This UidGenerator uses the Java UUID#randomUUID() implementation to generate UIDs. For ultimate security you can also use the secure flag to generate UIDs that are less crackable (from Moving away from UUIDs). Extract from RFC7986:

The description of the "UID" property in [RFC5545] contains some
recommendations on how the value can be constructed.  In particular,
it suggests use of host names, IP addresses, and domain names to
construct the value.  However, this is no longer considered good
practice, particularly from a security and privacy standpoint, since
use of such values can leak key information about a calendar user or
their client and network environment.  This specification updates
[RFC5545] by stating that "UID" values MUST NOT include any data that
might identify a user, host, domain, or any other security- or
privacy-sensitive information.  It is RECOMMENDED that calendar user
agents now generate "UID" values that are hex-encoded random
Universally Unique Identifier (UUID) values as defined in
Sections 4.4 and 4.5 of [RFC4122].

The following is an example of such a property value:

UID:5FC53010-1267-4F8E-BC28-1D7AE55A7C99