Thanks for your attention, but I have solved this with this code:
private void linkSitelinksToAdGroup(GoogleAdsClient googleAdsClient, List<String> sitelinkAssetResourceNames, AddSitelinkUsingAssetsParams params) {
// Creates AdGroupAssets representing the association between sitelinks and ad groups.
List<AdGroupAssetOperation> adGroupAssetOperations = sitelinkAssetResourceNames.stream().map(resName -> AdGroupAsset.newBuilder()
.setAsset(resName)
.setAdGroup(ResourceNames.adGroup(params.customerId, params.adGroupId))
.setFieldType(AssetFieldType.SITELINK)
.build())
// Creates an AdGroupAssetOperation to create the AdGroupAsset.
.map(a -> AdGroupAssetOperation.newBuilder().setCreate(a).build())
.collect(Collectors.toList());
// Creates the service client.
try (AdGroupAssetServiceClient client = googleAdsClient.getLatestVersion().createAdGroupAssetServiceClient()) {
// Sends the mutate request.
MutateAdGroupAssetsResponse response = client.mutateAdGroupAssets(String.valueOf(params.customerId), adGroupAssetOperations);
// Prints some information about the result.
for (MutateAdGroupAssetResult result : response.getResultsList()) {
System.out.printf("Linked sitelink to ad group with resource name '%s'.%n", result.getResourceName());
}
}
}