Files

File Class,Reading and Writing
Files,Random access File Operations
By
Sana Mateen

File Class
• File class of java.io package provides some methods to know
the properties of a file or a directory. We can create the File
class object by passing the filename or directory name to it.
• Java File class represents the files and directory pathnames
in an abstract manner. This class is used for creation of files
and directories, file searching, file deletion, etc.
• The File object represents the actual file/directory on the disk.
Following is the list of constructors to create a File object.
• File obj = new File (filename);
• File obj = new File (directoryname);
• File obj = new File ("path", filename);
• File obj = new File ("path", directoryname);

Methods also exist to mark files as readable, writable, and executable. Because
File implements the Comparable interface, the method compareTo( ) is also
supported. JDK 7 added a method to File called toPath( ), which is shown here:
Path toPath( ) toPath( ) returns a Path object that represents the file
encapsulated by the invoking File object.

Directories
• A directory is a File that contains a list of other files and
directories.
• When you create a File object that is a directory, the isDirectory( )
method will return true.
• In this case, you can call list( ) on that object to extract the list of
other files and directories inside.
• It has two forms. The first is shown here:
• String[ ] list( )
• The list of files is returned in an array of String objects.
• The program shown here illustrates how to use list( ) to examine
the contents of a directory:

Using FilenameFilter
• You will often want to limit the number of files returned by the list( )
method to include only those files that match a certain filename pattern,
or filter.
• To do this, you must use a second form of list( ), shown here:
• String[ ] list(FilenameFilter FFObj)
• In this form, FFObj is an object of a class that implements the
FilenameFilter interface.
• FilenameFilter defines only a single method, accept( ), which is called once
for each file in a list.
• Its general form is given here:
• boolean accept(File directory, String filename)
• The accept( ) method returns true for files in the directory specified by
directory that should be included in the list (that is, those that match the
filename argument) and returns false for those files that should be
excluded.

The listFiles( ) Alternative
• There is a variation to the list( ) method, called listFiles( ), which you might find
useful. The signatures for listFiles( ) are shown here:
– File[ ] listFiles( )
– File[ ] listFiles(FilenameFilter FFObj)
– File[ ] listFiles(FileFilter FObj)
• These methods return the file list as an array of File objects instead of strings.
• The first method returns all files, and the second returns those files that satisfy
the specified FilenameFilter.
• Aside from returning an array of File objects, these two versions of listFiles( )
work like their equivalent list( ) methods.
• The third version of listFiles( ) returns those files with path names that satisfy
the specified FileFilter.
• FileFilter defines only a single method, accept( ), which is called once for each
file in a list. Its general form is given here:
• boolean accept(File path)
• The accept( ) method returns true for files that should be included in the list
(that is, those that match the path argument) and false for those that should
be excluded.

Creating Directories
• Another two useful File utility methods are mkdir( ) and mkdirs( ).
• The mkdir( ) method creates a directory, returning true on success and
false on failure.
• Failure can occur for various reasons, such as the path specified in the File
object already exists, or the directory cannot be created because the
entire path does not exist yet.
• To create a directory for which no path exists, use the mkdirs( ) method.
It creates both a directory and all the parents of the directory.

Random Access File Operations
• Random access files permit nonsequential, or random, access to a file's
contents. To access a file randomly, you open the file, seek a particular
location, and read from or write to that file.

• The java.io.RandomAccessFile.seek(long pos) method sets the file-
pointer offset, measured from the beginning of this file, at which the next
read or write occurs.
• public void seek(long pos)
• IOException − If pos is less than 0 or if an I/O error occurs.

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