Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 0

这个程序出现Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 0这个下标越界错误，请教下怎么改。。多谢啦
package server;import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintStream;
import java.net.Socket;
import java.net.UnknownHostException;public class CilentSocketTest extends Thread{

private Socket sk;
private String name;

public CilentSocketTest(Socket _sk,String _name){
this.sk=_sk;
this.name=_name;
}

public void run(){ BufferedReader br;
try {
br = new BufferedReader(new InputStreamReader(sk.getInputStream()));
String str="";
while((str=br.readLine())!=null){
if (str.startsWith("exit")){
System.exit(0);
}
System.out.println(str);
}
br.close();
sk.close();


} catch (IOException e) {
e.printStackTrace();
}

}

public static void main(String[] args) {
try {
Socket client=new Socket("localhost",1999);
PrintStream ps=new PrintStream(client.getOutputStream());
CilentSocketTest cst=new CilentSocketTest(client,args[0]);
cst.start();
BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
String st="";
while((st=br.readLine())!=null){
ps.println(cst.name+":"+st);
ps.flush();
if (st.startsWith("exit")){
System.exit(0);
}
}
ps.close();
client.close();

} catch (UnknownHostException e) {

e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
}
}