One note - if you ever have to write middle ware (or a garbage collector), avoid any code calling custom implementations like hashCode/equals. A variant of Coll

One note - if you ever have to write middle ware (or a garbage collector), avoid any code calling custom implementations like hashCode/equals. A variant of Collections.newSetFromMap(IdentityHashMap...) should replaces all Sets in the example. That doesn't address any concurrency issues, of course.