Input/Output: Files

Dr Andy Evans

[Fullscreen]

• Files
• File types

   

• Dealing with files starts with encapsulating the idea of a file in an object

File locations

• java.io.File : Encapsulates a file on a drive.
• java.net.URL : Encapsulates a Uniform Resource Locator (URL), which could include internet addresses.

java.io.File

• Before we can read or write files we need to capture them. The File class represents an external file.

  	
  File(String pathname);
  File f = new File("e:/myFile.txt");
  
  

• However, we must remember that different OSs have different file systems.
• Note the use of a forward slash.
• Java copes with most of this, but "e:" wouldn't work in *NIX / a Mac / mobiles etc.

Getting file locations

• java.awt.FileDialog : Opens a "Open file" box with a directory tree in it. This stays open until the user chooses a file or cancels.
• Once chosen use FileDialog's getDirectory() and getFile() methods to get the directory and filename.

Getting file locations

	
import java.awt.*;
import java.io.*;

	
FileDialog fd = new FileDialog(new Frame());
fd.setVisible(true);
File f = null;

if((fd.getDirectory() != null)||
			    (fd.getFile() != null)) { 
    f = new File(fd.getDirectory() + fd.getFile());
}

Application Directory

• Each object has a java.lang.Class object associated with it. This represents the class loaded into the JVM.
• One use is to get resources local to the class, i.e. in the same directory as the .class file. We use a java.net.URL object to do this.

  	
  Class thisClass = getClass();
  URL url = thisClass.getResource("myFile.txt");
  
  

• We can then use URL's getPath() to return the file path as a String for the File constructor.

Useful methods

• exists(), canRead() and canWrite(): Test whether the file exists and can be read or written to.
• createNewFile() and createTempFile(): Create a new file, and create a new file in "temp" or "tmp".
• delete() and deleteOnExit(): Delete the file (if permissions are correct). Delete when JVM shutsdown.
• isDirectory() and listFiles(): Checks whether the File is a directory, and returns an array of Files representing the files in the directory. Can use a FilenameFilter object to limit the returned Files.

• Files
• File types

   

• As we'll see, the type of the file has a big effect on how we handle it.

Binary vs. Text files

• All files are really just binary 0 and 1 'bits'.
• In 'binary' files, data is stored in binary representations of the primitive types:

  8 bits (e.g. 00000000) = 1 byte

  	
  00000000 00000000 00000000 00000000 	= int 0
  00000000 00000000 00000000 00000001 	= int 1
  00000000 00000000 00000000 00000010		= int 2
  00000000 00000000 00000000 00000100		= int 4
  00000000 00000000 00000000 00110001 	= int 49
  00000000 00000000 00000000 01000001 	= int 65
  00000000 00000000 00000000 11111111 	= int 255
  
  

Binary vs. Text files

• In text files, which can be read in notepad++ etc. characters are stored in smaller 2-byte areas by code number:

  	
  00000000 01000001 =  code 65  = char  "A"
  00000000 01100001 =  code 97  = char  "a"
  
  

Characters

• All chars are part of a set of 16 bit international characters called Unicode.
• These extend the American Standard Code for Information Interchange (ASCII) , which are represented by the ints 0 to 127, and its superset, the 8 bit ISO-Latin 1 character set (0 to 255).
• There are some invisible characters used for things like the end of lines.

  	
  char back = 8;  // Try 7, "bell" as well.
  System.out.println("hello" + back + "world");
  
  

• The easiest way to use stuff like newline characters is to use escape characters (see website).

  	
  System.out.println("hello\nworld");
  
  

Binary vs. Text files

• Note that :

  	
  00000000 00110001 =  code 49  = char  "1"
  
  

  Seems much smaller - it only uses 2 bytes to store the character "1", whereas storing the int 1 takes 4 bytes.
• However each character takes this, so:

	
00000000 00110001 
=  code 49  = char  "1"
00000000 00110001 00000000 00110010 
=  code 49, 50  = char  "1" "2"
00000000 00110001 00000000 00110010 00000000 
00110111  = code 49, 50, 55  = char  "1" "2" "7"

• Whereas with an single int of 4 bytes, we can store 127, thus:

	
00000000 00000000 00000000 01111111 = int 127

Binary vs. Text files

• In short, it is much more efficient to store anything with a lot of numbers as binary (not text).
• However, as disk space is cheap, networks fast, and it is useful to be able to read data in notepad etc. increasingly people are using text formats like XML.
• As we'll see, the filetype determines how we deal with files.

Review

	
File f = new File("e:/myFile.txt");

• Three methods of getting file locations:
  1. Hardwiring
  2. FileDialog
  3. Class getResource()
• Need to decide the kind of file we want to deal with.