如何按降序对映射项的键值进行排序？

Question

I am relatively new to Java, and the whole sort by using comparators is a bit confusing to me.

Is it possible to sort the current output of the code below in descending order without using comparators?

Or is there an easier way for me to understand how to sort said values.

myRef.addChildEventListener(new ChildEventListener() {
    @Override
    public void onChildAdded(@NonNull DataSnapshot dataSnapshot , String s) {
        Map<String, Object> userInfo = (Map<String, Object>)dataSnapshot.getValue();
        String username = (String) userInfo.get("username");

        Map<String, Long> steps = (Map<String, Long>) userInfo.get("steps");

        long existingSteps = 0;
        for (Map.Entry<String, Long> step : steps.entrySet()) {
            existingSteps += Long.valueOf(step.getValue());
        }

        arrayList.add(new String( username + "  -  " + "steps:  " + existingSteps));

        arrayAdapter = new ArrayAdapter<>(Leaderboard.this , 
        android.R.layout.simple_list_item_1 , arrayList);

        listView.setAdapter(arrayAdapter);
}

The current output of this code is:

John Doe I - Steps: 79
John Doe II - Steps: 111
John Doe III - Steps: 0
John Doe IV - Steps: 88
John Doe V - Steps: 12
John Doe VI - Steps: 0

Is it possible to get this code to run the following output:

John Doe II - Steps: 111
John Doe IV - Steps: 88
John Doe I - Steps: 79
John Doe V - Steps: 12
John Doe III - Steps: 0
John Doe VI - Steps: 0

Database Structure

Answer 1

The simplest solution I can think of, is to add under each user object, the total number of steps, as a new property. In this case, there is one more thing that you need to do, which is to increment that property with the number of steps that you get every day. Your new schema should look like this:

Firebase-root
  |
  --- user
       |
       --- userId
            |
            --- steps
            |    |
            |    --- //daily steps
            |
            --- target: 100
            |
            --- username: "John Doe I"
            |
            --- totalNumberOfSteps: 111

To get the users in (ascending) order according to the totalNumberOfSteps property, please use the following lines of code:

DatabaseReference rootRef = FirebaseDatabase.getInstance().getReference();
DatabaseReference userRef = rootRef.child("user");
Query query = userRef.orderByChild("totalNumberOfSteps");
ValueEventListener valueEventListener = new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        for(DataSnapshot ds : dataSnapshot.getChildren()) {
            String username = ds.child("username").getValue(String.class);
            long totalNumberOfSteps = ds.child("totalNumberOfSteps").getValue(Long.class);
            Log.d(TAG, username + " - Steps: " + totalNumberOfSteps);
        }
    }

    @Override
    public void onCancelled(@NonNull DatabaseError databaseError) {
        Log.d(TAG, databaseError.getMessage()); //Don't ignore errors!
    }
};
query.addListenerForSingleValueEvent(valueEventListener);

Then to order it descending, please see my answer from the following post:

• How to arrange firebase database data in ascending or descending order?

Finally, the result in the logcat will be:

John Doe II - Steps: 111
John Doe IV - Steps: 88
John Doe I - Steps: 79
John Doe V - Steps: 12
John Doe III - Steps: 0
John Doe VI - Steps: 0