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.