If I have a Map<String, List<String>> and I want to get the List<String> value for a given key, and then append something to that list, why do I need to call map.put(key, list) to persist the update? Especially whereas a method like Arrays.sort(input_arr) does not require resetting the variable again like String[] sorted_arr = Arrays.sort(input_arr)? Using the below code as an example, is it because when we call List<String> list = map.getOrDefault(arr[i], new ArrayList<String>()), is the value stored in list not a reference to the ArrayList? Or is it a copy of the original ArrayList that gets set to the list variable?

HashMap<String, List<String>> map = new HashMap<>();
String[] arr = new String[]{"one", "two"};
for (int i=0; i<arr.length; i++) {
    List<String> list = map.getOrDefault(arr[i], new ArrayList<String>());
    if (arr[i].equals("one") {
        list.add("onetwo");
    }

    map.put(arr[i], list);
}