File Handling - An Introduction to Network Programming with Java - page 105

4.6 Serialisation [U.S. Spelling Seriali z ation]

As seen in the preceding sections, transferring data of the primitive types to and
from disc fi les is reasonably straightforward. Transferring string data presents a
little more of a challenge, but even this is not a particularly onerous task. However,
how do we go about transferring objects of classes? ( String is a class, of course, but
it is treated rather differently from other classes.) One way of saving an object to a
fi le would be to decompose the object into its constituent fi elds (strings and num-
bers) and write those individual data members to the fi le. Then, when reading the
values back from the fi le, we could re-create the original objects by supplying those
values to the appropriate constructors. However, this is a rather tedious and long-
winded method. In addition, since the data members of an object may themselves
include other objects (some of whose data members may include further objects,
some of whose members…), this method would not be generally applicable.
Unlike other common O-O languages, Java provides an inbuilt solution: seriali-
sation . Objects of any class that implements the Serializable interface may be trans-
ferred to and from disc fi les as whole objects, with no need for decomposition of
those objects. The Serializable interface is, in fact, nothing more than a marker to
tell Java that objects of this class may be transferred on an object stream to and from
fi les. Implementation of the Serializable interface need involve no implementation
of methods. The programmer merely has to ensure that the class to be used includes
the declaration ' implements Serializable ' in its header line.
Class ObjectOutputStream is used to save entire objects directly to disc, while
class ObjectInputStream is used to read them back from disc. For output, we wrap
an object of class ObjectOutputStream around an object of class FileOutputStream ,
which itself is wrapped around a File object or fi le name. Similarly, input requires
us to wrap an ObjectInputStream object around a FileInputStream object, which in
turn is wrapped around a File object or fi le name.

Examples

(i) ObjectOutputStream outStream =
new ObjectOutputStream(
new FileOutputStream("personnel.dat"));
(ii) ObjectInputStream inStream =
new ObjectInputStream(
new FileInputStream("personnel.dat"));
Methods writeObject and readObject are then used for the actual output and
input respectively. Since these methods write/read objects of class Object (the ulti-
mate superclass), any objects read back from fi le must be typecast into their origi-
nal class before we try to use them. For example:

Personnel person = (Personnel)inStream.readObject();
//(Assuming that inStream is as declared above.)