firstClassCollection2.java

public class LottoService {
  private static final int LOTTO_NUMBERS_SIZE = 6;
  
  public void createLottoNumber() {
    List<Long> lottoNumbers = createNonDuplicateNumbers();
    validateSize(lottoNumbers);
    validateDuplicate(lottoNumbers);
  }
  
  public void validateSize(List<Long> lottoNumbers) {
    if (lottoNumbers.size() != LOTTO_NUMBERS_SIZE) {
    
       throw new IllegalArgumentException("Only Six Numbers Allowed");
    }
  }
  
  private void validateDuplicate(List<Long> lottoNumbers) {
    Set<Long> nonDuplicateNumbers = new HashSet<>(lottoNumbers);
    
    if (nonDuplicateNumbers.size() != LOTTO_NUMBERS_SIZE) {
      throw new IllegalArgumentException("Numbers cannot be duplicate");
    }
  }
}