Retrieve multipe data from Firebase Database

Retrieve multipe data.java

package com.example.bahaaibrahim.firebasetest;

import android.content.Context;
import android.os.Bundle;
import android.support.annotation.NonNull;
import android.support.design.widget.FloatingActionButton;
import android.support.design.widget.Snackbar;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.View;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.ArrayAdapter;
import android.widget.Button;
import android.widget.ListView;
import android.widget.TextView;
import android.widget.Toast;

import com.google.android.gms.tasks.OnCompleteListener;
import com.google.android.gms.tasks.Task;
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 com.google.firebase.database.ValueEventListener;

import java.util.ArrayList;
import java.util.HashMap;

public class MainActivity extends AppCompatActivity {

    public DatabaseReference mDatabase;
    public ListView dataList;
    public ArrayList<String> dataArray;
    public ArrayList<String> mKeys;
    public ArrayAdapter<String> mAdapter;
    public String result;
    public String key;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

        mDatabase = FirebaseDatabase.getInstance().getReference();
        dataList = (ListView) findViewById(R.id.dataList);
        dataArray = new ArrayList<>();
        mKeys = new ArrayList<>();
        mAdapter = new ArrayAdapter<>(this, android.R.layout.simple_list_item_1, dataArray);


        dataList.setAdapter(mAdapter);


        mDatabase.addChildEventListener(new ChildEventListener() {
            @Override
            public void onChildAdded(DataSnapshot dataSnapshot, String s) {
                result = dataSnapshot.getValue().toString();
                key = dataSnapshot.getKey();

                dataArray.add(result);
                mKeys.add(key);
                mAdapter.notifyDataSetChanged();
                Toast.makeText(getApplicationContext(), "Accessed!", Toast.LENGTH_LONG).show();
            }

            @Override
            public void onChildChanged(DataSnapshot dataSnapshot, String s) {
                result = dataSnapshot.getValue().toString();
                key = dataSnapshot.getKey();

                int index = mKeys.indexOf(key);
                dataArray.set(index, result);
                mAdapter.notifyDataSetChanged();
            }

            @Override
            public void onChildRemoved(DataSnapshot dataSnapshot) {
                result = dataSnapshot.getValue().toString();
                dataArray.remove(result);
                mAdapter.notifyDataSetChanged();
            }

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

            }

            @Override
            public void onCancelled(DatabaseError databaseError) {
                Toast.makeText(getApplicationContext(), "Failed !", Toast.LENGTH_LONG).show();
            }
        });


        FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                        .setAction("Action", null).show();
            }
        });
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }
}