public class FirebaseInstanceIDService extends FirebaseInstanceIdService {

    @Override
    public void onTokenRefresh() {
        // Get updated InstanceID token.
        String refreshedToken = null;
        try {
            refreshedToken = FirebaseInstanceId.getInstance().getToken(getString(R.string.gcm_defaultSenderId),null);
            Log.d(TAG, "Token: " + refreshedToken);
        } catch (IOException e) {
            Log.d(TAG, "Token Err: " + e.toString());
            e.printStackTrace();
        }

        sendRegistrationToServer(refreshedToken);
    }

    private void sendRegistrationToServer(String refreshedToken) {
        //Do something with Token
    }
}

String refreshedToken = FirebaseInstanceId.getInstance().getToken();

<!-- FIREBASE SERVICES -->

    <service android:name=".services.firebase.AppFirebaseInstanceIdService">
        <intent-filter>
            <action android:name="com.google.firebase.INSTANCE_ID_EVENT" />
        </intent-filter>
    </service>

FirebaseApp.initializeApp(this);