Q:

ChildEventListener must implement onChildChanged

This is driving me insane. I'm using "onChildAdded" and if I switch it to "onChildChanged" then it asks me to switch back to "onChildAdded".  I have no idea why it's doing this.
Here's my code:
Query queryRecycler = mDatabase.limitToLast(5);
        queryRecycler.addChildEventListener(new ChildEventListener() {

            public void onChildAdded(DataSnapshot dataSnapshot, String previousKey) {
                messageList.add(dataSnapshot.getValue(Message.class));
                mMessageAdapter.notifyDataSetChanged();
            }

        });

And the complete error:

Class 'Anonymous class derived from ChildEventListener' must either be
  declared abstract or implement abstract method
  'onChildChanged(DataSnapshot, String)' in 'ChildEventListener'

A:

If you want to implement ChildEventListener, you should override onChildAdded,onChildChanged, onChildRemoved, onChildMoved. even you do not want it. (Code example from firebase)
ChildEventListener childEventListener = new ChildEventListener() {
    @Override
    public void onChildAdded(DataSnapshot dataSnapshot, String previousChildName) {
        Log.d(TAG, "onChildAdded:" + dataSnapshot.getKey());

        // A new comment has been added, add it to the displayed list
        Comment comment = dataSnapshot.getValue(Comment.class);

        // ...
    }

    @Override
    public void onChildChanged(DataSnapshot dataSnapshot, String previousChildName) {
        Log.d(TAG, "onChildChanged:" + dataSnapshot.getKey());

        // A comment has changed, use the key to determine if we are displaying this
        // comment and if so displayed the changed comment.
        Comment newComment = dataSnapshot.getValue(Comment.class);
        String commentKey = dataSnapshot.getKey();

        // ...
    }

    @Override
    public void onChildRemoved(DataSnapshot dataSnapshot) {
        Log.d(TAG, "onChildRemoved:" + dataSnapshot.getKey());

        // A comment has changed, use the key to determine if we are displaying this
        // comment and if so remove it.
        String commentKey = dataSnapshot.getKey();

        // ...
    }

    @Override
    public void onChildMoved(DataSnapshot dataSnapshot, String previousChildName) {
        Log.d(TAG, "onChildMoved:" + dataSnapshot.getKey());

        // A comment has changed position, use the key to determine if we are
        // displaying this comment and if so move it.
        Comment movedComment = dataSnapshot.getValue(Comment.class);
        String commentKey = dataSnapshot.getKey();

        // ...
    }

    @Override
    public void onCancelled(DatabaseError databaseError) {
        Log.w(TAG, "postComments:onCancelled", databaseError.toException());
        Toast.makeText(mContext, "Failed to load comments.",
                Toast.LENGTH_SHORT).show();
    }
};
ref.addChildEventListener(childEventListener);

