webservices 返回集合问题


//webservices 调用接口
public interface Help {      Collection<String> getAssetByRfid();

}//接口实现类
public class RfidHelpImp implements RfidHelp {
   
     public Collection<String> getAssetByRfid(){
            Collection<String> c = new ArrayList<String>();
            c.add("-2,,,,,,,");
   c.add("-3,,,,,,,");
            return c;
     
     }}
//测试类
public class WSClient {

public static void main(String[] args) {
try{    
     String serverURLStr = "http://localhost:8089/service1/service/RfidHelp?wsdl";
                        Client client = new Client(new URL( serverURLStr ));    
                        Object[] s= client.invoke("getAssetByRfid", new Object[0]);  
                        System.out.println(s.length);
                        for(int i=0;i<s.length;i++)
                            System.out.println(s[i].toString());
                     } catch(Exception e){    
                             throw new RuntimeException(e);   
                      }

}

}打印结果为:
1
[#document: null]什么原因.Collection集合因不需序列化啊