UDP - Java Network Programming - page 404

is the address of the machine that sent it (the source address). On the other hand, if the
datagram was created locally to be sent to a remote machine, this method returns the
address of the host to which the datagram is addressed (the destination address). This
method is most commonly used to determine the address of the host that sent a UDP
datagram, so that the recipient can reply.
public int getPort()
The getPort() method returns an integer specifying the remote port. If this datagram
was received from the Internet, this is the port on the host that sent the packet. If the
datagram was created locally to be sent to a remote host, this is the port to which the
packet is addressed on the remote machine.
public SocketAddress getSocketAddress()
The getSocketAddress() method returns a SocketAddress object containing the IP
address and port of the remote host. As is the case for getInetAddress() , if the datagram
was received from the Internet, the address returned is the address of the machine that
sent it (the source address). On the other hand, if the datagram was created locally to
be sent to a remote machine, this method returns the address of the host to which the
datagram is addressed (the destination address). You typically invoke this method to
determine the address and port of the host that sent a UDP datagram before you reply.
The net effect is not noticeably different than calling getAddress() and getPort() .
Also, if you're using nonblocking I/O, the DatagramChannel class accepts a SocketAd
dress but not an InetAddress and port.
public byte[] getData()
The getData() method returns a byte array containing the data from the datagram. It's
often necessary to convert the bytes into some other form of data before they'll be useful
to your program. One way to do this is to change the byte array into a String . For
example, given a DatagramPacket dp received from the network, you can convert it to
a UTF-8 String like this:
String s = new String ( dp . getData (), "UTF-8" );
If the datagram does not contain text, converting it to Java data is more difficult. One
approach is to convert the byte array returned by getData() into a ByteArrayInput
Stream . For example:
InputStream in = new ByteArrayInputStream ( packet . getData (),
packet . getOffset (), packet . getLength ());
You must specify the offset and the length when constructing the ByteArrayInput
Stream . Do not use the ByteArrayInputStream() constructor that takes only an array
as an argument. The array returned by packet.getData() probably has extra space in
it that was not filled with data from the network. This space will contain whatever