Generate Random number in a range with no repeatation

I am trying to randomize quiz questions from firebase. I am trying to generate random numbers from 0-50 add them in a array and once the number is used I want that number to be removed. But it goes forever with repeatation, here is piece of code that I use to generate random

 public void updateQuestion (){

    for (int i = 0; i < 50; i++) {
        questionNos.add(i);
    }
    Random r = new Random();
    index = r.nextInt(questionNos.size()-1);
    mQuestionNo = questionNos.remove(index);

and to switch activity

 if (questionNos == null ){
        Intent i = new Intent(qiozphy.this,Score.class);
        i.putExtra("Score",mScore);
        startActivity(i);

    }

I am calling updatequestion() on oncreate and 4 choice butons of the quiz.