I am doing an android App, but i am couldn't make it work

private randomText mRandomText = new randomText();
private getNewNumber mGetNewNumber = new getNewNumber();


private TextView mGameText;
private TextView mNewNumberWord;
private TextView mNewNumber;
private TextView mLevel;
private TextView mAnswerWord;
private EditText mAnswer;
private Button mPlayButton;

private int gameLevel = 0;

// the next line will generate random number from 0-9 <<<<<<<<<<<<<<<<<<<<<<<<<
private String randNumber = mGetNewNumber.getGenNumber();

private String numberList = "";

// I am trying to combine the new generated number to the previous generated number, so every click will add one digit to the numberList <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
        numberList += randNumber;  <<<<<<this line is not working!!


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


    mGameText = (TextView) findViewById(R.id.funText);
    mNewNumberWord = (TextView) findViewById(R.id.newNumberWord);
    mNewNumber = (TextView) findViewById(R.id.newNumber);
    mLevel = (TextView) findViewById(R.id.showLevelText);
    mAnswerWord = (TextView) findViewById(R.id.answerWord);
    mAnswer = (EditText) findViewById(R.id.answer);
    mPlayButton = (Button) findViewById(R.id.playButton);


    mNewNumber.setText(randNumber);
    mAnswerWord.setText(numberList);


    mPlayButton.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {

            gameProcess();

        }
    });
}


private void gameProcess() {

    String text = mRandomText.getRandomText();
    mGameText.setText(text);

    mNewNumber.setText(mGetNewNumber.getGenNumber());

    gameLevel++;
    mLevel.setText("Level: " + gameLevel);
}