Java writeObject() to always overwrite existing object, never append? -

i want serialize hashtable file using objectoutputstream.writeobject(), want writer overwrite existing object such single object exists:

fileoutputstream fout = new fileoutputstream( f); objectoutputstream writer = new objectoutputstream(fout); writer.writeobject( hashtable); writer.flush(); 

the hastable updated intermittently @ runtime, use helper method persist state:

private void persistobject( hashtable ht){    writer.writeobject( ht);    writer.flush(); } 

the problem every time call writeobject(), new hastable appended file; there way overwrite whatever in file single object ever persisted?

if want discard file contents , start writing @ beginning of file again, can obtain it's channel , reposition content. problem is, objectstreams uses internal headers, , if want "reset" stream need account that.

just after instantiating objectoutputstream, store initial position (writestreamheader writes 2 shorts - consumes 4 bytes, better generic sorry) can position channel without overwriting header.

final long initialposition = fout.getchannel().position(); 

then, every time have write something, skip initial bytes (by positioning channel), reset stream , write object:

//java 1.7 private void persistobject(hashtable ht){    fout.getchannel().position(initialposition);    fout.reset();    writer.writeobject(ht);    writer.flush(); } 

//java 1.6 private void persistobject(hashtable ht){    fout.getchannel().position(initialposition);    fout.getchannel().truncate(initialposition);    writer.writeobject(ht);    writer.flush(); } 

as @bgp mentioned, don't think opening / closing file introduces overhead... don't think truncating , flushing disk @ every write efficient, or keeping file streams open long periods of time practice (or safe), still quite fun hack objectoutputstream.