Checksum Builder - interface for checksum BUILDERs.
The PRODUCTs of these checksum builders are Checksum instances.
If not otherwise stated, concrete instances are NOT safe to be used concurrently by multiple threads.
Checksum Helper - the DIRECTOR in this BUILDER example.
Instances of this class create a checksum out of a given object array using a BUILDER.
These helper instances are not safe to be used concurrently by multiple threads.