How to update a textView from a questions Database randomly without knowing the total number of questions

I have a flashcard application that should dislplay a random question from a database that feeded by the user (so we don't have the total number of items which are the questions). I used a parameter z which the count of number of items at the runtime and I used the Random() method, but every time it gave me an IllegalArgumentException. here's my code how to count the parameter z:

while (resId1 != 0) {

        resId1 = getApplicationContext().getResources().getIdentifier(
                fquest + num1, "string", getPackageName());
        z = z + 1;
        num1 = num1 + 1;
    }

this code is working fine but the below code containing an exception I would like your assistance if you don't mind, here the bad code:

Random r1 = new Random(z);
    num1 = r1.nextInt(z);
    num1 = num1 + 1;

    resId1 = getApplicationContext().getResources().getIdentifier(
            fquest + num1, "string", getPackageName());
    quest1 = getResources().getString(resId1);
    q.setText(quest1);

the IllegalArgumentException is exactly at this line:

num1 = r1.nextInt(z);

please any help.