Dont judge me i am just starting

public static List<Integer> missingNumbers2(List<Integer> arr, List<Integer> brr) {
    if (arr.isEmpty() || brr.isEmpty() || arr.size() > brr.size()) return null;
    HashMap<Integer, Integer> frequency = new HashMap<>();
    HashMap<Integer, Integer> frequency2 = new HashMap<>();

    List<Integer> list = new ArrayList<>();
    for (int eArr : arr) {
        frequency2.put(eArr, frequency2.getOrDefault(eArr, 0) + 1);
    }
    for (int eBrr : brr) {
        frequency.put(eBrr, frequency.getOrDefault(eBrr, 0) + 1);
    }
    for (var entry : frequency.entrySet()){
        int key = entry.getKey();
        int value = entry.getValue();
        int value2 = frequency2.getOrDefault(key,0);
        if (value != value2){
            list.add(key);
        }
    }

    return list;
}