The Grouping feature in Lucene offers a way to group results together by a group field. Say you may want to list books by category so it'd be convenient to display results when the resulting documents are already grouped by the group field. Another example is a web search engine where results may be grouped by sites, so a site with excessive numbers of matching pages does not overwhelm the top result page. The search engine can show the top matching page from each site so there is more variety of quality matches.
Lucene has two implementations for grouping a single-pass search and a two-pass search. A single-pass search requires document grouping at index time and a two-pass search requires two searches to provide both group-level and document-level results.
To set up a single-pass search, we will add a document in document blocks by calling IndexWriter.addDocuments. We also need to add an end of the document marker, as Field, to the last document in each document block...
