[Chapter 6] 6.3 File Manipulation

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| Chapter 6 I/O |

## 6.3 File Manipulation

While streams are used to handle most types of I/O in Java, there are some nonstream-oriented classes in `java.io` that are provided for file manipulation. Namely, the `File` class represents a file on the local filesystem, while the `RandomAccessFile` class provides nonsequential access to data in a file. In addition, the `FilenameFilter` interface can be used to filter a list of filenames.

### File

The `File` class represents a file on the local filesystem. You can use an instance of the `File` class to identify a file, obtain information about the file, and even change information about the file. The easiest way to create a `File` is to pass a filename to the `File` constructor, like this:

    new File("readme.txt")

Although the methods that the `File` class provides for manipulating file information are relatively platform independent, filenames must follow the rules of the local filesystem. The `File` class does provide some information that can be helpful in interpreting filenames and path specifications. The variable `separatorChar` specifies the system-specific character used to separate the name of a directory from what follows.[3] In a Windows environment, this is a backslash (`\`), while in a UNIX or Macintosh environment it is a forward slash (`/`). You can create a `File` object that refers to a file called `readme.txt` in a directory called `myDir` as follows:

> [3] This information is also available as `System.getProperty('file.separator')`, which is how the `File` class gets it.

    new File("myDir" + File.separatorChar + "readme.txt")

The `File` class also provides some constructors that make this task easier. For example, there is a `File` constructor that takes two strings as arguments: the first string is the name of a directory and the second string is the name of a file. The following example does the exact same thing as the previous example:

    new File("myDir", "readme.txt")

The `File` class has another constructor that allows you to specify the directory of a file using a `File` object instead of a `String`:

    File dir = new File("myDir");
    File f = new File(dir, "readme.txt");

Sometimes a program needs to process a list of files that have been passed to it in a string. For example, such a list of files is passed to the Java environment by the `CLASSPATH` environment variable and can be accessed by the expression:

    System.getProperty("java.class.path")

This list contains one or more filenames separated by separator characters. In a Windows or Macintosh environment, the separator character is a semicolon (`;`), while in a UNIX environment, the separator character is a colon (`:`). The system-specific separator character is specified by the `pathSeparatorChar` variable. Thus, to turn the value of `CLASSPATH` into a collection of `File` objects, we can write:

    StringTokenizer s;
    Vector v = new Vector();
    s = new StringTokenizer(System.getProperty("java.class.path"),
                            File.pathSeparator);
    while (s.hasMoreTokens())
        v.addElement(new File(s.nextToken()));

You can retrieve the pathname of the file represented by a `File` object with `getPath()`, the filename without any path information with `getName()`, and the directory name with `getParent()`.

The `File` class also defines methods that return information about the actual file represented by a `File` object. Use `exists()` to check whether or not the file exists. `isDirectory()` and `isFile()` tell whether the file is a file or a directory. If the file is a directory, you can use `list()` to get an array of filenames for the files in that directory. The `canRead()` and `canWrite()` methods indicate whether or not a program is allowed to read from or write to a file. You can also retrieve the length of a file with `length()` and its last modified date with `lastModified()`.

A few `File` methods allow you to change the information about a file. For example, you can rename a file with `rename()` and delete it with `delete()`. The `mkdir()` and `mkdirs()` methods provide a way to create directories within the filesystem.

Many of these methods can throw a `SecurityException` if a program does not have permission to access the filesystem, or particular files within it. If a `SecurityManager` has been installed, the `checkRead()` and `checkWrite()` methods of the `SecurityManager` verify whether or not the program has permission to access the filesystem.

### FilenameFilter

The purpose of the `FilenameFilter` interface is to provide a way for an object to decide which filenames should be included in a list of filenames. A class that implements the `FilenameFilter` interface must define a method called `accept()`. This method is passed a `File` object that identifies a directory and a `String` that names a file. The `accept()` method is expected to return `true` if the specified file should be included in the list, or `false` if the file should not be included. Here is an example of a simple `FilenameFilter` class that only allows files with a specified suffix to be in a list:

    import java.io.File;
    import java.io.FilenameFilter;
    public class SuffixFilter implements FilenameFilter {
        private String suffix;
        public SuffixFilter(String suffix) {
            this.suffix = "." + suffix;
        }
        public boolean accept(File dir, String name) {
            return name.endsWith(suffix);
        }
    }

A `FilenameFilter` object can be passed as a parameter to the `list()` method of `File` to filter the list that it creates. You can also use a `FilenameFilter` to limit the choices shown in a `FileDialog`.

### RandomAccessFile

The `RandomAccessFile` class provides a way to read from and write to a file in a nonsequential manner. The `RandomAccessFile` class has two constructors that both take two arguments. The first argument specifies the file to open, either as a `String` or a `File` object. The second argument is a `String` that must be either `"r"` or `"rw"`. If the second argument is `"r"`, the file is opened for reading only. If the argument is `"rw"`, however, the file is opened for both reading and writing. The `close()` method closes the file. Both constructors and all the methods of the `RandomAccessFile` class can throw an `IOException` if they encounter an error.

The `RandomAccessFile` class defines three different `read()` methods for reading bytes from a file. The `RandomAccessFile` class also implements the `DataInput` interface, so it provides additional methods for reading from a file. Most of these additional methods are related to reading Java primitive types in a machine-independent way. Multibyte quantities are read assuming the most significant byte is first and the least significant byte is last. All of these methods handle an attempt to read past the end of file by throwing an `EOFException`.

The `RandomAccessFile` class also defines three different `write()` methods for writing bytes of output. The `RandomAccessFile` class also implements the `DataOutput` interface, so it provides additional methods for writing to a file. Most of these additional methods are related to writing Java primitive types in a machine-independent way. Again, multibyte quantities are written with the most significant byte first and the least significant byte last.

The `RandomAccessFile` class would not live up to its name if it did not provide a way to access a file in a nonsequential manner. The `getFilePointer()` method returns the current position in the file, while the `seek()` method provides a way to set the position. Finally, the `length()` method returns the length of the file in bytes.

Output Streams and Writers |  | Object Serialization