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(",")));
}