String fileName = "topic-map.xtm";

try{
  XTMSerializer xtmS = new XTMSerializer();

  FileOutputStream os = new FileOutputStream(fileName);

  xtmS.serialize(os, tm);
}
catch(TMAPIException tmae) {
  System.err.println("TMAPI Error");
} 
catch (FileNotFoundException fnfe) {
  System.err.println("Couldn't write file");
}
catch (SecurityException se) {
  System.err.println("No Permissions");
}