FirebaseLogin.java

public class FirebaseLogin implements ILoginAuthentication {

    private OnLoginCompleted listener;

    public FirebaseLogin(OnLoginCompleted listener){
        this.listener = listener;
    }

    @Override
    public void login(String username, String password) {
        //Firebase Login code here
        if (noError){
            listener.onLoginSuccess();
        } else {
            listener.onLoginFailed(new Exception("Failure message here"));
        }
    }
}

ILoginAuthentication.java

public interface ILoginAuthentication {
    void login(String username, String password);
}

LoginActivity.java

public class LoginActivity extends AppCompatActivity implements OnLoginCompleted {

    private ILoginAuthentication authentication = new FirebaseLogin(this);

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

    @Override
    public void onLoginSuccess() {
        //Do something when login is successful
        //Display a message and redirect to another activity or whatever else you want to do after successful login
    }

    @Override
    public void onLoginFailed(Exception e) {
        //When Login fails
        //Display a message
    }
}

OnLoginCompleted.java

public interface OnLoginCompleted {
    void onLoginSuccess();

    void onLoginFailed(Exception e);
}