File Processing - Building Java Programs: A Back to Basics Approach - page 387

Scanner objects are designed for file processing in a forward manner. They pro-
vide a great deal of flexibility for looking ahead in an input file, but no support for
reading the input backwards. There are no “previous” methods, and there's no mech-
anism for resetting a Scanner back to the beginning of the input. Instead, you would
have to construct a new Scanner object that would be positioned at the beginning
of the file. We will see an example of this technique in the case study at the end of
the chapter.

Scanner Parameters
Novices are sometimes surprised that the input cursor for a Scanner does not reset to
the beginning of the file when it is passed as a parameter to a method. For example,
consider the following variation of the ShowSum1 program. It has a method that takes
a Scanner as input and an integer specifying how many numbers to process:

1 // Demonstrates a Scanner as a parameter to a method that
2 // can consume an arbitrary number of tokens.
3
4 import java.io.*;
5 import java.util.*;
6
7 public class ShowSum3 {
8 public static void main(String[] args)
9 throws FileNotFoundException {
10 Scanner input = new Scanner( new File("numbers.dat"));
11 processTokens(input, 2);
12 processTokens(input, 3);
13 processTokens(input, 2);
14 }
15
16 public static void processTokens(Scanner input, int n) {
17 double sum = 0.0;
18 for ( int i = 1; i <= n; i++) {
19 double next = input.nextDouble();

20 System.out.println("number " + i + " = " + next);

21 sum += next;

22 }
23 System.out.println("Sum = " + sum);
24 System.out.println();
25 }
26 }

The main method creates a Scanner object that is tied to the numbers.dat file. It
then calls the processTokens method several times, indicating the number of tokens