Whatever.java

class Whatever {

    @Nullable
    private MessagesListener listener;
    
    @NonNull
    private ArrayList<String> localMessages = new ArrayList<>();
    
    public void setListener(@Nullable MessageListener callback) {
        listener = callback;
    }
    
    public void fetchMessageListFor(String activeUser) {

        ParseQuery<ParseObject> query1 = new ParseQuery<ParseObject>("Message");
        query1.whereEqualTo("sender",ParseUser.getCurrentUser().getUsername());
        query1.whereEqualTo("recepient",activeUser);

        ParseQuery<ParseObject> query2 = new ParseQuery<ParseObject>("Message");
        query2.whereEqualTo("recepient",ParseUser.getCurrentUser().getUsername());
        query2.whereEqualTo("sender",activeUser);

        List<ParseQuery<ParseObject>> queries = new ArrayList<>();
        queries.add(query1);
        queries.add(query2);

        ParseQuery<ParseObject> query = ParseQuery.or(queries);
            
        query.findInBackground(new FindCallback<ParseObject>() {
            @Override
            public void done(List<ParseObject> objects, ParseException e) {
                Log.d("XX", "Fetch: The size of this.localMessages is: " + this.localMessages.size());
                ArrayList<String> messages  = new ArrayList<>();
                
                if(e == null && objects.size() > 0 ){
                    for(ParseObject object : objects){
                       String messageContenct = object.getString("message");
                       if(! object.getString("sender").equals(ParseUser.getCurrentUser().getUsername()) ){
                            messageContenct = ">"+messageContenct;
                       }
                       messages.add(messageContenct);

                    }
                    
                    // Before notifying, update the local cache, beacuse onNewMessage needs it.
                    this.localMessages.clear(); 
                    this.localMessages.addAll(messages); //maybe use a different name for the "local" messages
                    
                    if (listener != null) {
                       listener.onMessageListAvailable(messages); // pass the new local list, but keep "this.messsages"
                    }
                }
            }
        });
    }
    
    public void onNewMessage(String sender, recipient, message) {
       final ParseObject message = new ParseObject("Message");
        message.put("sender" , sender);
        message.put("recepient", recipient);
        message.put("message", message);
        message.saveInBackground(new SaveCallback() {
            @Override
            public void done(ParseException e) {
                if (e == null) {
                    Log.d("XX", "OnNew: The size of this.localMessages is: " + this.localMessages.size());
                    localMessages.add(messageContent);
                    
                    if (listener != null) {
                       listener.onMessageListAvailable(localMessages);
                    }
                }
            }
        });
}
}