question: public FreeFormPointSet slice ( List < String > ids ) { FreeFormPointSet ret = new FreeFormPointSet ( ids . size ( ) ) ; HashSet < String > idsHashSet = new HashSet < String > ( ids ) ; ret . id = id ; ret . label = label ; ret . description = description ; int n = 0 ; for ( int i = 0 ; i < this . ids . length ; i ++ ) { if ( idsHashSet . contains ( this . ids [ i ] ) ) { ret . lats [ n ] = this . lats [ i ] ; ret . lons [ n ] = this . lons [ i ] ; ret . ids [ n ] = this . ids [ i ] ; ret . polygons [ n ] = this . polygons [ i ] ; n ++ ; } } return ret ; }
target:  Get a subset of this point set containing only the specified point IDs.