Subtraction of random numbers

Some times in result I get an unexpected answer for example if the Question is 10-8 the program is giving -9 as a correct answer.

public void questionAnswerGenerator(){

    Random random = new Random();

    int a = random.nextInt(21);
    int b = random.nextInt(21);

    if (a >= b) {

        questionBoard.setText(Integer.toString(a) + " - " + Integer.toString(b));

    }else {

        questionAnswerGenerator();
    }

    locationOfCorrectAnswer = random.nextInt(4);

    answer.clear();

    int incorrectAnswer;

    for (int i=0; i<4; i++){

        if (i == locationOfCorrectAnswer) {

            answer.add(a - b);

        }else {

            incorrectAnswer = random.nextInt(41);

            while (incorrectAnswer == a - b){

                incorrectAnswer = random.nextInt(41);
            }

            answer.add(incorrectAnswer);
        }
    }