import java.net.*;

import java.net.*;

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

public class Server {
public static void main(String args[]) {
ServerSocket server = null;
Socket you = null;
String str = null;
DataInputStream in = null;
DataOutputStream out = null;
try {
server = new ServerSocket(60000);// 绑定到服务器端口，给客户端
System.out.println("服务器启动");
} catch (IOException e1) {
System.out.println(e1);
}
try {
System.out.println("等待客户呼叫……");
you = server.accept();// 堵塞状态，除非有客户呼叫

Socket nextClient = server.accept();// 显示连接细节
System.out.println("收到消息的来源：" + nextClient.getInetAddress() + "\n" + "端口号：" + nextClient.getPort());// 获取消息来源信息

out = new DataOutputStream(you.getOutputStream());
in = new DataInputStream(you.getInputStream());
while (true) {
str = in.readUTF();// in读取客户放入“线路”里的信息，堵塞状态
int m = Integer.parseInt(str);
out.writeUTF("你好：我是服务器");
out.writeUTF("你说的数乘2后是：" + 2 * m);
System.out.println("服务器收到：" + str);
Thread.sleep(500);
}
} catch (Exception e) {
System.out.println("客户已断开！" + e);
}
}
}
数据类型匹配错误：以上是我修改后的代码。

DataOutputStream的构造方法不对吧 你查一下API吧

就in 和 out 写反了嘛~