Interacting with Networks and Databases - Beginning Java 7 - page 821

}
}
Listing9-7 ' s main() methodfirstcreatesa MulticastSocket instanceboundto
port 10000 via the MulticastSocket(int port) constructor.
It then obtains an InetAddress object that contains multicast group IP address
231.0.0.1, and uses this object to join the group at this address by calling Multic-
astSocket 's void joinGroup(InetAddress mcastaddr) method.
main() next receives ten datagram packets, prints their contents, and leaves the
group by calling MulticastSocket 's void leaveGroup(InetAddress
mcastaddr) method with the same multicast IP address as its argument.
Note joinGroup() and leaveGroup() throw IOException when an I/O
erroroccurswhileattemptingtojoinorleavethegroup,orwhentheIPaddressisnota
multicast IP address.
Because the client doesn't know exactly how long the arrays of bytes will be, it as-
sumes256bytestoensurethatthedatabufferwillholdtheentirearray.Ifittriedtoprint
outthereturnedarray,youwouldseealotofemptyspaceaftertheactualdatahadbeen
printed.Toeliminatethisspace,itinvokes DatagramPacket 's int getLength()
methodtoobtaintheactuallengthofthearray,createsasecondbytearray( buffer2 )
withthislength,anduses System.arraycopy() —discussedin Chapter4 —tocopy
thismanybytesto buffer2 .Afterconvertingthisbytearraytoa String object(via
the String(byte[] bytes) constructor,whichusestheplatform'sdefaultcharac-
terset—see Chapter8 tolearnaboutcharactersets),itprintstheresultingcharactersto
the standard output device.
Compile Listing 9-7 ( javac MCClient.java ) and run this application ( java
MCClient ). You should observe output similar to the following:
line 521103
line 521104
line 521105
line 521106
line 521107
line 521108
line 521109
line 521110