Chapter 20: Streams and Files

Quiz

1. What's the difference between a file and a filename? Which is more closely related to the java.io.File class?

   A file is the actual data located at a particular location on a disk. A filename is used to refer to the file. The name of the file can be changed without changing the data. The java.io.File class is closer to a filename than a file.

Exercises

1. Write a program to recursively list all the files and directories contained in a directory given on the command line.

   
   import java.io.File;
   
   
   public class rls {
   
     File dir;
   
     public static void main(String[] args)  {
     
       for (int i = 0; i < args.length; i++) {
         rls r = new rls(args[i]);
         r.list();
       }
     
     }
     
     public rls(String s) {
     
       dir = new File(s);
     
     }
     
     // This method doesn't do anything if dir isn't a directory
     public void list() {
     
       if (dir.isDirectory()) {
         System.out.println("\n" + dir + ":\n");
         String[] files = dir.list();
         for (int i = 0; i < files.length; i++) {
           System.out.println(files[i]);
         }
         for (int i = 0; i < files.length; i++) {
           rls r = new rls(dir + File.separator + files[i]);
           r.list();
         }
       
       } 
       
     }
   
   }
   

   This program can get stuck in an infinite loop if aliases or symbolic links point to a directory that's already been scanned. How would you fix this?

2. Modify the FileInfo program so that it runs as a GUI application. The user should select a file using a FileDialog and then be presented with a window that shows all the File info.

   import java.io.File;
   import java.io.IOException;
   import java.applet.Applet;
   import java.awt.Frame;
   import java.awt.TextArea;
   import java.awt.FileDialog;
   
   
   public class FileInfo extends Applet {
   
     TextArea info;
     File theFile;
   
   
     public static void main(String[] args) {
       Frame f = new Frame("File Info");
       FileInfo fi = new FileInfo();
       f.add("Center", fi);
       f.resize(200, 300);
       fi.init();
       f.show();
       fi.start();
       
     }
   
     
     public void init() {
     
       TextArea info = new TextArea(80, 40);
       add(info);
     }
     
     public void start() {
       FileDialog fd = new FileDialog(new Frame(), "Please choose a file: ", FileDialog.LOAD);
       fd.show();
       theFile = new File(fd.getFile());
       showInfo();  
     }
   
     
     
     public void showInfo() {
   
         if (theFile.exists()) {
           info.appendText("getName: " + theFile.getName());
           info.appendText("getPath: " + theFile.getPath());
           info.appendText("getAbsolutePath: " + 
             theFile.getAbsolutePath());
           info.appendText("getParent: " + theFile.getParent());
           if (theFile.canWrite()) info.appendText(theFile.getName() + 
             " is writable.");
           if (theFile.canRead()) info.appendText(theFile.getName() + 
             " is readable.");
           if (theFile.isFile()) {
              info.appendText(theFile.getName() + " is a file.");
           }
           else if (theFile.isDirectory()) {
             info.appendText(theFile.getName() + " is a directory.");
           }
           else {
             info.appendText("What is this?");
           }
           if (theFile.isAbsolute()) {
             info.appendText(theFile.getName() + 
               " is an absolute path.");
           }
           else {
             info.appendText(theFile.getName() + 
               " is not an absolute path.");
           }
           info.appendText("Last Modified" + theFile.lastModified());
           info.appendText(theFile.getName() + " is " + theFile.length() + " bytes.");
           info.appendText(theFile.getName() + " is " + theFile.length() + " bytes.");
         }  
         else {
            info.appendText("I'm sorry. I can't find the file " + theFile);
         }
         
       }
   
   }
   

3. Add a File menu to the above program. It should have functional New..., Open..., Close, Save and Quit methods. It should allow the user to get information about any file on their hard drive, display that information and save that information to a text file.