BINARY I/O - Introduction to Java Programming: Comprehensive Version - page 694

4 public static void main(String[] args) throws IOException {

5 try ( // Create an output stream for file object.dat

6 ObjectOutputStream output =

output stream

7 new ObjectOutputStream( new FileOutputStream( "object.dat" ));

8 ) {
9 // Write a string, double value, and object to the file
10 output.writeUTF( "John" );
11 output.writeDouble( 85.5 );
12

output string

output.writeObject( new java.util.Date()); output object

13 }
14 }
15 }

An ObjectOutputStream is created to write data into the file object.dat in lines 6 and 7.
A string, a double value, and an object are written to the file in lines 10-12. To improve
performance, you may add a buffer in the stream using the following statement to replace
lines 6 and 7:

ObjectOutputStream output = new ObjectOutputStream(
new BufferedOutputStream( new FileOutputStream( "object.dat" )));

Multiple objects or primitives can be written to the stream. The objects must be read back
from the corresponding ObjectInputStream with the same types and in the same order as
they were written. Java's safe casting should be used to get the desired type. Listing 17.6 reads
data from object.dat .

L ISTING 17.6

TestObjectInputStream.java
1 import java.io.*;
2
3 public class TestObjectInputStream {
4 public static void main(String[] args)

5 throws ClassNotFoundException, IOException {

6 try ( // Create an input stream for file object.dat

input stream

7 ObjectInputStream input =

8 new ObjectInputStream( new FileInputStream( "object.dat" ));

9 ) {
10 // Read a string, double value, and object from the file
11 String name = input.readUTF();
12 double score = input.readDouble();
13 java.util.Date date = (java.util.Date)(input.readObject());
14 System.out.println(name + " " + score + " " + date);
15 }
16 }
17 }

input string

input object

John 85.5 Sun Dec 04 10:35:31 EST 2011

The readObject() method may throw java.lang.ClassNotFoundException , because
when the JVM restores an object, it first loads the class for the object if the class has not
been loaded. Since ClassNotFoundException is a checked exception, the main method
declares to throw it in line 5. An ObjectInputStream is created to read input from
object.dat in lines 7 and 8. You have to read the data from the file in the same order and
format as they were written to the file. A string, a double value, and an object are read in
lines 11-13. Since readObject() returns an Object , it is cast into Date and assigned to a
Date variable in line 13.

ClassNotFoundException