Some quesstion about elasticsearch index optimization

Elastic Stack Elasticsearch
1

Hi all
I Just beginning to learn elasticsearch，have some quesstion as

  1. how can i set index optimize threshold in configuration files ?
  2. i send an order as
    curl -XPOST 'http://localhost:9200/gindex/_optimize' -d '{"max_num_segments":"1","wait_for_merge":"false"}'
    ,but no change in the index,it can't optimize.
  3. i use java api,how can i traversal SearchHit.getHighlightFields() ?

Thank you.!