code: public List<String> keysForBucket(String bucket) {
          AmazonS3 s3 = getClient();
          List<String> ret = new ArrayList<>();
          ListObjectsRequest listObjectsRequest = new ListObjectsRequest().withBucketName(bucket);
          ObjectListing objectListing;
  
          do {
              objectListing = s3.listObjects(listObjectsRequest);
              for (S3ObjectSummary objectSummary : objectListing.getObjectSummaries()) {
                  ret.add(objectSummary.getKey());
              }
              listObjectsRequest.setMarker(objectListing.getNextMarker());
          } while (objectListing.isTruncated());
  
          return ret;
      }
docs: Return the keys for a bucket
  @param bucket the bucket to get the keys for
  @return the bucket's keys