一个最简单的服务器与客户机的乱码问题

就是不能够用传递中文信息，出现乱码"?????",这个问题我想了几天也没有搞出来，求大家帮帮我！！！（附源程序）//服务器程序
import java.io.*;
import java.net.*;
public class tcpserver
{
public static void main(String[] args) throws IOException
{
ServerSocket svrsoc=null;
Socket soc=null;
DataInputStream in=null;
PrintStream out=null;
InetAddress clientIP=null;
String str=null;
try
{
svrsoc=new ServerSocket(5555);
System.out.println("Server start....");
soc=svrsoc.accept();in=new DataInputStream(soc.getInputStream());
out=new PrintStream(soc.getOutputStream());
clientIP=soc.getInetAddress();
System.out.println("Client's IP address:"+clientIP);
System.out.println("Port:"+soc.getLocalPort());
System.out.println("Client's Name:"+soc.getInetAddress().getHostName());
out.println("welcome.....");
in.readLine();
while (!str.equals("quit"))
{
System.out.println("Client said:"+str);
str=in.readLine();
}
System.out.println("Client want to leave");
}
catch(Exception e)
{
System.out.println("error:"+e);
}
finally
{
in.close();
out.close();
soc.close();
svrsoc.close();
System.exit(0);
}
}
}
//客户机程序
import java.io.*;
import java.net.*;
public class tcpclient
{
public static void main(String[] args) throws IOException
{
Socket soc=null;
DataInputStream in=null;
PrintStream out=null;
DataInputStream sysin=null;
String strin=null;
String strout=null;
try
{
soc=new Socket("127.0.0.1",5555);
System.out.println("Connecting to the Server");
in=new DataInputStream(soc.getInputStream());
out=new PrintStream(soc.getOutputStream());
strin=in.readLine();
System.out.println("Server said:"+strin);
sysin=new DataInputStream(System.in);
strout=sysin.readLine();
while (!strout.equals("quit"))
{
out.println(strout);
strout=sysin.readLine();
}
out.println(strout);
}
catch(Exception e)
{
System.out.println("error:"+e);
}
finally
{
in.close();
out.close();
sysin.close();
soc.close();
System.exit(0);
}
}
}