The java.io Package - File Class

• used to access file and directory objects using the file-naming, path conventions of the implementing operating system
• the class has three constructors

  File(String pathname)
  File(String parent, String child)
  File(File parent, String child)
  
  where,
      parent      is the pathname
      child       is the filename
  

• used to create an instance of a File BUT does not actually create a file

  // does not create a file on the system
  new File("test.txt"); 
  

• however, you can use the createNewFile() method

  File f = new File("test.txt");
  // returns 'false' if file exists
  f.createNewFile();          
  

• or, the method createTempFile() which creates the file in the default temporary directory using specified file extensions
• the class has four CONSTANTS which define properties of the file conventions on the operating system

  char separatorChar

  the field is initialized to hold the system separator
  / for UNIX
  \ for Win32
  : for Mac

  String separator

  a string representation of the separatorChar

  char pathSeparator

  initialized to hold the character used by the system to separate file names in a list
  : for UNIX
  ; for Win32

  String pathSeparator

  string representation of the pathSeparator character

FileName Methods

• there are a number of methods for retreiving filenames, paths, etc

  getAbsolutePath()       getAbsoluteFile() 
  getCanonicalPath()      getCanonicalFile() 
  getName()                                 
  getParent()             getParentFile()
  getPath()
  compareTo()
  toURL()
  

• the absolute path is system dependent and may include relative indicators

  For example, the following code creates a file 
  'test2.txt' in the directory directly above the
  current directory:
  
  File f1 = new File("..", "test2.txt")
  f1.createNewFile();
  
  System.out.println( f1.getAbsolutePath() );
  
  Output (on Win98):
  D:\Java\jeg\io\..\test2.txt
  

• the canonical path is the same as the absolute path BUT all relative indicators are resolved

  For example, 
  
  System.out.println( f1.getCanonicalPath() );
  
  Output (on Win98):
  // '..' in absolute path is resolved 
  D:\Java\jeg\test2.txt 
  

• toURL() will construct a valid URL identifier for the File

  System.out.println( f.toURL() ); 
  
  Output:
  file:/D:/Java/jeg/io/test1.txt
  

File Status Methods

• there are methods to check the status of a file

  canRead()        lastModified()      isDirectory()
  canWrite()       setLastModified()   isFile()
  exists()         setReadOnly()       isHidden()
  length()                             isAbsolute()
  

Modifiying Files and Directories

• there are a number of methods for modifiying files and creating directories

  delete()             mkdir()         listFiles()
  deleteOnExit()       mkdirs()        listRoots()
  renameTo()                                  
  

• list() and listFiles() can be used with FilenameFilters ie '*'
• listRoots() returns the system drives
• while renameTo() will change the name of the file on the system, the reference will return the original path and name

  // File object reference
  File f  = new File("test.txt"); 
  f.createNewFile();      // creates the file
      
  // new File reference
  File f2 = new File("testRename.txt"); 
  f.renameTo(f2);     // renames the file
      
  System.out.println( f.getAbsolutePath() );
      
  Output (on Win98):
  D:\Java\jeg\io\test1.txt // original path for 'f' 
      
  And if you check to see which file actually 
  exists on the system:
  
  System.out.println( f.exists() );
  System.out.println( f2.exists() );
      
  Output:
    false
    true        
  

Security

• many of the above methods will work correctly only if they are allowed by the security permissions
• for example, an Applet would probably not be allowed to create a new file

Source Code for Examples

• TestFileClass.java