Java Print Streams - Formatter
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In fact, printf() is a little more general than System.out (though that’s its primary justification). Besides printf(), the PrintStreamclass also has aformat()method:
public PrintStream format(String format, Object... args)
This does exactly the same thing asprintf(). That is, the previous example could be rewritten like this and produce identical output:
for (double degrees = 0.0; degrees < 360.0; degrees++) {
double radians = Math.PI * degrees / 180.0;
double grads = 400 * degrees / 360;
System.out.format("%5.1f %5.1f %5.1f\n", degrees , radians, grads);
}
Why two methods, then? Theformat()method is used not just byPrintStreambut also by thejava.util.Formatterclass:
public class Formatter implements Flushable, Closeable
printf()is there solely to make C programmers feel nostalgic.
Formatteris the object-oriented equivalent ofsprintf()andfprintf()in C. Rather than writing its output onto the console, it writes it into a string, a file, or an output stream. Pass the object you want to write into to theFormatterconstructor. For example, this code fragment creates aFormatterthat writes data into a file named angles.txt:
Formatter formatter = new Formatter("angles.txt");
Once you’ve created aFormatterobject, you can write to it using theformat()method just as you would withSystem.out.format(), except that the output goes into the file rather than onto the console:
for (double degrees = 0.0; degrees < 360.0; degrees++) {
double radians = Math.PI * degrees / 180.0;
double grads = 400 * degrees / 360;
formatter.format("%5.1f %5.1f %5.1f\n", degrees , radians, grads);
}
Formatters are not output streams, but they can and should be flushed and closed just the same:
formatter.flush();
formatter.close();
Please check back next week for the continuation of this article.
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 This article is excerpted from chapter seven of Java I/O, Second Edition, written by Elliotte Rusty Harold (O'Reilly, 2006; ISBN: 0596527500). Check it out today at your favorite bookstore. Buy this book now.
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