Input Files

Objectives

1. Be able to read data from an input file.

Notes

• This is from Section 11.2
  • For now, we will only read data from files
  • And we will assume our files are in the proper format.
  • But we will change all of that later.
• A file is a collection of data stored on permanent media
  • For us this will probably be a hard drive.
  • But it could be usb, cd, dvd, network storage ...
• Files have a name.
  • Usually a name (like input.dat)
  • And a path like (C:\users\dbennett\java)
  • We will work with files in the local directory, so we don't need to worry about paths.
• Files can be of many types
  • Text only
  • Binary
  • Other formats.
  • For now we will read text only files.
• We will assume that there are no errors in our files for now.
  • But we will change that later too.
• Reading from a file consists of two parts
  • Opening the file.
  • Associating the file with a file reader.
  • This will be a simple modification to the Scanner for now.
  • Scanner myScanner = new Scanner (new File(filename));
  • But now we definitely need to close the scanner when we are done.
  • myScanner.close()
• We will use the type File
  • This comes from java.io.*
  • So we will need import java.io.*; at the top of our code.
• The exception
  • If the file does not exist, a FileNotFoundException will be thrown
  • We should catch this exception, and exit the program.
• A simple example
  • The file "MyNumber.txt" contains a single integer. Print it out.

  • import java.io.*;
    import java.util.Scanner;
    
    public class MyNumberReader {
        public static void main(String[] args){
             Scanner myScanner;
             int number;
    
             try {
                 myScanner = new Scanner(new File("MyNumber.txt"));
             } catch (FileNotFoundException e){
                 System.out.println("Unable to open the file");
                 return;
             }
    
             number = myScanner.nextInt();
    
             System.out.printf("The number is %d\n", number);
    
             myScanner.close();
    
        }
    }