The I/O Package - The Java Programming Language - page 812

When the serialized hash map is deserialized, the two analogous entries
in the new copy of the hash map will have references to a single copy of
the rose.jpg object, not references to two separate copies of rose.jpg . [2]
[2] The first key field is the word "rose" in Tibetan.
Sometimes, however, sharing objects in this way is not what is desired.
In that case you can use ObjectOutputStream 's writeUnshared method to
write the object as a new distinct object, rather than using a reference
to an existing serialization of that object. Any object written into the
graph by writeUnshared will only ever have one reference to it in the seri-
alized data. The readUnshared method of ObjectInputStream reads an ob-
ject that is expected to be unique. If the object is actually a reference to
an existing deserialized object then an ObjectStreamException is thrown;
similarly, if the deserialization process later tries to create a second ref-
erence to an object returned by readUnshared , an ObjectStreamException is
thrown. These uniqueness checks only apply to the actual object passed
to writeUnshared or read by readUnshared , not to any objects they refer to.
20.8.2. Making Your Classes Serializable
When an ObjectOutputStream writes a serialized object, the object must
implement the Serializable marker interface. This marker interface de-
clares that the class is designed to have its objects serialized.