Firebase Read /Write simple data

Firebase_RW.java

public class MainActivity extends AppCompatActivity {
    public Button mSendButton;
    public DatabaseReference mDatabase;
    public HashMap<String, String> dataMap;
    public TextView retData;
    public String name;





    @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().child("Bahaa");
        mSendButton = (Button)findViewById(R.id.Send);
        dataMap= new HashMap<>();
        dataMap.put("Username", "Bahaa Ibrahim");
        dataMap.put("Email", "Bahaasco@gmail.com");
        dataMap.put("Mobile", "01009540399");



        mSendButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                mDatabase.setValue(dataMap).addOnCompleteListener(new OnCompleteListener<Void>() {
                    @Override
                    public void onComplete(@NonNull Task<Void> task) {
                        if(task.isSuccessful()){
                            Toast.makeText(MainActivity.this, "Sent !", Toast.LENGTH_LONG).show();
                        }else {
                            Toast.makeText(MainActivity.this, "Failed !", Toast.LENGTH_LONG).show();

                        }
                    }
                });
            }
        });

        mDatabase = FirebaseDatabase.getInstance().getReference().child("Bahaa").child("Username");
        retData = (TextView)findViewById(R.id.retData);

        mDatabase.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {
                name = dataSnapshot.getValue().toString();
                retData.setText("Name is: " + name);
            }

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

            }
        });