How can i get Multiple values of multiple nodes using Datasnapshot from the Firebase Database?

Question

I want to display dayVisitors of all the dates. Hence i want to access all the dayVisitors under Noida Sec1/ all dates. Database structure This is what I've done but it gives null pointer error: Attempt to invoke virtual method 'java.lang.String com.example.thehighbrow.visitormanagement.DayVisitor.getName()' on a null object reference.

Answer 1

If you want to specifically access just the dayVisitors under Noida Sec 1 you can simply implement this using:

final FirebaseDatabase db = FirebaseDatabase.getInstance();
DatabaseReference ref = db.getReference("Noida Sec1");
    ref.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {

                for (DataSnapshot childSnapshot : dataSnapshot.getChildren()) {
                        if(childSnapshot.hasChild("dayVisitor")) {
                            for (DataSnapshot visitorSnapshot : childSnapshot.child("dayVisitor").getChildren()) {
                                 Visitor visitorObject = visitorSnapshot.getValue(Visitor.class); //or whatever your dayVisitor object is 
                                //now you can access visitorObject with the fields you created and do whatever like add it to an arraylist
                            }
                        }
                }

        }  @Override
        public void onCancelled(DatabaseError databaseError) {
            Log.e("READ FAILED", databaseError.getMessage());
        }
    });

I'm not sure what your code is doing, however, I do suggest, if possible, you try to format your database layout to be as flat as possible because nesting data in this manner can get very messy and inefficient. Maybe make dayVisitor a field in visitor rather than its own child node.

Answer 2

First of all get the reference to the children of Noida Sec 1 like:

DatabaseReference mNoidaReference = mFirebaseDatabase.getReference().child("Noida Sec1");

Now make a childEventListener for it and loop through it to find the dayVisitor child

ChildEventListener  mChildEventListener = new ChildEventListener() {
        @Override
        public void onChildAdded(@NonNull DataSnapshot dataSnapshot, @Nullable String s) {
            for(Datasnapshot data: dataSnapshot.child("dayVisitor")){
              String dayVisitor = data.getValue();
            }
        }

        @Override
        public void onChildChanged(@NonNull DataSnapshot dataSnapshot, @Nullable String s) {

        }

        @Override
        public void onChildRemoved(@NonNull DataSnapshot dataSnapshot) {

        }

        @Override
        public void onChildMoved(@NonNull DataSnapshot dataSnapshot, @Nullable String s) {

        }

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

        }
    };
mNoidaReference.addChildEventListener(mChildEventListener);