A class that implements java.util.Comparator and is used to construct a sorted collection needs to be serializable. An ordered collection (such as a java.util.TreeMap) that is constructed using a comparator serializes successfully only if the comparator is serializable.

The Collections in the Java Standard Library that require a comparator (TreeSet, TreeMap, PriorityQueue) all call ObjectOutputStream.defaultWriteObject, which tries to serialize every non-static, non-transient field in the class. As the comparator is stored in a field in these collections, the attempt to serialize a non-serializable comparator throws a java.io.NotSerializableException.

Comparators should be serializable if they are used in sorted collections that may be serialized. In most cases, simply changing the comparator so it also implements Serializable is enough. Comparators that have internal state may require additional changes (for example, custom writeObject and readObject methods). In these cases, it is best to follow general best practices for serializable objects (see references below).

In the following example, WrongComparator is not serializable because it does not implement Serializable. However, StringComparator is serializable because it does implement Serializable.

Java API Documentation: Comparator, ObjectOutputStream, Serializable.