The client code:

The code for the client is shown below:

import java.net. *;
import java.io. *;

public class DatagramClient
{
// number of bytes in a UDP packet
static final int UDP_PACKET_SIZE = 512;

// port number of UDP service
static final int PORT_NUMBER = 4000;
private DatagramSocket datagramSocket;

// constructor
public DatagramClient ( )
{

try
{

// create a socket 
datagramSocket = new DatagramSocket( ); 

}

catch (IOException e)

{
System.out.println("Trouble setting up datagram socket" + e);
}
}
// send a datagram containing a message
public void sendDatagramPacket (String message)
{
DatagramPacket packet; // UDP packet

// convert string to bytes
byte [] toBeSent = new byte [UDP_PACKET_SIZE ];
toBeSent = message.getBytes( );
try
{
/ * UDP packet contains the data, total length of data, destination IP address and destination port. */
packet = new DatagramPacket(toBeSent, toBeSent.length, InetAddress.getLocalHost( ), PORT_NUMBER);
datagramSocket.send(packet);
}
catch (IOException e)
{
System.out.println("Trouble sending datagram packet" + e);
}
}
}

The DatagramClient has a simple constructor, which sets up a datagram socket to be used for sending datagrams. Because UDP does not set up a connection (unlike TCP) this socket does not require any details of the destination. Only the individual packets contain the destination details, as we shall see later.

The client class has one method, sendDatagramPacket, which has a string argument specifying the text of the message to be sent. The packet data is a string containing the message text. The first part of the method formats the data appropriately and stores the string in byte format in the buffer toBeSent. We then create a datagram packet containing this data together with the packet header information required by the UDP protocol. This header information includes the destination IP address and port number as well as the number of bytes of data. The packet is then sent via the datagram socket to the specified IP address and port number. In this example, we are assuming the server is on the local host, as we specify the destination address using the static method InetAddress.getLocalHost. This is an alternative to specifying the loopback address "127.0.0.1".

We test this class using the main method of class TestDatagramClient, which is as follows:

public class TestDatagramClient
{
public static void main (String [] args)
{
// message to tell the server we are quitting
final String CLIENT_QUITTING = "Client quitting";

DatagramClient dc = new DatagramClient( );
// create and send some packets

dc.sendDatagramPacket("Wish you were here");

dc.sendDatagramPacket("Weather is lovely");

// send a packet to tell the server we are quitting
dc.sendDatagramPacket(CLIENT_QUITTING);
}
}

This tests the class by creating a DatagramClient object and sending several messages, terminating with the message CLIENT_QUITTING. This is a special message to indicate to the server that the client has finished sending datagrams. Because UDP is connectionless, there is no need to close anything before the client program terminates - we just exit.

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