In the page https://lucene.apache.org/core/5_5_0/demo/overview-summary.html#overview_description in the subsection titled "Indexing Files" the command suggested is:
java org.apache.lucene.demo.IndexFiles -docs {path-to-lucene}/src
I believe the file structure in the source file downloads has changed
and the command should be edited to:
java org.apache.lucene.demo.IndexFiles -docs {path-to-lucene}
to avoid confusion.
