Reading and Displaying Text Files using Java

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Reading and Displaying Text Files using Java

Example 3-3 shows the FileViewer class. It combines the use of the File class and I/O streams to read the contents of a text file with GUI techniques to display those contents. FileViewer uses a java.awt.TextArea component to display file contents, as shown in Figure 3-1.Example 3-3 uses graphical user interface techniques that are introduced in Chapter 11. If you have not yet read that chapter or do not already have AWT programming experience, you probably won’t understand all the code in the example. That’s okay; just concentrate on the I/O code, which is the main focus of this chapter.

Figure 3-1. A FileViewer window

The FileViewer constructor concerns itself mainly with the mechanics of setting up the necessary GUI. There are some interesting uses of theFile object at the end of this constructor, however. The heart of this example is the setFile( ) method. This is where the file contents are loaded and displayed. Because the file contents are to be displayed in a TextArea component, the legitimate assumption is that the file contains characters. Thus, we use a character input stream, a FileReader, instead of the byte input stream used in the FileCopy program ofExample 3-2. Once again, use a finally clause to ensure that the FileReader stream is properly closed.

The actionPerformed( ) method handles GUI events. If the user clicks on the Open File button, this method creates a FileDialog object to prompt for a new file to display. Note how the default directory is set before the dialog is displayed and then retrieved after the user makes a selection. This is possible because the show( ) method actually blocks until the user selects a file and dismisses the dialog.

The FileViewer class is designed to be used by other classes. It also has its own main( ) method, however, so that it can be run as a standalone program.

Example 3-3. FileViewer.java

1. Example 3-3. FileViewer.java
   
2. package je3.io;
   
3. import java.awt.*;
   
4. import java.awt.event.*;
   
5. import java.io.*;
   
6. 
   
7. /**
   
8. * This class creates and displays a window containing a TextArea,
   
9. * in which the contents of a text file are displayed.
   
10. **/
    
11. public class FileViewer extends Frame implements ActionListener {
    
12.     String directory;  // The default directory to display in the FileDialog
    
13.     TextArea textarea; // The area to display the file contents into
    
14.    
    
15.     /** Convenience constructor: file viewer starts out blank */
    
16.     public FileViewer( ) { this(null, null); }
    
17.     /** Convenience constructor: display file from current directory */
    
18.     public FileViewer(String filename) { this(null, filename); }
    
19.    
    
20.     /**
    
21.      * The real constructor.  Create a FileViewer object to display the
    
22.      * specified file from the specified directory
    
23.      **/
    
24.     public FileViewer(String directory, String filename) {
    
25.         super( );  // Create the frame
    
26.         
    
27.         // Destroy the window when the user requests it
    
28.         addWindowListener(new WindowAdapter( ) {
    
29.                 public void windowClosing(WindowEvent e) { dispose( ); }
    
30.             });
    
31. 
    
32.         // Create a TextArea to display the contents of the file in
    
33.         textarea = new TextArea("", 24, 80);
    
34.         textarea.setFont(new Font("MonoSpaced", Font.PLAIN, 12));
    
35.         textarea.setEditable(false);
    
36.         this.add(textarea, "Center");
    
37.         
    
38.         // Create a bottom panel to hold a couple of buttons in
    
39.         Panel p = new Panel( );
    
40.         p.setLayout(new FlowLayout(FlowLayout.RIGHT, 10, 5));
    
41.         this.add(p, "South");
    
42.         
    
43.         // Create the buttons and arrange to handle button clicks
    
44.         Font font = new Font("SansSerif", Font.BOLD, 14);
    
45.         Button openfile = new Button("Open File");
    
46.         Button close = new Button("Close");
    
47.         openfile.addActionListener(this);
    
48.         openfile.setActionCommand("open");
    
49.         openfile.setFont(font);
    
50.         close.addActionListener(this);
    
51.         close.setActionCommand("close");
    
52.         close.setFont(font);
    
53.         p.add(openfile);
    
54.         p.add(close);
    
55.         
    
