It first depends on what they define as secure file deletion. If they mean
overwriting the contents of the file in a secure way, then I suspect its
just a matter of writing carefully picked bit patterns across the file
length X number of times. But the problem is you can not necessarily
guarantee that it write on the same sectors as the file originally was
stored. For example, some file systems move the physical files around to
prevent fragmentation. Others employ other techniques. So to make a claim
of secure file deletion you have to understand the underlying file system
supported and you have to use either the kernel api, the fs driver or raw
disk access directly
In any case, I think this rules out a pure java implementation, you could
use a jni implementation, but then you are not far off using a c version
instead. In any case the program is bound to the specific system and file
system so there is no added value in using java.
FYI, there are companies that specialise in dismantling a hard drive and
with special tools read data of individual physical sectors of the actual
discs in the drive. One company I know of,
www.ibas.com, has been able to
read data of a disk sector where the data had been overwritten ten times
already. They sell a software tool that almost guarantees secure deletion.
That is why the military disposes of their most sensitive hard disks with
the help of an acid bath instead.