UDP - Java Network Programming - page 411

public DatagramSocket(int port, InetAddress interface) throws SocketException
This constructor is primarily used on multihomed hosts; it creates a socket that listens
for incoming datagrams on a specific port and network interface. The port argument
is the port on which this socket listens for datagrams. As with TCP sockets, you need
to be root on a Unix system to create a DatagramSocket on a port below 1024. The
address argument is an InetAddress object matching one of the host's network ad‐
dresses. A SocketException is thrown if the socket can't be created. There are three
common reasons for this constructor to fail: the specified port is already occupied, you
are trying to connect to a port below 1024 and you're not root on a Unix system, or
address is not the address of one of the system's network interfaces.
public DatagramSocket(SocketAddress interface) throws SocketException
This constructor is similar to the previous one except that the network interface address
and port are read from a SocketAddress . For example, this code fragment creates a
socket that only listens on the local loopback address:
SocketAddress address = new InetSocketAddress ( "127.0.0.1" , 9999 );
DatagramSocket socket = new DatagramSocket ( address );
protected DatagramSocket(DatagramSocketImpl impl) throws SocketException
This constructor enables subclasses to provide their own implementation of the UDP
protocol, rather than blindly accepting the default. Unlike sockets created by the other
four constructors, this socket is not initially bound to a port. Before using it, you have
to bind it to a SocketAddress using the bind() method:
public void bind ( SocketAddress addr ) throws SocketException
You can pass null to this method, binding the socket to any available address and port.
Sending and Receiving Datagrams
The primary task of the DatagramSocket class is to send and receive UDP datagrams.
One socket can both send and receive. Indeed, it can send and receive to and from
multiple hosts at the same time.
public void send(DatagramPacket dp) throws IOException
Once a DatagramPacket is created and a DatagramSocket is constructed, send the packet
by passing it to the socket's send() method. For example, if theSocket is a Datagram
Socket object and theOutput is a DatagramPacket object, send theOutput using the
Socket like this:
theSocket . send ( theOutput );
If there's a problem sending the data, this method may throw an IOException . However,
this is less common with DatagramSocket than Socket or ServerSocket , because the
unreliable nature of UDP means you won't get an exception just because the packet