private void writeObject(ObjectOutputStream out) throws IOException {
  out.writeObject(mapType);
  out.writeInt(num);
  for (  Entry e : this) {
    out.writeObject(e.getKey());
    out.writeObject(e.getValue());
  }
}

private void writeObject ( ObjectOutputStream out ) throws IOException { out . writeObject ( mapType ) ; out . writeInt ( num ) ; for ( Entry e : this ) { out . writeObject ( e . getKey ( ) ) ; out . writeObject ( e . getValue ( ) ) ; } }
format : maptype , num , ( key , value ) pairs
