question: public static String extractMusicIDFromMediaID ( @ NonNull String mediaID ) { int pos = mediaID . indexOf ( LEAF_SEPARATOR ) ; if ( pos >= 0 ) { return mediaID . substring ( pos + 1 ) ; } return null ; }
target:  Extracts unique musicID from the mediaID. mediaID is, by this sample's convention, a concatenation of category (eg "by_genre"), categoryValue (eg "Classical") and unique musicID. This is necessary so we know where the user selected the music from, when the music exists in more than one music list, and thus we are able to correctly build the playing queue.