File Handling - An Introduction to Network Programming with Java - page 117

4.9 ArrayLists and Serialisation

It is much more effi cient to save a single ArrayList to disc than it is to save a series
of individual objects. Placing a series of objects into a single ArrayList is a very neat
way of packaging and transferring our objects. This technique carries another sig-
nifi cant advantage: we shall have some form of random access , via the ArrayList
class's get method (albeit based on knowing each element's position within the
ArrayList ). Without this, we have the considerable disadvantage of being restricted
to serial access only.

Example

This example creates three objects of class Personnel (as featured in the example at
the end of Sect. 4.6 ) and uses the add method of class ArrayList to place the
objects into an ArrayList . It then employs a 'for-each' loop and the 'get' meth-
ods of class Personnel to retrieve the data properties of the three objects.
We could use the same ArrayList object for sending objects out to the fi le and
for receiving them back from the fi le, but two ArrayList objects have been used
below simply to demonstrate beyond any doubt that the values have been read back
in (and are not simply the original values, still held in the ArrayList object).
import java.io.*;
import java.util.*;
public class ArrayListSerialise
{
public static void main(String[] args)
throws IOException, ClassNotFoundException
{
ObjectOutputStream outStream =
new ObjectOutputStream(
new FileOutputStream("personnelList.dat"));
ArrayList<Personnel> staffListOut =
new ArrayList<>();
ArrayList<Personnel> staffListIn =
new ArrayList<>();
Personnel[] staff =
{new Personnel(123456,"Smith", "John"),
new Personnel(234567,"Jones", "Sally Ann"),
new Personnel(999999,"Black", "James Paul")};

for (int i=0; i<staff.length; i++)
staffListOut.add(staff[i]);
outStream.writeObject(staffListOut);
outStream.close();
ObjectInputStream inStream =