code: public Observable<ServiceResponse<IPAddressAvailabilityResultInner>> checkIPAddressAvailabilityWithServiceResponseAsync(String resourceGroupName, String virtualNetworkName) {
          if (resourceGroupName == null) {
              throw new IllegalArgumentException("Parameter resourceGroupName is required and cannot be null.");
          }
          if (virtualNetworkName == null) {
              throw new IllegalArgumentException("Parameter virtualNetworkName is required and cannot be null.");
          }
          if (this.client.subscriptionId() == null) {
              throw new IllegalArgumentException("Parameter this.client.subscriptionId() is required and cannot be null.");
          }
          final String apiVersion = "2018-07-01";
          final String ipAddress = null;
          return service.checkIPAddressAvailability(resourceGroupName, virtualNetworkName, this.client.subscriptionId(), ipAddress, apiVersion, this.client.acceptLanguage(), this.client.userAgent())
              .flatMap(new Func1<Response<ResponseBody>, Observable<ServiceResponse<IPAddressAvailabilityResultInner>>>() {
                  @Override
                  public Observable<ServiceResponse<IPAddressAvailabilityResultInner>> call(Response<ResponseBody> response) {
                      try {
                          ServiceResponse<IPAddressAvailabilityResultInner> clientResponse = checkIPAddressAvailabilityDelegate(response);
                          return Observable.just(clientResponse);
                      } catch (Throwable t) {
                          return Observable.error(t);
                      }
                  }
              });
      }
docs: Checks whether a private IP address is available for use.
  
  @param resourceGroupName The name of the resource group.
  @param virtualNetworkName The name of the virtual network.
  @throws IllegalArgumentException thrown if parameters fail the validation
  @return the observable to the IPAddressAvailabilityResultInner object