public Subscription(
     User subscriber, Frequency frequency, Severity severity, Collection<String> topics) {
   this(
       subscriber,
       frequency,
       severity,
       topics.stream().map(String::trim).collect(Collectors.joining(",")));
 }
 public void addTopics(Collection<String> topicCollection) {
   addTopics(topicCollection.stream().map(String::trim).collect(Collectors.joining(",")));
 }