Android Firebase Retrieving data as a list from multiple nods

Question

I am using firebase realtime database in my first android app. I'm trying to retrieve data from firebase as an arraylist of "username" but i'm constantly having the Query returning a null result. i'm very new to firebase so i'm really struggling with data retrieval errors.

"Marchand" : {
    "Taha" : {
      "adress" : "58 avenu St Eugène ",
      "blockage" : false,
      "latitude" : 35.7498635,
      "longitude" : -0.5566705,
      "mail" : "mailmail@gmail.com ",
      "password" : "123456",
      "signalement" : 0,
      "telephone" : "0666666666",
      "username" : "Taha"
    },
    "Yasmine" : {
      "adress" : "Address kkdndbdk ",
      "blockage" : false,
      "latitude" : 35.7498636,
      "longitude" : -0.5566704,
      "mail" : "randommail@gmail.com",
      "password" : "bobo",
      "signalement" : 0,
      "telephone" : "06999999",
      "username" : "Yasmine"
    }
.
.
.

This is one of my attempts, i tried to use the order-by-child method to iterate through the multiple nods that i have but i'm sure that it's not the right way to do this, this gives me null poiter exceptions because the datasnapshot is returning a NULL value :

 referenceMarchand = rootNode.getReference("Marchand");
final ArrayList<String> types = new ArrayList<>();
        Query users =  referenceMarchand.orderByChild("username");

        users.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {

                types.add(dataSnapshot.child("username").toString());

            }

            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) {

            }
        });

Answer 1

lets make use of some functions of firebase, Lets see it works for you.

part 1)

we will save it another way around

"Merchant" : {"Name" : "Taha"
"ID" :"-KSMCMCMnnkd"},//Firebase generated id
{"Name" : "Yasmine"
"ID" :"-KSMCMccccnkd"},//Firebase generated Id
...........

Part 2)

And Save the Details

"Details" : {
      "ID" : "-KSMCMCMnnkd" //Firebase generated id
      "adress" : "58 avenu St Eugène ",
      "blockage" : false,
      "latitude" : 35.7498635,
      "longitude" : -0.5566705,
      "mail" : "mailmail@gmail.com ",
      "password" : "123456",
      "signalement" : 0,
      "telephone" : "0666666666",
      "username" : "Taha"},
      {
      "ID":"-KSMCMccccnkd" //Firebase generated id
      "adress" : "Address kkdndbdk ",
      "blockage" : false,
      "latitude" : 35.7498636,
      "longitude" : -0.5566704,
      "mail" : "randommail@gmail.com",
      "password" : "bobo",
      "signalement" : 0,
      "telephone" : "06999999",
      "username" : "Yasmine"}
  

In Part 1)

DatabaseRefrence ref = FirebaseDatabase.getInstance("/Path");
String id = ref.child("Merchant").push().getKey(); //Firebase generated id
Merchant merchant = new Merchant();
merchant.setId(id);
merchant.setName("SomeName");
ref.child("Merchant").child(id).setValue(merchant);

Part 2)

Detail detail = new Detail();
detail.setAddress("Some Address");
detail.setBlockage("false");
detail.setLatitude("SomeValue");
detail.setEmail("someText@gmail.com");
......
ref.child("Details").child(id).setValue(detail);

Now

List<Merchant> merchants = new ArrayList<>();
List<Detail> details = new ArrayList<>();
ref.child("Merchant").addValueEventListener(new ValueEventListener(){
@Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {

               for(Datasnapshot snapshot:dataSnapshot){
                  Merchant merchant = snapshot.getValue(Merchant.class);
                  merchants.add(merchant);
}

            }

            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) {

            }
});

ref.child("Details").addValueEventListener(new ValueEventListener(){
    @Override
                public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
    
                   for(Datasnapshot snapshot:dataSnapshot){
                      Detail detail = snapshot.getValue(Detail.class);
                      details.add(detail);
    }
    
                }
    
                @Override
                public void onCancelled(@NonNull DatabaseError databaseError) {
    
                }
    });

Now you can get the details for some merchant or save it in local database for later querying.