/**
* <p>Expert: Collectors are primarily meant to be used to
* gather raw results from a search, and implement sorting
* or custom result filtering, collation, etc. </p>
*
* <p>Lucene's core collectors are derived from {@link Collector}
* and {@link SimpleCollector}. Likely your application can
* use one of these classes, or subclass {@link TopDocsCollector},
* instead of implementing Collector directly:
*
* <ul>
*
* <li>{@link TopDocsCollector} is an abstract base class
* that assumes you will retrieve the top N docs,
* according to some criteria, after collection is
* done. </li>
*
* <li>{@link TopScoreDocCollector} is a concrete subclass
* {@link TopDocsCollector} and sorts according to score +
* docID. This is used internally by the {@link
* IndexSearcher} search methods that do not take an
* explicit {@link Sort}. It is likely the most frequently
* used collector.</li>
*
* <li>{@link TopFieldCollector} subclasses {@link
* TopDocsCollector} and sorts according to a specified
* {@link Sort} object (sort by field). This is used
* internally by the {@link IndexSearcher} search methods
* that take an explicit {@link Sort}.
可以随意定制需要获取的内容。