Why do we pass an adapter to the Section Indexer object in android?
It actually says in android documentation but basically it stands for fast scrolling.I recommend you to read https://developer.android.com/reference/android/widget/SectionIndexer.html and if you want to implement http://www.survivingwithandroid.com/2012/12/android-listview-sectionindexer-fastscroll.html this link.However android developer site explains more detailed.