Managing Input/Output Files in Java

The FileDialog class

. Is part of the java.awt package. You will need an import statement to access the class with its abbreviated name.

. Has many ancestor classes as indicated by the following diagram:

. Can be instantiated to display a modal dialog window from which the user can select a file. Because it is a modal dialog, once an application makes a FileDialog object visible, it cannot continue until the user has chosen a file.

. Has two static fields that indicate whether the file will be used for input or output.

They are

. Has several constructors. The most frequently used is demonstrated by

FileDialog fd = new FileDialog(this, "Choose file", FileDialog.LOAD);
fd.setVisible(true);

which constructs and displays a file dialog window for the current (this) object. The title of the file dialog window will be "Choose file", and the chosen file will be used for input.

. Has a number of methods. The most frequently used are as follows:

Consult the Java API documentation for more details.

Sample program

The following menu-driven, Windows program uses the File and FileDialog classes to display a variety of information about a file selected by the user:

import java.awt.*;
import java.awt.event.*;
import java.util.*;
import java.io.*;

public class App extends Frame implements ActionListener
{

// Object references.

MenuBar mainBar;
Menu fileMenu;
MenuItem find;
MenuItem exit;
TextArea fileInfo;

// Processing starts here.

public static void main(String[] args)
{
App myWindow = new App("File Analyzer");
myWindow.setSize(500, 200);
myWindow.setVisible(true);
}

// Class constructor.

public App(String title)
{

// Usual boilerplate for startup and shutdown.

super(title);
//addWindowListener();
new WindowAdapter()
{
public void windowClosing(WindowEvent e)
{
dispose();
System.exit(0);
}
}
);

// Create the frame's menu bar.

mainBar = new MenuBar();
setMenuBar(mainBar);
fileMenu = new Menu("File");
find = new MenuItem("Find");
find.addActionListener(this);
fileMenu.add(find);
fileMenu.addSeparator();
exit = new MenuItem("Exit");
exit.addActionListener(this);
fileMenu.add(exit);
mainBar.add(fileMenu);

// Create the file information text area.

fileInfo = new TextArea(" Use the menu bar to find a file...",
10, 40, TextArea.SCROLLBARS_NONE);
fileInfo.setFont(new Font("Monospaced", Font.PLAIN, 12));
fileInfo.setEditable(false);
add(fileInfo);
}

// Required method of the ActionListener interface.

public void actionPerformed(ActionEvent e)
{

// If the user selected "File" | "Find" from the menu, let them
// choose a file to be analyzed. Then, display its information.

if (e.getSource().equals(find))
{

// Create and display a modal file dialog box through which the
// user can choose the file they want to analyze.

FileDialog fd = new FileDialog(this, "Choose file", FileDialog.LOAD);
fd.setVisible(true);

// Construct a File object for the file the user selected.

File f = new File(fd.getDirectory() + File.separator + fd.getFile());

// Display information about the file the user selected.

fileInfo.setText(" FILE INFORMATION ");
fileInfo.append(" Path: " + f.getPath());
fileInfo.append(" Directory: " + fd.getDirectory());
fileInfo.append(" File: " + f.getName());
fileInfo.append(" Last modified: " +
new Date(f.lastModified()).toString());
fileInfo.append(" Byte length: " + f.length());
fileInfo.setCaretPosition(0);
}

// If the user selected "File" | "Exit" from the menu, terminate
// the application.

if (e.getSource().equals(exit)) {
dispose();
System.exit(0);
}
}
}

Output