How delete rows in table with free disk space?

[Ekrame Ayari]

Hi

I wish to remove table rows to allow user decrease space disk.
I try to delete table rows for free disk space with this code :

"Connect connect = DriverManager.getConnection("jdbc:derby:GenoCensusMin2");
Statement stat = connect.createStatement();
stat.executeUpdate("DELETE FROM Project WHERE id="+idProjet);"

But disk space size of my system directory is the same before and after 
the deletion.

Can you help me?

Sincerly

E'Krame Jacoby
IntegraGen SA (France)