@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;
}

@ 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 ; }
count number of values of given column .
