largeobject.write(byte[] buf)

I am having problems with large objects. I am using jdbc:postgresql driver
on linux.

I can insert a large object using PreparedStatement.setBytes(byte[] buf)
I can read the large object using ASCIISTream ( they are text objects)

BUT when I use large.object.write(byte[]buf), it overwrites my original
object once, then will not overwrite again.
for eg. if the large object is text "aaaaaaaaaaaaa", then I write "bbb" to
the object, the result is "bbbaaaaaaaaaa", correct.
THEN, if I write "ccccc" to the same object, the answer is "bbbccaaaaaaaa".
It overwrites the part of the object that was the original, but not the part
I overwrote at first.

If this is not fixable, I can unlink and create a new largeobject, but when
I use \lo_list, the unlinked objects are still there.
Can I get rid of them off my machine ??

Thanks, PW