Section 10.2
Files

THE DATA AND PROGRAMS IN A COMPUTER'S MAIN MEMORY survive only as long as the power is on. For more permanent storage, computers use files, which are collections of data stored on a hard disk, on a floppy disk, on a CD-ROM, or on some other type of storage device. Files are organized into directories (sometimes called "folders"). A directory can hold other directories, as well as files. Both directories and files have names that are used to identify them.

Programs can read data from existing files. They can create new files and can write data to files. In Java, such input and output is done using streams. Human-readable character data is read from a file using an object belonging to the class FileReader, which is a subclass of Reader. Similarly, data is written to a file in human-readable format through an object of type FileWriter, a subclass of Writer. For files that store data in machine format, the appropriate I/O classes are FileInputStream and FileOutputStream. In this section, I will only discuss character-oriented file I/O using the FileReader and FileWriter classes. However, FileInputStream and FileOutputStream are used in an exactly parallel fashion. All these classes are defined in the java.io package.

It's worth noting right at the start that applets which are downloaded over a network connection are generally not allowed to access files. This is a security consideration. You can download and run an applet just by visiting a Web page with your browser. If downloaded applets had access to the files on your computer, it would be easy to write an applet that would destroy all the data on a computer that downloads it. To prevent such possibilities, there are a number of things that downloaded applets are not allowed to do. Accessing files is one of those forbidden things. Standalone programs written in Java, however, have the same access to your files as any other program. When you write a standalone Java application, you can use all the file operations described in this section.

The FileReader class has a constructor which takes the name of a file as a parameter and creates an input stream that can be used for reading from that file. This constructor will throw an exception of type FileNotFoundException if the file doesn't exist. This exception type requires mandatory exception handling, so you have to call the constructor in a try statement (or inside a routine that is declared to throw FileNotFoundException). For example, suppose you have a file named "data.txt", and you want your program to read data from that file. You could do the following to create an input stream for the file:

     FileReader data;   // (Declare the variable before the
                        //   try statement, or else the variable
                        //   is local to the try block and you won't
                        //   be able to use it later in the program.)
                             
     try {
        data = new FileReader("data.txt");  // create the stream
     }
     catch (FileNotFoundException e) {
        ... // do something to handle the error -- maybe, end the program
     }

The FileNotFoundException class is a subclass of IOException, so it would be acceptable to catch IOExceptions in the above try...catch statement. More generally, just about any error that can occur during input/output operations can be caught by a catch clause that handles IOException.

Once you have successfully created a FileReader, you can start reading data from it. But since FileReaders have only the primitive input methods inherited from the basic Reader class, you will probably want to wrap your FileReader in a TextReader object or in some other wrapper class. (The TextReader class is not a standard part of Java; it is described in the previous section.) To create a TextReader for reading from a file named data.dat, you could say:

       TextReader data;
       
       try {
          data = new TextReader(new FileReader("data.dat"));
       }
       catch (FileNotFoundException e) {
          ... // handle the exception
       }

Once you have a TextReader named data, you can read from it using such methods as data.getInt() and data.getWord(), exactly as you would from any other TextReader.

Working with output files is no more difficult than this. You simply create an object belonging to the class FileWriter. You will probably want to wrap this output stream in an object of type PrintWriter. For example, suppose you want to write data to a file named "result.dat". Since the constructor for FileWriter can throw an exception of type IOException, you should use a try statement:

       PrintWriter result;
       
       try {
          result = new PrintWriter(new FileWriter("result.dat"));
       }
       catch (IOException e) {
          ... // handle the exception
       }

If no file named result.dat exists, a new file will be created. If the file already exists, then the current contents of the file will be erased and replaced with the data that your program writes to the file. An IOException might occur if, for example, you are trying to create a file on a disk that is "write-protected," meaning that it cannot be modified.

