updating CLOBs and BLOBs

I use Orcale 8.1.5 with JDBC thin driver.  When I want to update a CLOB column in a table, I make something like that:
 out = clob.getAsciiOutputStream();
 out.write(source); // where source is a byte[]  out.close();

 That works but if I had previously a longer data in the database, the end of the data remains.
 Example:
 I store "abcdef"
 Then I store "xx"
 When I get back the value, I obtain "xxcdef"

 Should I override the data with an empty_clob before storing the new value ?

The same problem occurs with BLOBs.

 Any ideas ?
 Thanks.

-- 
leriche_at_abaksystemes.fr
ABAK Systèmes, ZI Le Rachat - BP 5 - 35890 LAILLE
Tel: +33(0) 299.42.30.10, Fax: +33(0) 299.42.31.57