integer. Then you can just sort the entries with a comparator you prefer.

var result = scores.entrySet().stream()
    .flatMap(
        entry -> entry.getValue().stream().map(v -> Map.entry(entry.getKey(), v))
    )
    .sorted(
        Map.Entry.<String, Integer>comparingByValue().reversed()
            // if you would like to also break ties on the values by comparing the keys
            //.thenComparing(Map.Entry.comparingByKey())
    )
    .toList();

result is a List<Entry<String, Integer>>. Printing result gives you

[3=300, 1=200, 2=200, 1=100]

Also note that your if statement for adding the user input into the map can be simplified to the following using computeIfAbsent:

scores.computeIfAbsent(UID, x -> new ArrayList<>()).add(score);