Ok this is how it finally worked.
I deserialized everything in the file to an ArrayList; added the new object and while serializing, I added each element of the ArrayList to the file [erasing the previous entries by using (new FileOutputStream(new File("Vehicule.txt"),false) ].
Finally I added a null explicitly to the file so as to help with the deserializing.
Also while creating the file for the first time with createNewFile, I added a null to the file.
