Javaネットワークプログラミングの簡単なアプリケーションルーチン

import java.io.*;
import java.net.*;
public class HelloServer{
    public static void main(String args[]) throws IOException
    {
        ServerSocket serverSocket = null;
        PrintWriter out = null;
        try{
            serverSocket = new ServerSocket(9999);
        }
        catch(IOException e)
        {
            System.err.println("Counld not listen on port:9999");
            System.exit(1);
        }
        Socket clientSocket = null;
        try{
            clientSocket = serverSocket.accept();
        }
        catch(IOException e)
        {
            System.err.println("Accept failed");
            System.exit(1);
        }
        out = new PrintWriter(clientSocket.getOutputStream(),true);
        out.println("hello world!");
        clientSocket.close();
        serverSocket.close();
    }
}

import java.io.*;
import java.net.*;
public class HelloClient{
    public static void main(String args[]) throws IOException
    {
        Socket helloSocket = null;
        BufferedReader in  = null;
        try{
            helloSocket = new Socket("172.20.223.73",9999);
            in = new BufferedReader(new InputStreamReader(helloSocket.getInputStream()));
        }
        catch(UnknownHostException e){
            System.err.println("Don't know about host:localhost!");
            System.exit(1);
        }
        catch(IOException e)
        {
            System.err.println("Couldn't get I/O for the connection");
            System.exit(1);
        }
        System.out.println(in.readLine());
        in.close();
        helloSocket.close();
    }
}