Comparators should be "Serializable"

These classes implement the Comparator interface. You should also implement the Serializable interface. If a comparator is used to construct an ordered collection such as a TreeMap, then the TreeMap will be serializable only if the comparator is also serializable. As most comparators have little or no state, making them serializable is generally easy and good defensive programming.

A non-serializable Comparator can prevent an otherwise-Serializable ordered collection from being serializable. Since the overhead to make a Comparator serializable is usually low, doing so can be considered good defensive programming.