File Processing - Building Java Programs: A Back to Basics Approach - page 400

instead of writing it to the console. There are many ways to accomplish this task. The
simplest approach is to take advantage of what you already know. You've already
learned all about how print and println statements work, and you can leverage
that knowledge to easily create output files.
If you look at Sun's Java documentation, you will find that System.out is a vari-
able that stores a reference to an object of type PrintStream . The print and
println statements you've been writing are calls on methods that are part of the
PrintStream class. The variable System.out stores a reference to a special
PrintStream object that is tied to the console window. However, you can construct
other PrintStream objects that send their output to other places. Suppose, for exam-
ple, that you want to send output to a file called results.txt . You can construct a
PrintStream object as follows:

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

This line of code looks a lot like the one we used to construct a Scanner tied
to an input file. In this case, the computer is creating an output file. If no such
file already exists, the program creates it. If such a file does exist, the computer
overwrites the current version. Initially, the file will be empty. It will end up con-
taining whatever output you tell it to produce through calls on print and
println .
The line of code that constructs a PrintStream object can generate an exception if
Java is unable to create the file you've described. There are many reasons that this
might happen: You might not have permission to write to the directory, or the file might
be locked because another program is using it. Like the line of code that creates a file-
based Scanner , this line of code potentially throws a FileNotFoundException .
Therefore, Java requires you to include the throws clause in whatever method contains
this line of code. The simplest approach is to put this line in main . In fact, it is common
practice to have the main method begin with the lines of code that deal with the input
and output files.
Once you have constructed a PrintStream object, how do you use it? You
should already have a good idea of what to do. We have been making calls on
System.out.print and System.out.println since Chapter 1. If you recall
everything you know about System.out you'll have a good idea of what to do, but
for this program, you will call output.print instead of System.out.print and
output.println instead of System.out.println .
As a simple example, remember that in Chapter 1 we looked at the following
variation of the simple “hello world” program that produces several lines of
output:

1 public class Hello2 {
2 public static void main(String[] args) {

3 System.out.println("Hello, world!");

4 System.out.println();