code: public ServiceFuture<P2SVpnGatewayInner> createOrUpdateAsync(String resourceGroupName, String gatewayName, P2SVpnGatewayInner p2SVpnGatewayParameters, final ServiceCallback<P2SVpnGatewayInner> serviceCallback) {
          return ServiceFuture.fromResponse(createOrUpdateWithServiceResponseAsync(resourceGroupName, gatewayName, p2SVpnGatewayParameters), serviceCallback);
      }
docs: Creates a virtual wan p2s vpn gateway if it doesn't exist else updates the existing gateway.
  
  @param resourceGroupName The resource group name of the P2SVpnGateway.
  @param gatewayName The name of the gateway.
  @param p2SVpnGatewayParameters Parameters supplied to create or Update a virtual wan p2s vpn gateway.
  @param serviceCallback the async ServiceCallback to handle successful and failed responses.
  @throws IllegalArgumentException thrown if parameters fail the validation
  @return the {@link ServiceFuture} object