CSCI 161 - Lecture 9 - File Reading Basics Token-Based Line-Based & Advanced File Processing

Instructor's Class Notes

1. File Reading Basics
• File - A collection of information that is stored on a computer and assigned a particular name.
• Common file extensions:
  
  

  Extension	Description
  .txt	text file
  .java	Java source code file
  .class	compiled Java bytecode file
  .doc	Microsoft Word file
  .xls	Microsoft Excel file
  .pdf	Adobe Portable Document File
  .mp3	audio file
  .jpg	image file
  .zip	compressed archive
  .html	hypertext markup language file (web page)
  .exe	executable file

• File object
• import java.io.*
  
  

                        
  import java.io.*;   // for File                                        
                        
                        

• Many useful methods of the File object
  
  

  Method	Description
  canRead()	Whether or not this file exists and can be read
  delete()	Deletes the given file
  exists()	Whether or not this file exists on the system
  getAbsolutePath()	The full path where this file is located
  getName()	The name of the file as a string without directory attached
  isDirectory()	Whether this file represents a directory/folder on the system
  isFile()	Whether this file represents a file (nonfolder) on the system
  length()	The number of characters in this file (size of file)
  renameTo()	Changes this file's name to the given file's name

• Scanner object reading from file
• CountWords - Example that counts words in hamlet.txt file.
• Checked Exception - An exception that must be caught or specifically declared in the header of the method that might generate it.
• throws Clause - A declaration that a method will not attempt to handle a particular type of exception.
• Common Programming Error - Reading beyond end-of-file. A NoSuchElementException will result if trying to read past the end of file (no more tokens lines). Moral know your file format use a conditional with hasNext...() if not sure.

 

2. Token-Based Processing - Processing input token by token (i.e. one word at a time or one number at a time.)
• Chapter 3 review - nextInt() - get next integer nextDouble() - get next double next() - get next token as a string
• ShowSum1 example from book - This example is much like our Lab examples where the user is asked the number of items in the series.
• ShowSum2 example from book - Reads from a file processing all doubles within (not a specific known number of items).
• Common Programming Error - Reading the wrong token. An InputMismatchException will result if trying to read the wrong type of token. Moral know your file format use a conditional with hasNext...() if not sure.
• Common Programming Error - Forgetting new File(...). If you forget to include wrap the filename in a new File object constructor then the Scanner object takes the filename as a literal string and scans that.

 

3. Line-Based Processing - The practice of processing input line-by-line (i.e. reading in entire lines of input at a time). This practice usually requires additional processing of each line seperately.
• nextLine() - Use nextLine() to get and hasNextLine() to check if another line is available.
• HoursWorked2 - Example that combines line reading and token scanning.

 

4. Advanced File Processing - File output and guaranteeing a file may be read
• PrintStream object - Used for file output
• Declare a PrintStream object constructed with a new File object specifying the output file:
  
  

                       
  PrintStream output = new PrintStream(new File("output.txt"));                     
                       
                       

  Then instead of using System.out methods use the same like-named methods of the PrintStream class object
  
  

                       
  output.println("Hello world!");                   
                       
                       

• HoursWorked3 - Example Reads hours from a file and outputs summary to the screen an an output file.
• Guaranteeing that Files can be read - canRead()method
• CountWords2 - Example counts words in input file entered by user.
• Note the use of getInput method for getting an input file name. That is a good boilerplate method.
• Boilerplate Code - Code that tends to be the same from one program to another.
• Paths and Directories
• File Path - A description of a file's location on a computer starting with a drive (or volume) and including the path from the root directory to the directory where the file is stored.
• Current Directory (a.k.a. Working Directory) - The directory that Java uses as the default when a program uses a simple file name (sans file path).
  

                      
  Scanner input = new Scanner(new File("C:/My Documents/My Data/data.txt"));                    
                      
                      

• Relative Path - A path that is relative to the Current Directory typically starts with ./
  

                      
  Scanner input = new Scanner(new File("./data/hours.dat"));