IF statement count++ in android studio using java firebase

How can i put a function regarding this one on java android fire-base. I am creating a chat app and i have a existing fire-base structure like this.let say i send a message "Hello".

Chat:
    Author:"name"
    Chat:"Hello"

I want to put push a function that it will count the number of message in every user. therefore it will be like this. let say i send a message saying hi? and Be right back

Chat:
    Author:"name"
    Chat:"hi?"
    numMessages: 1

Chat:
    Author:"name"
    Chat:"Be right back"
    numMessages: 2

I think it is if statement count++? but i try everything regarding this.ugh. Java and Android masters hope you can give me some suggestions sir.

Here is my mainactivity.

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        Firebase.setAndroidContext(this);
        final Firebase firebase = new Firebase("http://ift.tt/1oSgYe5");


        editText = (EditText) findViewById(R.id.editText);
        listView = (ListView) findViewById(R.id.listView);

        listView.setAdapter(new MessageAdapter(this, Message.class, R.layout.fragment_message, firebase.child("chat")));

        Button button = (Button) findViewById(R.id.button);

        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Message message = new Message();

                message.setMessage(editText.getText().toString());
                editText.setText("");
                message.setAuthor("Name");
                firebase.child("chat").push().setValue(message);


            }
        });

    }



}