<services>
 <service type="0">...</service>
 <service type="1">...</service>
 <service type="1">...</service>
</services>

public enum Type {
 FILE("0","File"){
    @Override
    public Service createServiceInstance(ServiceAttributes attributes){
      return FileService.createInstance(attributes);
    }
 },
 NETWORK("1","Network"){
    // overridden createServiceInstance method
 }
 // declaration of string type variables for typeCode and typeName
 // constructor declaration with two arguments typeCode and typeName
 public abstract Service createServiceInstance(ServiceAttribtes attributes);
}

public class Service {
  //declaration of variables representing common data;
  private Type type;
  // getters and setters
}

public class NetworkService extends Service {
  private Link link;
  // getters and setters
  public static Service createInstance(ServiceAttributes attributes) {
     NetworkService ns = new NetworkService();
     ns.setName(attributes.getName());
     ns.setType(attributes.getType())
     ns.setLink(attributes.getLink())
     return ns;
  }
}

public class ServiceTypeAdapter extends XmlAdapter<ServiceTypeAdapter.ServiceAttributes, Service> {
 public ServiceAttributes marshal(Service service) throws Exception {
   // currently I need only the unmarshalling method
   // And I know this will never be required. What should I do here?
   // return null or throw new UnsupportedOperationException() ?
   return null;
 }
 public Service unmarshal(ServiceAttributes serviceAttributes) {
    Type serviceType = serviceAttributes.getType();
    return serviceType.createServiceInstance(serviceAttributes);
 }

 public class ServiceAttributes {
   private String name;
   @XmlAttribute
   private Type type;
   // declaration of attributes that will be present in
   // each of service classes

   // getters and setters
 }
}