Reports objects which are a subtype of java.util.Set or java.util.Map and which may contain java.net.URL objects. Adding java.net.URL objects to such collections can cause performance problems because of calls to the equals() and hashCode() methods of java.net.URL. java.net.URL's equals() and hashCode() method use a DNS lookup, which depending on the availability of the network and the speed of the DNS server can cause significant delays.