This inspection reports on subclasses of java.lang.Comparator which are not also Serializable. Objects of java.util.TreeMap or java.util.TreeSet will become non-Serializable if instantated with such Comparators. This can result in unexpected and difficult-to-diagnose bugs. Since subclasses of java.lang.Comparator are often stateless, simply marking them Serializable is a small cost to avoid such issues.