Simple problem in running a Server class

[Roger Fed]

Hello,
I've developed a client/server application to send datagram packets and receive them through the UDP protocol using two classes
this is the Server class

package client_server_udp;
import java.io.*;
import java.net.*;
public class Server {
    public static  void main(String [] args) throws SocketException, IOException{
        //Create a datagram socket at port 9876
        DatagramSocket serverSocket=new DatagramSocket(9876);
        byte [] receiveData=new byte[1024];
        byte [] sendData=new byte[1024];
        while(true)
        {
            //Create space for received datagram
            DatagramPacket receivePacket=new DatagramPacket(receiveData, receiveData.length);
            //Receive datagram
            serverSocket.receive(receivePacket);
            String sentence=new String(receivePacket.getData());
            //Get IP address and port number from the sender
            InetAddress ipAddress=receivePacket.getAddress();
            int port=receivePacket.getPort();
            String capitalizedSentence=sentence.toUpperCase();
            sendData=capitalizedSentence.getBytes();
            // Create datagram to send to client
            DatagramPacket sendPacket=new DatagramPacket(sendData,sendData.length, ipAddress, port);
            //Write out datagram to socket
            serverSocket.send(sendPacket);
        }
        //End of while loop, loop back and wait for another datagram
    }

}

this is the Client class

package client_server_udp;
import java.io.*;
import java.net.*;
public class Client {

    public static void main(String [] args) throws SocketException, UnknownHostException, IOException{
        //Create input stream
        BufferedReader inFromUser=new BufferedReader(new InputStreamReader(System.in));
        //Create a client socket
        DatagramSocket clientSocket=new DatagramSocket();
        //Translate hostname to IP address using dns
        InetAddress ipAddress=InetAddress.getByName("localhost");
        byte [] sendData=new byte[1024];
        byte [] receiveData=new byte[1024];
        String sentence=inFromUser.readLine();
        sendData=sentence.getBytes();
        //Create datagram packet with data-to-send, length, IP address, port
        DatagramPacket sendPacket=new DatagramPacket(sendData, sendData.length, ipAddress,9876);
        //Send datagram to server
        clientSocket.send(sendPacket);
        //Create a datagram packet to receive packets from server
        DatagramPacket receivePacket=new DatagramPacket(receiveData, receiveData.length);
        //Read datagram from server
        clientSocket.receive(receivePacket);
        String modifiedSentence=new String(receivePacket.getData());
        System.out.println("FROM SERVER: "+modifiedSentence);
        clientSocket.close();



    }

}

The problem is that when i try to run the server class from the command prompt :
javac Server.java
// this works well but when i try to run it using this command:
java Server
//it throws this exception:
Exception in thread "main" java.net.SocketException: Unrecognized Windows Sockets error: 0: Cannot bind
at java.net.PlainDatagramSocketImpl.bind0(Native Method)
at java.net.PlainDatagramSocketImpl.bind(PlainDatagramSocketImpl.java:82)
at java.net.DatagramSocket.bind(DatagramSocket.java:368)
at java.net.DatagramSocket.<init>(DatagramSocket.java:210)
at java.net.DatagramSocket.<init>(DatagramSocket.java:261)
at java.net.DatagramSocket.<init>(DatagramSocket.java:234)
at Server.main(Server.java:8)
How can i get rid of this problem?
thanks in advance

[Tom Reilly]

I Googled the error and a couple of items suggested that the port is already in use. I noticed that your server loops forever and does not clean up after itself (by calling DatagramSocket.close). Could it be that another instance is already running? Or maybe not cleaning up caused the port to remain open? Or perhaps another application is using that port? Try using another port or rebooting to close the existing port (both easy tests).

I tried running your server and got a different error (Address already in use) when I tried running it a second time but I'm not running on Windows.

[Roger Fed]

Thanks tom , this is already the case
I've reboot it and it it works well