java调用webservice，方法里有这样的一个参数out string retV

调用webservice方法： public int getMA(string UN, string UP,out string retV)out string retV:这个是返回值参数，（不知道理解的对不对）代码：
            String username="111";
            String password="123456";
            String service_url = "http://111.111.111.111:8080/qq.asmx";
            
            Service service = new Service();
            Call call = (Call) service.createCall();
            call.setTargetEndpointAddress(new java.net.URL(service_url));
            
            call.setOperationName(new QName("http://tempuri.org/", "getMA"));            call.addParameter("UN", org.apache.axis.encoding.XMLType.XSD_STRING,
                    javax.xml.rpc.ParameterMode.IN);            call.addParameter("UP", org.apache.axis.encoding.XMLType.XSD_STRING,
                    javax.xml.rpc.ParameterMode.IN);            call.addParameter("retV", org.apache.axis.encoding.XMLType.XSD_STRING,
                  javax.xml.rpc.ParameterMode.INOUT);
            
            call.setReturnType(org.apache.axis.encoding.XMLType.XSD_STRING);            call.setUseSOAPAction(true);            call.setSOAPActionURI("http://tempuri.org/getMA");            String qq = "";            
           String res = (String)call.invoke(
                    new Object[]{
                     username,password,qq
                    }
            );
            System.out.println(res);
到了最后的调用方法，异常提示：服务器不能处理请求，值不能为空。异常：
AxisFault
 faultCode: {http://schemas.xmlsoap.org/soap/envelope/}Server
 faultSubcode: 
 faultString: Server was unable to process request. ---> Value cannot be null.
Parameter name: input
 faultActor: 
 faultNode: 
 faultDetail: 
{http://xml.apache.org/axis/}stackTrace:Server was unable to process request. ---> Value cannot be null.
at org.apache.axis.message.SOAPFaultBuilder.createFault(SOAPFaultBuilder.java:222)
at org.apache.axis.message.SOAPFaultBuilder.endElement(SOAPFaultBuilder.java:129)
at org.apache.axis.encoding.DeserializationContext.endElement(DeserializationContext.java:1087)
at com.sun.org.apache.xerces.internal.parsers.AbstractSAXParser.endElement(AbstractSAXParser.java:633)
at com.sun.org.apache.xerces.internal.impl.XMLNSDocumentScannerImpl.scanEndElement(XMLNSDocumentScannerImpl.java:713)
at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl$FragmentContentDispatcher.dispatch(XMLDocumentFragmentScannerImpl.java:1685)
at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl.scanDocument(XMLDocumentFragmentScannerImpl.java:368)
at com.sun.org.apache.xerces.internal.parsers.XML11Configuration.parse(XML11Configuration.java:834)
at com.sun.org.apache.xerces.internal.parsers.XML11Configuration.parse(XML11Configuration.java:764)
at com.sun.org.apache.xerces.internal.parsers.XMLParser.parse(XMLParser.java:148)
at com.sun.org.apache.xerces.internal.parsers.AbstractSAXParser.parse(AbstractSAXParser.java:1242)
at javax.xml.parsers.SAXParser.parse(SAXParser.java:375)
at org.apache.axis.encoding.DeserializationContext.parse(DeserializationContext.java:227)
at org.apache.axis.SOAPPart.getAsSOAPEnvelope(SOAPPart.java:696)
at org.apache.axis.Message.getSOAPEnvelope(Message.java:435)
at org.apache.axis.handlers.soap.MustUnderstandChecker.invoke(MustUnderstandChecker.java:62)
at org.apache.axis.client.AxisClient.invoke(AxisClient.java:206)
at org.apache.axis.client.Call.invokeEngine(Call.java:2784)
at org.apache.axis.client.Call.invoke(Call.java:2767)
at org.apache.axis.client.Call.invoke(Call.java:2443)
at org.apache.axis.client.Call.invoke(Call.java:2366)
at org.apache.axis.client.Call.invoke(Call.java:1812)