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The File class encapsulates the attributes and behavior needed to represent a file or a directory on the native system. The File class is not concerned with the contents of the file; it is concerned with the storage of the file on disk.

You can use an instance of the File class to perform certain operations on a corresponding file on the native system:

• delete()
• renameTo()
• mkdir() and mkdirs()

Note that, while you can create a directory using the File class (mkdir()), you do not create a file through the File class. Creating a file is an operation performed by the stream classes, which we'll discuss later.

Using Class File

Here's an example of using File to rename a file:

package inputOutput;

import java.io.File;

public class RenameFile
{
    public static void main(String[] args)
    {
        File fromFile = new File("myFile.txt");
        File toFile   = new File("yourFile.txt");
        boolean renamed = fromFile.renameTo(toFile);
        if (renamed)
            System.out.println("File was successfully renamed");
        else
            System.out.println("File was not renamed");
    }
}

Note that no exception is thrown when the rename fails; you have to check the return value. The delete() method has the same return value convention.

Combining File Path and Name

If you wish to write portable Java code, do not do the following:

package inputOutput;

import java.io.File;

/**
 *  This class creates an instance of File in a non-portable way.
 */
public class FilePathName
{
    public static void main(String[] args)
    {
        File file = new File("path/path1/name.txt");
        
        // ...
    }
}

The above is UNIX-specific (although it will also work on Microsoft Windows). It can't be assumed to work on other platforms.

Class File has the following, which you might think would help:

• public static final String separator -- set to the value of the System property file.separator
• public static final char separatorChar -- set to the value of the System property file.separator

(Note that File also has the following:

• public static final String pathSeparator -- set to the value of the System property path.separator
• public static final char pathSeparatorChar -- set to the value of the System property path.separator

but these relate to CLASSPATH separator characters, not file path separator characters.)

For example, you might think that something like:

package inputOutput;

import java.io.File;

/**
 *  This class creates an instance of File in a non-portable way.
 */
public class FilePathName
{
    public static void main(String[] args)
    {
        File file = new File("path" + File.separator +
                             "path1" + File.separator +
                             "name.txt");
        
        // ...
    }
}

would make things portable, but it doesn't. There are some computer systems which do not use the pure file separator convention to separate directories from the files they contain.

How can we make things more portable? Well, take a look at the constructors available in the File class:

• public File(String path)
• public File(String path, String name)
• public File(File dir, String name)

Let's see how we can use these constructors to solve the portability problem (at least to the extent that we can):

package inputOutput;

import java.io.File;

/**
 *  This class creates an instance of File in a portable way.
 */
public class FilePathName
{
    public static void main(String[] args)
    {
        File path = new File("path");
        path = new File(path, "path1");
        path = new File(path, "name.txt");
        System.out.println("File path = '" + 
                           path.getAbsolutePath() + "'");
        
        // ...
    }
}

Notice how no explicit separators are used at all. If you type this in (or copy it from this page and paste it into an editor), you'll find that the println produces a full (absolute) path specification (despite the fact that there is probably no such file on your system). Note that we entered a relative path specification.

What File Is Not

It is important to note that an instance of class File merely represents a file or a directory. Creating an instance of class File does not create a corresponding file on the native system. Further, an instance of class File may exist without there necessarily existing a corresponding file. However, using an instance of class File, you can discover a number of things about the corresponding file, including whether it exists or not:

• canRead() and canWrite()
• exists()
• getName() and getPath()
• isAbsolute()
• getAbsolutePath() and getCanonicalPath()
• isFile() and isDirectory()
• lastModified()
• length()