Modifying table structure within a transaction?
As DDL implicitly commits, the only way to "rollback" your changes is to construct the reverse operation and apply it to revert the change, as a_horse_with_no_name states. 

Constructing such a rollback won't always be straightforward however. If data could be written to the table between type modifications (varchar2(10) -> varchar2(50), number -> varchar2) and rolling this back then you'll also have to check the new data will be valid when reverting to the original type (or perform some conversion). Be aware that dropping columns on large tables could take some time and generate large amounts of redo.

You'll also have to be wary of invalidating any stored procedures on your database and other application dependencies as a result of these changes.

The flashback option won't help you in this instance. Once you've made DDL changes to a table, you can't restore it to it's previous state using flashback. Trying to do so will give you the error:

ORA-01466: unable to read data - table definition has changed

Flashing-back your full database would be overkill and also not possible from the Java app - you need to shut down and then mount the database to complete this operation. 

Which all raises the question of what your tool is for. If you just need a GUI for people to edit tables, then something like Oracle SQL Data Modeler can do this and generate DDL scripts for you. These can then be validated, tested, an appropriate rollback constructed and applied to the database. Modifying the structure of a (production) database should be done with care and tested to ensure that all changes are valid!