How to change the name in the upper bar dynamically when a fragment is selected?

I want the name in the upper bar to change according to the fragment that is selected, for example, I chose the list of clients list, then on the bar it would appear "List Clients", but I can not do it, it only gets the initial name. Here is the code for my Activity with NavigationDrawer:

public class MainActivity extends AppCompatActivity
    implements NavigationView.OnNavigationItemSelectedListener {

private String barra;
int id;
FragmentTransaction fragmentTransaction;


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

    DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
    ActionBarDrawerToggle toggle = new ActionBarDrawerToggle(
            this, drawer, toolbar, R.string.navigation_drawer_open, R.string.navigation_drawer_close);
    drawer.setDrawerListener(toggle);
    toggle.syncState();

    NavigationView navigationView = (NavigationView) findViewById(R.id.nav_view);
    navigationView.setNavigationItemSelectedListener(this);

    fragmentTransaction = getSupportFragmentManager().beginTransaction();
    fragmentTransaction.add(R.id.main_container, new TelaCadastrar());
    fragmentTransaction.commit();
}

@Override
public void onBackPressed() {
    DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
    if (drawer.isDrawerOpen(GravityCompat.START)) {
        drawer.closeDrawer(GravityCompat.START);
    } else {
        super.onBackPressed();
    }
}

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

@SuppressWarnings("StatementWithEmptyBody")
@Override
public boolean onNavigationItemSelected(MenuItem item) {
    // Handle navigation view item clicks here.
    id = item.getItemId();

    if (id == R.id.nav_CadastrarCliente) {
        fragmentTransaction = getSupportFragmentManager().beginTransaction();
        fragmentTransaction.replace(R.id.main_container, new TelaCadastrar());
        barra = "Cadastrar Cliente";
        fragmentTransaction.commit();
    } else if (id == R.id.nav_ListarCliente) {
        fragmentTransaction = getSupportFragmentManager().beginTransaction();
        fragmentTransaction.replace(R.id.main_container, new TelaListar());
        barra = "Listar Clientes";
        fragmentTransaction.commit();
    } else if (id == R.id.nav_Configs) {
        fragmentTransaction = getSupportFragmentManager().beginTransaction();
        fragmentTransaction.replace(R.id.main_container, new TelaConfig());
        barra = "Configurações";
        fragmentTransaction.commit();

    }

    DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
    drawer.closeDrawer(GravityCompat.START);
    return true;
}

}