code: public ServiceFuture<List<VpnConnectionInner>> listByVpnGatewayAsync(final String resourceGroupName, final String gatewayName, final ListOperationCallback<VpnConnectionInner> serviceCallback) {
          return AzureServiceFuture.fromPageResponse(
              listByVpnGatewaySinglePageAsync(resourceGroupName, gatewayName),
              new Func1<String, Observable<ServiceResponse<Page<VpnConnectionInner>>>>() {
                  @Override
                  public Observable<ServiceResponse<Page<VpnConnectionInner>>> call(String nextPageLink) {
                      return listByVpnGatewayNextSinglePageAsync(nextPageLink);
                  }
              },
              serviceCallback);
      }
docs: Retrieves all vpn connections for a particular virtual wan vpn gateway.
  
  @param resourceGroupName The resource group name of the VpnGateway.
  @param gatewayName The name of the 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