> ### 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:

> - `data.dat` -- on any computer, this would be a file named data.dat in the current directory.
> - `/home/eck/java/examples/data.dat` -- This is an absolute path name in the UNIX operating system. It refers to a file named data.dat in a directory named examples, which is in turn in a directory named java,....
> - `C:\eck\java\examples\data.dat` -- An absolute path name on a DOS or Windows computer.
> - `Hard Drive:java:examples:data.dat` -- Assuming that "Hard Drive" is the name of a disk drive, this would be an absolute path name on a Macintosh computer.
> - `examples/data.dat` -- a relative path name under UNIX. "Examples" is the name of a directory that is contained within the current directory, and data.data is a file in that directory. The corresponding relative path names for Windows and Macintosh would be `examples\data.dat` and `examples:data.dat`.

> 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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