java小服务端问题？有程序请高手指点
import java.net.*;import java.io.*;
import java.util.*;
public class ChatServer{
static final int PORT = 4000;//设置服务端口
private byte[] buf = new byte[1000];
private DatagramPacket dgp =new DatagramPacket(buf,buf.length);
private DatagramSocket sk;
//方法说明：服务端构造器，实现读取用户输入和通讯
public ChatServer(){
try{
//实例化数据报
sk = new DatagramSocket(PORT);
System.out.println("Server started");
while(true){
//等待接收
sk.receive(dgp);
//获取接收信息
String rcvd = new String(dgp.getData(),0,dgp.getLength())+
", from address: "+ dgp.getAddress()+ ", port: "+ dgp.getPort();
System.out.println(rcvd);
String outMessage ="";
//读取输入
BufferedReader stdin = new BufferedReader(new InputStreamReader(System.in));
try{
outMessage = stdin.readLine();
}catch(IOException ie){ System.err.println("IO error!"); }
String outString = "Server say: "+ outMessage;
//拷贝字符到缓存
byte[] buf = outString.getBytes();
//打包数据，发送回信息。
DatagramPacket out = new DatagramPacket(buf,buf.length,dgp.getAddress(),dgp.getPort());
sk.send(out);
}
}catch(SocketException e){
System.err.println("Can't open socket");
System.exit(1);
}catch(IOException e){
System.err.println("Communication error");
e.printStackTrace();
System.exit(1);
}
}
public static void main(String[] args){ new ChatServer(); }
}
哪位能告诉我为什么不能打开socket?
我运行了显示时 can't open socket!
哪位高手指点一下？
