public class MainActivity extends AppCompatActivity {
    private FirebaseAuth mAuth;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        getSupportActionBar().setDisplayShowTitleEnabled(false);
        mAuth = FirebaseAuth.getInstance();
        String s = mAuth.getCurrentUser().getUid();
        TextView tV = (TextView) findViewById(R.id.uID);
        tV.setText(s);
    }