class LotteryNumbers {

  private final Set<Integer> numbers;

  public static final int MIN_NUMBER = 1;
  public static final int MAX_NUMBER = 20;
  public static final int NUM_NUMBERS = 4;

  /**
   * Constructor. Creates random lottery numbers.
   */
  private LotteryNumbers() {
    numbers = new HashSet<>();
    generateRandomNumbers();
  }

  /**
   * Constructor. Uses given numbers.
   */
  private LotteryNumbers(Set<Integer> givenNumbers) {
    numbers = new HashSet<>();
    numbers.addAll(givenNumbers);
  }

  /**
   * Creates a random lottery number.
   *
   * @return random LotteryNumbers
   */
  public static LotteryNumbers createRandom() {
    return new LotteryNumbers();
  }

  /**
   * Creates lottery number from given set of numbers.
   *
   * @return given LotteryNumbers
   */
  public static LotteryNumbers create(Set<Integer> givenNumbers) {
    return new LotteryNumbers(givenNumbers);
  }

  /**
   * Get numbers.
   *
   * @return lottery numbers
   */
  public Set<Integer> getNumbers() {
    return Collections.unmodifiableSet(numbers);
  }

  /**
   * Get numbers as string.
   *
   * @return numbers as comma separated string
   */
  public String getNumbersAsString() {
    return Joiner.on(',').join(numbers);
  }

  /**
   * Generates 4 unique random numbers between 1-20 into numbers set.
   */
  private void generateRandomNumbers() {<FILL_FUNCTION_BODY>}

  /**
   * Helper class for generating random numbers.
   */
  private static class RandomNumberGenerator {

    private final PrimitiveIterator.OfInt randomIterator;

    /**
     * Initialize a new random number generator that generates random numbers in the range [min,
     * max].
     *
     * @param min the min value (inclusive)
     * @param max the max value (inclusive)
     */
    public RandomNumberGenerator(int min, int max) {
      randomIterator = new SecureRandom().ints(min, max + 1).iterator();
    }

    /**
     * Gets next random integer in [min, max] range.
     *
     * @return a random number in the range (min, max)
     */
    public int nextInt() {
      return randomIterator.nextInt();
    }
  }

}

    numbers.clear();
    var generator = new RandomNumberGenerator(MIN_NUMBER, MAX_NUMBER);
    while (numbers.size() < NUM_NUMBERS) {
      var num = generator.nextInt();
      numbers.add(num);
    }
  
<no_super_class>