How to bach upsert?

Java SDK
1

public List incrementJsonIds(String generatorId, String idPrefix, List contents)
{
if(contents == null)
{
return null;
}
List rjd = new ArrayList(contents.size());

	Observable.from(contents).subscribe(new Action1<String>() {

		@Override
		public void call(String content)
		{
			String id = idPrefix + generatorId + bucket.async().counter(generatorId, 1)
					.map(jsonLongDocument -> {
						return jsonLongDocument.content();
					}).toBlocking().single(); 
			LOGGER.debug("\n generatorId:" + id + "\n content:" + content);
			
			bucket.async().upsert(RawJsonDocument.create(id, content));
			
		}});
	
	return rjd;
}

I don’t know how to return RawJsonDocuemnt after upsert