question: @ Override public Object compute ( ArrayList < Map < String , Object > > rows ) throws Exception { if ( column . equals ( "*" ) ) { return rows . size ( ) ; } long count = 0 ; for ( Map < String , Object > row : rows ) { if ( row . containsKey ( column ) && ( row . get ( column ) != null ) ) { count ++ ; } } return count ; }
target:  Count number of values of given column.