File I/O - Absolute Java - page 627

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, 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
Display 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 such as
SomeClass , 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

serializable

Display 10.18 A Serializable Class

1 import java.io.Serializable;

2 public class SomeClass implements Serializable
3 {
4 private int number;
5 private char letter;

6 public SomeClass()
7 {
8 number = 0;
9 letter = 'A';
10 }

11 public SomeClass( int theNumber, char theLetter)
12 {
13 number = theNumber;
14 letter = theLetter;
15 }
16 public String toString()
17 {
18 return "Number = " + number
19 + " Letter = " + letter;
20 }
21 }