After you are finished using a file, it's a good idea to close the file, to tell the operating system that you are finished using it. (If you forget to do this, the file will ordinarily be closed automatically when the program terminates or when the file stream object is garbage collected, but it's best to close a file as soon as you are done with it.) You can close a file by calling the close() method of the associated stream. Once a file has been closed, it is no longer possible to read data from it or write data to it, unless you open it again as a new stream. (Note that for most stream classes, the close() method can throw an IOException, which must be handled; however, both PrintWriter and TextReader override this method so that it cannot throw such exceptions.)

As a complete example, here is a program that will read numbers from a file named data.dat, and will then write out the same numbers in reverse order to another file named result.dat. It is assumed that data.dat contains only one number on each line, and that there are no more than 1000 numbers altogether. Exception-handling is used to check for problems along the way. Although the application is not a particularly useful one, this program demonstrates the basics of working with files. (By the way, at the end of this program, you'll find our first example of a finally clause in a try statement. When the computer executes a try statement, the commands in its finally clause are guaranteed to be executed, no matter what.)

   import java.io.*;
   // (The TextReader class must also be available to this program.)
   
   public class ReverseFile {
   
      public static void main(String[] args) {
   
         TextReader data;     // Character input stream for reading data.
         PrintWriter result;  // Character output stream for writing data.
         
         double[] number = new double[1000];  // An array to hold all
                                              //   the numbers that are
                                              //   read from the file.

         int numberCt;  // Number of items actually stored in the array.
         
         try {  // Create the input stream.
            data = new TextReader(new FileReader("data.dat"));
         }
         catch (FileNotFoundException e) {
            System.out.println("Can't find file data.dat!");
            return;  // End the program by returning from main().
         }
   
         try {  // Create the output stream.
            result = new PrintWriter(new FileWriter("result.dat"));
         }
         catch (IOException e) {
            System.out.println("Can't open file result.dat!");
            System.out.println(e.toString());
            data.close();  // Close the input file.
            return;        // End the program.
         }
         
         try {
         
             // Read the data from the input file.
             
             numberCt = 0;
             while (data.eof() == false) {  // Read until end-of-file.
                number[numberCt] = data.getlnDouble();
                numberCt++;
             }
          
             // Output the numbers in reverse order.
             
             for (int i = numberCt-1; i >= 0; i--)
                result.println(number[i]);
                
             System.out.println("Done!");
   
          }
          catch (TextReader.Error e) {
             // Some problem reading the data from the input file.
             System.out.println("Input Error: " + e.getMessage());
          }
          catch (IndexOutOfBoundsException e) {
             // Must have tried to put too many numbers in the array.
             System.out.println("Too many numbers in data file.");
             System.out.println("Processing has been aborted.");
          }
          finally {
             // Finish by closing the files, 
             //     whatever else may have happened.
             data.close();
             result.close();
          }
            
      }  // end of main()
   
   } // end of class

File Names, Directories, and File Dialogs

The subject of file names is actually more complicated than I've let on so far. To fully specify a file, you have to give both the name of the file and the name of the directory where that file is located. A simple file name like "data.dat" or "result.dat" is taken to refer to a file in a directory that is called the current directory (or "default directory" or "working directory"). The current directory is not a permanent thing. It can be changed by the user or by a program. Files not in the current directory must be referred to by a path name, which includes both the name of the file and information about the directory where it can be found.

To complicate matters even further, there are two types of path names, absolute path names and relative path names. An absolute path name uniquely identifies one file among all the files available to the computer. It contains full information about which directory the file is in and what its name is. A relative path name tells the computer how to locate the file, starting from the current directory.

Unfortunately, the syntax for file names and path names varies quite a bit from one type of computer to another. Here are some examples:

Similarly, the rules for determining which directory is the current directory are different for different types of computers. It's reasonably safe to say, though, that if you stick to using simple file names only, and if the files are stored in the same directory with the program that will use them, then you will be OK.

In many cases, however, you would like the user to be able to select the file that is going to be used for input or output. If your program lets the user type in the file name, you will just have to assume that the user understands how to work with files and directories. But in a graphical user interface, the user expects to be able to select files using a file dialog box, which is a special window that a program can open when it wants the user to select a file for input or output. Java provides a platform-independent method for using file dialog boxes in the form of a class called FileDialog. This class is part of the package java.awt.

