Java Reference
In-Depth Information
If the class is not found or cannot be loaded for any reason,
readObject
will throw a
ClassNotFoundException
.
20.8.4. Customized Serialization
The default serialization methods work for many classes but not for all
of them. For some classes default deserialization may be improper or in-
efficient. The
HashMap
class is an example of both problems. Default seri-
alization would write all the data structures for the hash map, including
the hash codes of the entries. This serialization is both wrong and inef-
ficient.
It is wrong because hash codes may be different for deserialized entries.
This will be true, for example, of entries using the default
hashCode
im-
plementation.
It is inefficient because a hash map typically has a significant number of
empty buckets. There is no point in serializing empty buckets. It would
be more efficient to serialize the referenced keys and entries and rebuild
a hash map from them than to serialize the entire data structure of the
map.
For these reasons,
java.util.HashMap
provides private
writeObject
and
and
ObjectInputStream
, respectively, when it is time to serialize or deseri-
alize a
HashMap
object. These methods are invoked only on classes that
provide them, and the methods are responsible only for the class's own
state, including any state from non-serializable superclasses. A class's
writeObject
and
readObject
methods, if provided, should
not
invoke the
superclass's
readObject
or
writeObject
method. Object serialization differs
in this way from
clone
and
finalize
.
[3]
These methods are
private
because they should never be overridden and they should never be
invoked by anyone using or subclassing your class. The serialization mechanism gains access to
