FireBase LiveChat code

MainActivity
======================
import android.content.DialogInterface;
import android.content.Intent;
import android.support.v7.app.AlertDialog;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.Button;
import android.widget.EditText;
import android.widget.ListView;
import android.widget.TextView;


import com.google.firebase.database.DataSnapshot;
import com.google.firebase.database.DatabaseError;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;
import com.google.firebase.database.ValueEventListener;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Iterator;
import java.util.Map;
import java.util.Set;

public class MainActivity extends AppCompatActivity {


    private Button add_user;
    private EditText room_name;

    private ListView listView;
    private ArrayAdapter<String> arrayAdapter;
    private ArrayList<String> list_of_users = new ArrayList<>();
    private String name;
    private DatabaseReference root = FirebaseDatabase.getInstance().getReference().getRoot();

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

    private void setPointer() {
        add_user = (Button) findViewById(R.id.btn_add_room);
        room_name = (EditText) findViewById(R.id.room_name_edittext);
        listView = (ListView) findViewById(R.id.listView);

        arrayAdapter = new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1, list_of_users);

        listView.setAdapter(arrayAdapter);

        request_user_name();

        add_user.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {

                Map<String, Object> map = new HashMap<String, Object>();
                map.put(room_name.getText().toString(), "");
                root.updateChildren(map);
                room_name.setText("");


            }
        });

        root.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {

                Set<String> set = new HashSet<String>();
                Iterator i = dataSnapshot.getChildren().iterator();

                while (i.hasNext()) {
                    set.add(((DataSnapshot) i.next()).getKey());
                }

                list_of_users.clear();
                list_of_users.addAll(set);

                arrayAdapter.notifyDataSetChanged();
            }

            @Override
            public void onCancelled(DatabaseError databaseError) {

            }
        });

        listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> adapterView, View view, int i, long l) {

                Intent intent = new Intent(getApplicationContext(), Chat_Room.class);
                intent.putExtra("room_name", ((TextView) view).getText().toString());
                intent.putExtra("user_name", "(" + name + ") ");
                startActivity(intent);
            }
        });

    }

    private void request_user_name() {
        AlertDialog.Builder builder = new AlertDialog.Builder(this);
        builder.setTitle("Enter name:");

        final EditText input_field = new EditText(this);
        builder.setIcon(R.drawable.chat);
        builder.setView(input_field);
        builder.setPositiveButton("OK", new DialogInterface.OnClickListener() {
            @Override
            public void onClick(DialogInterface dialogInterface, int i) {
                name = input_field.getText().toString();
            }
        });

        builder.setNegativeButton("Cancel", new DialogInterface.OnClickListener() {
            @Override
            public void onClick(DialogInterface dialogInterface, int i) {
                dialogInterface.cancel();
                request_user_name();
            }
        });

        builder.show();
    }


}

Chat_Users
====================
import android.os.Bundle;
import android.support.annotation.Nullable;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;

import com.google.firebase.database.ChildEventListener;
import com.google.firebase.database.DataSnapshot;
import com.google.firebase.database.DatabaseError;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;

import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;


class Chat_Room extends AppCompatActivity{

    private Button btn_send_msg;
    private EditText input_msg;
    private TextView chat_conversation;

    private String user_name,room_name;
    private DatabaseReference root ;
    private String temp_key;

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

        btn_send_msg = (Button) findViewById(R.id.btn_send);
        input_msg = (EditText) findViewById(R.id.msg_input);
        chat_conversation = (TextView) findViewById(R.id.textView);

        user_name = getIntent().getExtras().get("user_name").toString();
        room_name = getIntent().getExtras().get("room_name").toString();
        setTitle(" User - "+room_name);

        root = FirebaseDatabase.getInstance().getReference().child(room_name);

        btn_send_msg.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {

                Map<String,Object> map = new HashMap<String, Object>();
                temp_key = root.push().getKey();
                root.updateChildren(map);

                DatabaseReference message_root = root.child(temp_key);
                Map<String,Object> map2 = new HashMap<String, Object>();
                map2.put("name",user_name);
                map2.put("msg",input_msg.getText().toString());
                input_msg.setText("");
                message_root.updateChildren(map2);
            }
        });

        root.addChildEventListener(new ChildEventListener() {
            @Override
            public void onChildAdded(DataSnapshot dataSnapshot, String s) {

                append_chat_conversation(dataSnapshot);
            }

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

                append_chat_conversation(dataSnapshot);

            }

            @Override
            public void onChildRemoved(DataSnapshot dataSnapshot) {

            }

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

            }

            @Override
            public void onCancelled(DatabaseError databaseError) {

            }
        });

    }

    private String chat_msg,chat_user_name;

    private void append_chat_conversation(DataSnapshot dataSnapshot) {

        Iterator i = dataSnapshot.getChildren().iterator();

        while (i.hasNext()){

            chat_msg = (String) ((DataSnapshot)i.next()).getValue();
            chat_user_name = (String) ((DataSnapshot)i.next()).getValue();

            chat_conversation.append(chat_user_name +" : "+chat_msg +" \n");
        }


    }
}