Realtime Database and lists

Lists are compelling data structures and they help in numerous use cases. Firebase has excellent support for HashMap and lists. Users can append the data according to the unique key from Firebase, or you can create your logic to create a unique identifier. Using the push() method a user can insert the data, and there are many ways to filter and match the data pushed. Let's see how the push() method helps in creating a list. As usual first grab the reference to the database and then using the push() method get the unique key. Using the push() method we can add a new child:

// Write a message to the database
mDatabase = FirebaseDatabase.getInstance();
mDbRef = mDatabase.getReference("Donor/Name");

//Setting firebase unique key for Hashmap list
String key = mDbRef.push().getKey();

mDbRef.child(key).setValue("First item");

Apart from allowing a database to create a list, it is also necessary to receive a data-changed notification from the list. This can be achieved through adding child event listeners. These listeners will notify the app when there is a new child added. We need to implement a couple of callbacks when we use this listener. Most commonly there is the onChildAdded() method when a child is added, and it sends a new data snapshot with data added. Note that onChildChanged() is called when there is an update to the existing node, and onChildRemoved() is called when a child node is removed. However onChildMoved() is called when any alterations change the list order:

ChildEventListener childListener = new ChildEventListener() {
    @Override
    public void onChildAdded(DataSnapshot dataSnapshot, String s) {
    }
    @Override
    public void onChildChanged(DataSnapshot dataSnapshot, String s) {
    }
    @Override
    public void onChildMoved(DataSnapshot dataSnapshot, String s) {
    }
    @Override
    public void onChildRemoved(DataSnapshot dataSnapshot) {
    }
    @Override
    public void onCancelled(DatabaseError databaseError) {
    }
};

mDbRef.addChildEventListener(childListener);

There are many ways to perform the query on the list. Firebase has a class named Query to access the database inside the application on specified criteria: 

Query mQuery = mDbRef.orderByKey();

ValueEventListener mQueryValueListener = new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        Iterable<DataSnapshot> snapshotIterator = dataSnapshot.getChildren();
        Iterator<DataSnapshot> iterator = snapshotIterator.iterator();
        while (iterator.hasNext()) {
            DataSnapshot next = (DataSnapshot) iterator.next();
            Log.i(TAG, "Value = " + next.child("name").getValue());
        }
    }
    @Override
    public void onCancelled(DatabaseError databaseError) {
    }
};

mQuery.addListenerForSingleValueEvent(mQueryValueListener);