Managing Input/Output Files in Java

The ObjectOutputStream class

. Is part of the java.io package

. Is an extension of the OutputStream class, an abstract class that describes the behavior of an output stream

. Is a high-level class that can be used to send primitive values and "serializable" objects to a stream. All that is needed for an object to be serializable, is that its class must implement the Serializable interface. For example, if a Customer class is to be serializable, its header may be coded

public class Customer implements Serializable

The interface requires no methods.

Many packaged classes are serializable including all wrapper classes, String and Stringbuffer classes, Vector and Array classes, and the collection classes. In other words, an entire collection, such as a SortedMap, can be stored as an object on disk!

. Has overloaded constructors but the most useful "chains" to an object that descends from the OutputStream class (such as a FileOutputStream object). For example, if fd is the reference of a File object

ObjectOutputStream out = new ObjectOutputStream(new FileOutputStream(fd));

It will construct a ObjectOutputStream object chained to a FileOutputStream object for sending primitive values and serializable objects to the file. Because a checked, IOException may occur, the statement should be enclosed in a try block with an appropriate catch.

. Has many useful methods as follows:

Because a checked, IOException may occur, calls to these methods should be enclosed in a try block with an appropriate catch. Consult the Java API documentation for more details.

. Example. The following program can be used to create a file of multiple array objects on disk.

import java.io.*;
public class App
{
public static void main(String[] args)
{

// Local variables and object references.

char again = 'y';
String[] array;
File fd;
ObjectOutputStream out;

// Get the path name from the user.

System.out.print("Enter the file's complete path name: ");
fd = new File(Keyboard.readString());

// Try to write data to the output stream.

try
{

// Open an output stream for the file.

out = new ObjectOutputStream(new FileOutputStream(fd));

// This loop constructs an array with values entered by the user
// and writes the array to the file. The user is given the option
// of repeating the loop to construct and store another array.

while (again == 'Y' || again == 'y')
{
System.out.print("How many strings will you enter? ");
array = new String[Keyboard.readInt()];
for (int i = 0; i < array.length; i++) {
System.out.print("Enter a string: ");
array[i] = Keyboard.readString();
}
out.writeObject(array);
System.out.print("Again? (Y/N) ");
again = Keyboard.readChar();
}

// Close the stream.

out.close();
System.out.println("Closed - " + fd.getPath());
}

// Catch an IOException if one is thrown.

catch (IOException e) {
System.out.println(e.getMessage());
}
}
}