What is ObjectStreamClass used for?

The ObjectStreamClass is used to check if the version of a serialized class (.ser file) is compatibile with the version of a class in memory/loadable from the CLASSPATH. See How can I programmatically obtain the serialVersionUID for a class? for an example of its usage.

It can also be used to get a descriptor for the fields in the stream. See the Serialization Specification for more information.