How shrink datafiles in a database?

set pages 1000
set lines 250
column value new_val blksize

select value from v$parameter where name = 'db_block_size';

select 'alter database datafile '''||file_name||''' resize ' ||
       ceil( (nvl(hwm,1)*&&blksize)/1024/1024 )  || 'm;' SQL_STMT
from dba_data_files a,
     ( select file_id, max(block_id+blocks-1) hwm
         from dba_extents
        group by file_id ) b
where a.file_id = b.file_id(+)
  and ceil( blocks*&&blksize/1024/1024) -
      ceil( (nvl(hwm,1)*&&blksize)/1024/1024 ) > 0;