udp socket in jave , the receiver not receive message

package UDP_Socket;

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

public class PacketReceiver {
    public static void main(String[] args) {
        try {
            
              
// Create a datagram socket, bound to the specific port 9999
            DatagramSocket socket = new DatagramSocket(9999);
// Create a datagram packet, containing a maximum buffer of 256 bytes
            DatagramPacket packet = new DatagramPacket(new byte[256], 256);
// Receive a packet - remember by default this is a blocking operation
            //socket.setSoTimeout(10000);
            socket.receive(packet);
            System.out.println("Packet received!");
// Display packet information
            InetAddress remote_addr = packet.getAddress();
            System.out.println("Sent by host: " + remote_addr.getHostAddress());
            System.out.println("Sent from port: " + packet.getPort());
// Display packet contents, by reading from byte array
            ByteArrayInputStream bin = new ByteArrayInputStream(packet.getData());
// Display only up to the length of the original UDP packet
            for (int i = 0; i < packet.getLength(); i++) {
                int data = bin.read();
                if (data == -1)
                    break;
                else
                    System.out.print((char)data);
            }
            socket.close();
        } catch (IOException ioe) {
            System.err.println("Error - " + ioe);
        }
    }
}

package UDP_Socket;

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

public class PacketSender {
    public static void main(String[] args) {

        try {
// Hostname/IP address of the remote server  
            String hostname = "localhost";
// Lookup the specified hostname, and get an InetAddress
            System.out.println("Looking up hostname " + hostname);
            InetAddress remote_addr = InetAddress.getByName(hostname);
            System.out.println("Hostname resolved as " + remote_addr.getHostAddress());
// Create a datagram socket, bound to any available local port
            DatagramSocket socket = new DatagramSocket();
// Create a message to send using a UDP packet
            ByteArrayOutputStream bout = new ByteArrayOutputStream();
            PrintStream pout = new PrintStream(bout);
            pout.print("Greetings!");
// Get the contents of our message as an array of bytes
            byte[] barray = bout.toByteArray();
// Create a datagram packet, containing our byte array
            DatagramPacket packet = new DatagramPacket(barray, barray.length);
// Set the destination address of the packet 
            packet.setAddress(remote_addr);
// Set port number to 9999
            packet.setPort(9999);
// Send the packet - remember no guarantee of delivery
            socket.send(packet);
            System.out.println("Packet sent!");
            socket.close();
        } catch (UnknownHostException uhe) {
            System.err.println("Can't find host " + uhe.getMessage());
        } catch (IOException ioe) {
            System.err.println("Error - " + ioe);
        }
    }
}