网络基础

在java中，客户机和服务器实现的具体方法是什么？它们是怎么运行的？以下面的代码来解释一下……
package test4;

import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.IOException;
import java.net.ServerSocket;
import java.net.Socket;

public class ServerDemo {
public static void main(String[] args) {
ServerSocket server = null;
Socket clientSocket = null;
String str = null;
DataOutputStream out = null;
DataInputStream in = null;
try {
server = new ServerSocket(4331);
clientSocket = server.accept();
in = new DataInputStream(clientSocket.getInputStream());
out = new DataOutputStream(clientSocket.getOutputStream());
while(true){
str = in.readUTF();//从内存读取
out.writeUTF("hello，我是服务器");//写入内存
out.writeUTF(str);//在将读到的值写入内存
System.out.println("服务器收到："+str);
Thread.sleep(1000);
}
} catch (IOException e) {
e.printStackTrace();
} catch (InterruptedException e) {
e.printStackTrace();
}

}
}
package test4;

import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.IOException;
import java.net.Socket;
import java.net.UnknownHostException;

public class SocketUtil {
public static void main(String[] args) {
String str =null;
Socket clientSocket;
DataInputStream in ;
DataOutputStream out;
try {
clientSocket = new Socket("localhost",4331);
in = new DataInputStream(clientSocket.getInputStream());
out = new DataOutputStream(clientSocket.getOutputStream());
out.writeUTF("你好");//写入内存
while(true){
str = in.readUTF();//从内存读取数据
out.writeUTF(((int)(Math.random()*10)+1)+"");//写入内存
System.out.println("客户端收到："+str);
Thread.sleep(1000);
}
} catch (UnknownHostException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
} catch (InterruptedException e) {
e.printStackTrace();
}
}

}
谢谢高手赐教……