Thanks for the hint, but the problem isn't yet solved.
It further it led me to an article where accessing nested tables seems
to work with Oracle 9i, since you can create Java objects within the DB.
Unfortunately I am using Oracle 8i and I haven't found any way to access
nested tables without creating Java objects in the DB, which is only
supported by 9i. Though I am using Java and JDBC to access the DB I
cannot create Java objects inside the DB in the first place.
Accessing PL*SQL generated objects that reside within the nested table
would be my next problem but Oracle 8i and JDBC seem to quit with the
nested tables already. I'm going to do the PL*SQL logic in Java and just
fire some SELECTs over JDBC to the DB, since I don't see any other way
around this issue.