Learning Java - Chapter 9 : Java

We earlier discussed using the Scanner class to obtain input from the console. can read from a file just as easily as it can from the console. The example program ScanFileApp expects an input file of the type produced by FormatWriteFileApp containing the output shown for FormatWriteApp discussed earlier.

The program uses Scanner to scan past the text at the beginning of the file and then reads each of the primitive type values. There are many options with the pattern matching capabilities of Scanner to jump past the initial text. We choose a simple technique of looking for the first primitive type value, which is a boolean type. So we loop over calls to hasNextBoolean() until we find a boolean value.

This method, and the similar ones for other primitive types, look ahead at the next token and return true or false depending on whether the token is of the type indicated. It does not jump past the token. So, if the next token is not a boolean, we use the next() method to skip this token and examine the next one. When we find the boolean we break out of the loop.

ScanFileApp.java
Resources: textOutput.txt

import java.io.*;
import java.util.*;

/** Demonstrate using Scanner to read a file. **/
public class ScanFileApp
{
  public static void main (String arg[]) {

    File file = null;

    // Get the file from the argument line.
    if (arg.length > 0) file = new File (arg[0]);
    // or use the default
    if (file == null ) {
      file = new File ("textOutput.txt");
    }

    Scanner scanner = null;
    try {
       // Create a scanner to read the file
       scanner = new Scanner (file);
    } catch (FileNotFoundException e) {
      System.out.println ("File not found!");
      // Stop program if no file found
      System.exit (0);
    }

    try {
      // Skip tokens until the boolean is found.
      while (true) {
        if (scanner.hasNextBoolean ()) break;
        scanner.next ();
      }

      System.out.printf ("Skip strings at start of %s %n", file.toString ());
      System.out.printf ("and then read the primitive type values: %n%n");

      // Read and print the boolean
      System.out.printf (" boolean = %9b %n",  scanner.nextBoolean ());

      // and then the set of numbers
      System.out.printf (" int     = %9d %n"  ,scanner.nextInt ());
      System.out.printf (" int     = %9d %n"  ,scanner.nextInt ());
      System.out.printf (" int     = %9d %n"  ,scanner.nextInt ());
      System.out.printf (" long    = %9d %n"  ,scanner.nextLong ());
      System.out.printf (" float   = %9.1f %n",scanner.nextFloat ());
      System.out.printf (" double  = %9.2e %n",scanner.nextFloat ());

    }
    catch (InputMismatchException e) {
      System.out.println ("Mismatch exception:" + e );
    }
  } // main

} // class ScanFileApp

The output of the program goes as: