File I/O - Java - page 572

Display 10.17 Using EOFException (part 2 of 2)

Sample Dialogue
Reading numbers in numbers.dat
0
1
2
3
4
No more numbers in the file.

Assumes the program in
Display 10.13 was run to
create the file numbers.dat .

We will discuss interfaces in general in Chapter 13. However, the Serializable
interface is particularly easy to use and requires no knowledge of interfaces. All you
need to do to make a class implement the Serializable interface is add the two words
implements Serializable to the heading of the class definition, as in the following
example:

Serial-izable
interface

public class Person implements Serializable
{

The Serializable interface is in the same java.io package that contains all the I/O
classes we have discussed in this chapter. For example, in Display 10.18 we define a toy
class named SomeClass that implements the Serializable interface. We will explain the
effect of the Serializable interface a bit later in this chapter, but first let's see how you
do binary file I/O with a serializable class such as this class SomeClass in Display 10.18.
Display 10.19 illustrates how class objects can be written to and read from a binary
file. To write an object of a class such as SomeClass to a binary file, you simply use the
method writeObject of the class ObjectOutputStream . You use writeObject in the
same way that you use the other methods of the class ObjectOutputStream , such as
writeInt , but you use an object as the argument.
If an object is written to a file with writeObject , then it can be read back out of the
file with readObject of the stream class ObjectInputStream , as also illustrated in Dis-
play 10.19. The method readObject returns its value as an object of type Object . If
you want to use the values retuned by readObject as an object of a class like Some-
Class , you must do a type cast, as shown in Display 10.19.

writeObject

readObject

The Serializable Interface
A class that implements the Serializable interface is said to be a serializable class. To
use objects of a class with writeObject and readObject , that class must be serializable.
But to make the class serializable, we change nothing in the class. All we do is add the
phrase implements Serializable . This phrase tells the run-time system that it is OK
to treat objects of the class in a particular way when doing file I/O. If a class is serializ-
able, Java assigns a serial number to each object of the class that it writes to a stream of

serializable