PinGenerator.java

public static String generatePin(int n, int k, Random random) {
    StringBuilder sb = new StringBuilder();

    while (k-- > 0) {
        sb.append("0123456789");
    }

    char[] result = new char[n];

    for (int i = 0; i < n; ++i) {
        int index = random.nextInt(sb.length());
        result[i] = sb.charAt(index);
        sb.deleteCharAt(index);
    }

    return new String(result);
}

// usage

String pin = generatePin(6, 3, new SecureRandom());