Files & Directories : Java Glossary

Because java.io.File can represent a file, wildcard, or directory node, this class has many more uses than its name would suggest. File is a misnomer because File does not represent a file at all, but rather the name of a file or directory that may or may not actually exist.

Using new File( name )

File myFile = new File( "C:\\temp\\abc.txt" );

File myDir = new File( "/home/fundir" );

Using new File( dir, name )

new File ( ".", "C:/fileInRoot.txt" )

new File ( "myDir", "/otherDir" )

File( "myDir/OtherDir" )

File( "/OtherDir" );

new File ( "species", "clan/individual.txt" )

new File ( "species/clan/individual.txt" );

Under the hood, File is surprisingly lightweight. It is just a String and an int to record the prefix length. The prefix is the drive specifier, or the lead /. There is no OS (Operating System) file handle of any kind. Since the current directory cannot change throughout the execution of a program, the File constructor does not need to embed it in each File object.

// this does not work.
File base = new File ( "mydir/myfile.txt" );
File f2 = new File( base, "myfile2.txt"  );

Filename not URL (Uniform Resource Locator)

// if E:\mindprod is a directory, then these
// four different ways of specifying it are
// all equivalent in Windows.
// You can use / or \ (coded as \\)
// You can optionally have a trailing / or \.
File dir1 = new File ( "E:\\mindprod" );
File dir2 = new File ( "E:\\mindprod\\" );
File dir3 = new File ( "E:/mindprod" );
File dir4 = new File ( "E:/mindprod/" );

If you need an URL or URI (Uniform Resource Indicator), you can use File.toURL or File. toURI.

Sample Code

Methods For Manipulating Filenames and Files

String f = null;
if ( new File( f + ".ext" ) .exists() )  ...

Manipulating Directories

String[] fileNames = file.list();

String[] fileNames = file.list( someFilter );

// Creating several levels of directory all in one pop as needed.
// Note mkdirs is an instance method, so you specify the directory tree
// to create in the File constructor.
final boolean success = new File( "C:/temp/wombats" ).mkdirs();

// That is single line is equivalent to:
final File dirs = new File( "C:/temp/wombats" );

// true if created dir, false if could not create, or already created.
final boolean success = dirs.mkdirs();

Taking Apart A Filename:

Combining Parts of A Filename:

Permanent Files

• Use Java Web Start. It has a mechanism to allocate you a unique directory to place persistent files.
• For small amounts of data, use the Preferences mechanism that stores the data in the Windows registry.
• Ask the end user via JFileChooser (good for files or directories, new or existing) for the name of a suitable director. It is best to offer a sensible default drive and directory since most users have no clue what a drive or directory is.
• Sniff about the drives using File.listRoots and File. getFreeSpace looking for a partition that has lots of free space and is writable and just create a unique directory. Techies will be alarmed if you use this approach.
• Look at system properties such as java.io.tmpdir, os.name, user.dir, user.home and user.name to concoct a suitable directory name. E.g. on Windows, C:\Program Files\MyCompany\MyProduct\ is a good bet.

Temporary Files

Tips

• There are many different strings you might want for a filename e.g. J:\mindprod\jgloss\filesanddirectories.html
  filesanddirectories.html
  jgloss/filesanddirectories.html
  filesanddirectories.html
  filesanddirectories
  file:///filesanddirectories.html
  http://mindprod.com/jgloss/filesanddirectories.html
  From Java’s point of view, they are all Strings. It is very common if you start passing such Strings between methods that you will feed a method the wrong flavour. It is best to pass a File. Then Java type checking can help you co-ordinate. When you want one of the other formats, create it on the spot from the File. Unfortunately, there are no built-in methods to get the bald filename without the extension or just the extension. On top of that, add some asserts to assure Strings representing filenames are as you expect.
• There are no tools to test or change file attributes such as hidden, system or archive, except for setReadOnly and setLastModified. The java.io.FilePermission class can’t be used for changing permissions of operating system files. It is used for specifying the permissions of files within a Java program. Java does not support the hidden attribute because not all platforms have it. Unix and Linux use a convention of hiding files whose names begin with a ..
• new File does not actually create a file. The file does not get created until you open some sort of stream or reader. You can create one explicitly with File.createNewFile(). If the file already exists it does nothing. It does not empty the file.
• Java does not let you change the current directory, though you can discover it with new File(.).getCanonicalPath(). The concept of current does not really make sense in a multithreaded environment. You can keep track of your own simulated current directory string (possibly on a per thread or per Object basis), and use new File(dir, filename) to construct a fully qualified filename. The constructor is not very bright. It basically just glues the filename onto the end of the dir name with a file separator. It does not cleverly merge the directory name and the filename the way a command processor would merge the current directory name and a partly qualified filename to get a fully qualified name. You can’t, for example, provide the constructor with a fully qualified filename as the file and have it completely override the dir. The best way to understand its limitations is to experiment and dump your results with File.getCanonicalPath. Writing a smart merge would have to consider the naming conventions of each platform. Another option is to write your own JNI CD (Compact Disc) native method that uses the C chdir or _chdir function and GetStringUTFChars. You could also use exec to spawn a cd command.
• There are no tools to look for an extension. You have to do that yourself with code like this: File offers no built-in way to get the extension or the filename without
• File also handles directories. You can use it to delete, rename and test directories. File.list will give you a list of files in this directory as an array of Strings. However, File cannot handle wildcards. You have to use FilenameFilter.
• File is not final. You can extend it with your own features, e.g. methods to decompose the filename into its composite pieces.

File and Record Locking

The easiest way to deal with this is to use an SQL database manager which can deal with concurrent updates.

Platform Specific Filenames

• lower case only.
• no punctuation.
• no spaces.
• no accented letters.
• no control characters.

Real World Platforms

Console I/O

Applets

Learning More