There are really two types of file dialog windows: one for selecting an existing file to be used for input, and one for specifying a file for output. You can specify which type of file dialog you want in the constructor for the FileDialog object, which has the form

        public FileDialog(Frame parent, String title, int mode)

where parent is presumably the main window of the program, title is meant to be a short string describing the dialog box, and mode is one of the constants FileDialog.SAVE or FileDialog.LOAD. Use FileDialog.SAVE if you want an output file, and use FileDialog.LOAD if you want a file for input. You can actually omit the mode parameter, which is equivalent to using FileDialog.LOAD.

Once you have a FileDialog object, you can use its show() method to make it appear on the screen. It will stay on the screen until the user either selects a file or cancels the request. The instance method getFile() can then be called to retrieve the name of the file selected by the user. If the user has canceled the file dialog, then the String value returned by getFile() will be null. Since the user can select a file that is not in the current directory, you will also need directory information, which can be retrieved by calling the method getDirectory(). For example, if you want the user to select a file for output, and if the main window for your application is mainWin, you could say:

       FileDialog fd = new FileDialog(mainWin, "Select output file",
                                                      FileDialog.SAVE);
       fd.show();
       String fileName = fd.getFile();
       String directory = fd.getDirectory();

       if (fileName != null) {  // (otherwise, the user canceled the request)

          ...  // open the file, save the data, then close the file

       }

Once you have the file name and directory information, you will have to combine them into a usable file specification. The best way to do this is to create an object of type File. The File object can then be used as a parameter in a constructor for a FileReader, FileWriter, FileInputStream, or FileOutputStream. For example, the body of the if statement in the above example could include:

        try {
            File file = new File(directory, fileName);
            PrintWriter out = new PrintWriter(new FileWriter(file));
            ... // write the data to the output stream, out
            out.close();
         }
         catch { IOException e } 
            ... // respond to the exception
         }

The File class has other uses as well. An object of type File really represents a file name rather than a file. The file to which the name refers might or might not exist. If it does exist, it might actually be a directory rather than a regular file. A File object can be constructed from a directory and a file name, as in the above example. The directory can be specified as a String or as an object of type File that refers to a directory. A File object can also be created just from a file name, as in the constructor call new File("data.txt"). In that case, the directory is assumed to be the current directory.

File objects contain several useful instance methods. Assuming that file is a variable of type File, here are some of the methods that are available:

file.exists() -- This boolean-valued function returns true if the file named by the File object already exists. You could use this method if you wanted to avoid overwriting the contents of an existing file when you create a new FileWriter.

file.isDirectory() -- This boolean-valued function returns true if the File object refers to a directory. It returns false if it refers to a regular file or if no file with the given name exists.

file.delete() -- Deletes the file, if it exists.

file.list() -- If the File object refers to a directory, this function returns an array of type String[] containing the names of the files in this directory. Otherwise, it returns null.

Here, for example, is a program that will list the names of all the files in a directory specified by the user:

      import java.io.File;

      public class DirectoryList {
      
         /* This program lists the files in a directory specified by
            the user.  The user is asked to type in a directory name.
            If the name entered by the user is not a directory, a
            message is printed and the program ends.
         */
         
         public static void main(String[] args) {
         
            String directoryName;  // Directory name entered by the user.
            File directory;        // File object referring to the directory.
            String[] files;        // Array of file names in the directory.
         
            TextIO.put("Enter a directory name: ");
            directoryName = TextIO.getln().trim();
            directory = new File(directoryName);
            
            if (directory.isDirectory() == false) {
                if (directory.exists() == false)
                   TextIO.putln("There is no such directory!");
                else
                   TextIO.putln("That file is not a directory.");
            }
            else {
                files = directory.list();
                TextIO.putln("Files in directory \"" + directory + "\":");
                for (int i = 0; i < files.length; i++)
                   TextIO.putln("   " + files[i]);
            }
         
         } // end main()
      
      } // end class DirectoryList

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