java.net.DatgramSocket and MulticastSocket have support for pre JDK 1.4 DatgramSocketImpl implementations, specifically to support DatagramSocketImpls that were compiled with JDK 1.3 or older, which do not have support for connected sockets, for peeking at received datagrams, and for joining and leaving a group at a specific interface. This support is legacy. Such implementations do not compile with JDK 1.4 or newer, so it should be relatively safe to drop this support.