56.         this.pack( );
    
57.         
    
58.         // Figure out the directory, from filename or current dir, if necessary
    
59.         if (directory == null) {
    
60.             File f;
    
61.             if ((filename != null)&& (f = new File(filename)).isAbsolute( )) {
    
62.                 directory = f.getParent( );
    
63.                 filename = f.getName( );
    
64.             }
    
65.             else directory = System.getProperty("user.dir");
    
66.         }
    
67.         
    
68.         this.directory = directory;   // Remember the directory, for FileDialog
    
69.         setFile(directory, filename); // Now load and display the file
    
70.     }
    
71.    
    
72.     /**
    
73.      * Load and display the specified file from the specified directory
    
74.      **/
    
75.     public void setFile(String directory, String filename) {
    
76.         if ((filename == null) || (filename.length( ) == 0)) return;
    
77.         File f;
    
78.         FileReader in = null;
    
79.         // Read and display the file contents.  Since we're reading text, we
    
80.         // use a FileReader instead of a FileInputStream.
    
81.         try {
    
82.             f = new File(directory, filename); // Create a file object
    
83.             in = new FileReader(f);            // And a char stream to read  it
    
84.             char[  ] buffer = new char[4096];    // Read 4K characters at a time
    
85.             int len;                           // How many chars read each time
    
86.             textarea.setText("");              // Clear the text area
    
87.             while((len = in.read(buffer)) != -1) { // Read a batch of chars
    
88.                 String s = new String(buffer, 0, len); // Convert to a string
    
89.                 textarea.append(s);                    // And display them
    
90.             }
    
91.             this.setTitle("FileViewer: " + filename);  // Set the window title
    
92.             textarea.setCaretPosition(0);              // Go to start of file
    
93.         }
    
94.         // Display messages if something goes wrong
    
95.         catch (IOException e) {
    
96.             textarea.setText(e.getClass( ).getName( ) + ": " + e.getMessage( ));
    
97.             this.setTitle("FileViewer: " + filename + ": I/O Exception");
    
98.         }
    
99.         // Always be sure to close the input stream!
    
100.         finally { try { if (in!=null) in.close( ); } catch (IOException e) {  } }
     
101.     }
     
102. 
     
103.     /**
     
104.      * Handle button clicks
     
105.      **/
     
106.     public void actionPerformed(ActionEvent e) {
     
107.         String cmd = e.getActionCommand( );
     
108.         if (cmd.equals("open")) {          // If user clicked "Open" button
     
109.             // Create a file dialog box to prompt for a new file to display
     
110.             FileDialog f = new FileDialog(this, "Open File", FileDialog.LOAD);
     
111.             f.setDirectory(directory);       // Set the default directory
     
112. 
     
113.             // Display the dialog and wait for the user's response
     
114.             f.show( );                        
     
115. 
     
116.             directory = f.getDirectory( );    // Remember new default directory
     
117.             setFile(directory, f.getFile( )); // Load and display selection
     
118.             f.dispose( );                     // Get rid of the dialog box
     
119.         }
     
120.         else if (cmd.equals("close"))      // If user clicked "Close" button,
     
121.             this.dispose( );                  //    then close the window
     
122.     }
     
123.    
     
124.     /**
     
125.      * The FileViewer can be used by other classes, or it can be
     
126.      * used standalone with this main( ) method.
     
127.      **/
     
128.     static public void main(String[  ] args) throws IOException {
     
129.         // Create a FileViewer object
     
130.         Frame f = new FileViewer((args.length == 1)?args[0]:null);
     
131.         // Arrange to exit when the FileViewer window closes
     
132.         f.addWindowListener(new WindowAdapter( ) {
     
133.                 public void windowClosed(WindowEvent e) { System.exit(0); }
     
134.             });
     
135.         // And pop the window up
     
136.         f.show( );
     
137.     }
     
138. }

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