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