File class

■ File resides in the java.io package and a File object is immutable.

■ Some things you can do with File's methods:

Delete files with the delete( ) method.

Delete an empty directory. If the directory was not empty it takes no action and returns false.

Compare path or file names

Extract the file name from a full pathname – getName( )

Obtain a file’s time last modified

List the filenames in a directory - list( ) Returns a String array of the names.

Create a new file - createNewFile( ) No action and returns false if file already exists.

Rename a file - renameTo(File obj)

Set a file to read only – setReadOnly( )

Create a new directory – mkdir( )

Create a new directory with necessary higher level parent directories– mkdirs( )

Change a filepathname to a URL

Test for presence of a directory - isDirectory( ), or a regular file - isFile( )

Check if you can read or write to a file, without actually doing so and without throwing an exception

■ Things you CAN’T do with File:

You can't change the current directory with File. You can get the current directory with:

String currentdir = System.getProperty("user.dir");

You can't rename a directory

You can't set a file to write access if it is Read Only

■ In Windows, each directory in a File constructor requires double backslashes. i.e.

File f = new File("c:\\test");

and

File f = new File("c:\\test\\subtest");

■ The filepathname can be two parts, separated by a comma. i.e.

File f = new File("c:", "\test\\subtest\\brandnewfile.htm"); for Windows

File f = new File(“/”, “autoexec.bat”); for Unix

to create a new file or directory

■ For a new empty file:

import java.io.*;

File f = new File( "testfile.txt" );

try { f.createNewFile( ); }

catch (IOException ex) { System.out.println( "File creation error." ); }

if (f.exists( )) { System.out.println( "File " + f.getName( ) + " created OK." ); }

■ For a new empty directory:

import java.io.*;

File f = new File( "c:\\testdirectory" );

try { f.mkdir( ); }

catch (Exception ex) { System.out.println( "Dir creation error." ); }

if (f.exists( )) { System.out.println( "Dir " + f.getName( ) + " created OK." ); }

to write over an existing file

File file = new File("c:\\111\\test.txt");

// at this point you simply don't do a file.createNewFile( );

PrintWriter pw = null;

try

{

pw = new PrintWriter( new FileOutputStream(file), true);

pw.println("Ode to a Lamb");

pw.println(" ");

pw.println("Mary had a little lamb.");

pw.println("Its fleece was white as snow.");

pw.println("And everywhere that Mary went");

pw.println("Her lamb was sure to go.");

System.out.println("All records written");

}

catch (IOException ex)

{

System.out.println(" IOException!");

}

finally

{

// won't write records if you don't close it

if (pw != null) pw.close( );

}

to delete an existing file or directory

■ The .delete( ) method below works for deleting directories too, but directories must be emptied beforehand.

import java.io.*;

// for a file

File f = new File( "testfile.txt" );

try { f.delete( ); }

catch (Exception ex) { System.out.println( "File deletion error." ); }

if (!f.exists( )) { System.out.println( "File " + f.getName( ) + " deleted OK." ); }

import java.io.*;

// for a directory

File f = new File( "c:\\mytestdirectory" );

try { f.delete( ); }

catch (Exception ex) { System.out.println( "Dir deletion error." ); }

if (!f.exists( )) { System.out.println( "Dir " + f.getName( ) + " deleted OK." ); }

to list files in a directory

■ .list( ) and .listFiles( )

Returns an Array of filenames – a String[ ] array for list( ) and a File[ ] array for listFiles( ) i.e.

File f = new File("c:\\test"); File[ ] array = f.listFiles( );

or

File f = new File("c:\\test"); String[ ] array = f.list( );

Filenames are relative if the pathname was relative, or absolute if the pathname was absolute.

Get the number of files with System.out.println(array.length); or System.out.println(java.lang.reflect.Array.getLength(array));

This snippet prints a directory’s file names:

import java.io.*;

import java.util.*;

File f = new File("c:\\tempdir");

String s[ ] = f.list( );

System.out.println( Arrays.asList( s ));

to determine if a file exists

 ■  .exists( )

Returns true if a file in a File object filepathname exists. Else it returns false.  i.e. where  f  is a pre-existing File object here:

if(f.exists( ))  System.out.println(f + " exists! ");

to find the length of a file

■ .length( )

Returns a long character length of a file in a File filepathname. i.e.

long l = fileObject.length( );

to rename a file

■ .renameTo( File )

Assuming f1 exists here:

import java.io.*;

File f1 = new File( "test.txt" );

File f2 = new File( "test1234.txt" );

try {

if (!f1.renameTo(f2)) System.out.println("File rename error"); }

catch (Exception ex) { System.out.println( "File rename exception." ); }

■ .canRead( ) and .canWrite( )

Return true if the current program can read from or can write to a file in a File filepathname. False if not, or false if file doesn’t exist.

■ .createNewFile( )

Creates the file named in the File object filepathname and returns true if successful.

The new file is empty with a .length( ) of zero

Throws checked IOException so .createNewFile( ) must go in a try-catch block

■ .isFile( ) and .isDirectory( )

Returns true if filepathname represents a file or a directory.

■ .listRoots( )

Returns a File[ ] array of the Windows drives. i.e.

File[ ] array = new File[0];

array = java.io.File.listRoots( );