按降序对 Firebase 数据进行排序[重复]

Question

我想创建一个高分 ListView，条目按降序显示，使用 orderByValue 我将它们按升序排序，但是当我的 arraylist 使用 Collections.reverse() 时，条目仍然没有反转列表视图。

 private Query reference; //database reference
private ListView ListView; //creating listview
private ArrayList<String> arrayList = new ArrayList<>();

private ArrayAdapter<String> adapter; //creating array adapter
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_high_scores);


    reference = FirebaseDatabase.getInstance().getReference("Highscores").orderByValue(); //reference the highscore section in database



    ListView = (ListView) findViewById(R.id.list_view); //find the list view in the xml


    adapter = new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1, arrayList); // set the adapter to the list
    Collections.reverse(arrayList);

    ListView.setAdapter(adapter);

    reference.addChildEventListener(new ChildEventListener() { // add a child listener for the database
        @Override
        public void onChildAdded(@NonNull DataSnapshot dataSnapshot, @Nullable String s) {

            String value = dataSnapshot.getValue().toString(); // convert the values in the databse to string
            arrayList.add(value); //add them to the arraylist
            adapter.notifyDataSetChanged(); //notify that the dataset has changed



        }

数据库中的数据是

{ “-M4FPEiJ_DVc9kaQrebz”：0，

“-M4FPIuMbArT6KjtrDtO”：6，

“-M4FPP5EdC1CaKgOVECX”：17，

“-M4FPTWg7JkKRvWakRkH”：12，

“-M4FPYPNVZIjhBM6gVWW”：11，

“-M4FPbL9I-fsQwRvVIFX”：5 }

答案：

通过简单地添加到 0 索引处的 arraylist 来解决这个问题，所有其他元素都简单地向右移动，导致按降序排列的列表视图。

            String value = dataSnapshot.getValue().toString(); // convert the values in the databse to string
            arrayList.add(0, value); //add them to the arraylist
            adapter.notifyDataSetChanged(); //notify that the dataset has changed

Answer 1

reference.addChildEventListener(new ChildEventListener() { // 为数据库添加一个子监听 @覆盖 public void onChildAdded(@NonNull DataSnapshot dataSnapshot, @Nullable String s) {

        String value = dataSnapshot.getValue().toString(); // convert the values in the databse to string
        arrayList.add(value); //add them to the arraylist
        Collections.reverse(arrayList);  //add this line for short solution

        adapter.notifyDataSetChanged(); //notify that the dataset has changed



    }