The client must identify parameter data types

<namesp1:sayHelloTo xmlns:namesp1="urn:HelloServer">
  <c-gensym3 xsi:type="xsd:string">Tim O'Reilly</c-gensym3>
</namesp1:sayHelloTo>

The server must identify return value data types

 <ns1:sayHelloToResponse 
 soapenv:encodingStyle="http://schemas.xmlsoap.org/soap/encoding/" 
   xmlns:ns1="urn:HelloServer">
   <sayHelloToReturn xsi:type="xsd:string">Hello Tim O'Reilly</sayHelloToReturn>
  </ns1:sayHelloToResponse>

The class definition's data types

public class HelloServer {
    public String sayHelloTo(String name) {
         return "Hello " + name;
    }
}

Study the request and response envelope contents for a moment here. Note that SOAP parameter and return types of the class (both Java String's) corresponds directly to the SOAP types in the request and response ("xsd:string").