Java IO.pdf - Nguyen Dang Binh

Chapter 16. Formatted I/O with java.text
Java I/O
One of the most obvious differences between Java and C is that Java has no equivalent of
printf() or scanf(). Part of the reason is that Java doesn't support the variable length
argument lists on which these functions depend. However, the real reason Java doesn't have
equivalents to C's formatted I/O routines is a difference in philosophy. C's printf() and the
like combine number formatting with I/O in an inflexible manner. Java separates number
formatting and I/O into separate packages and by so doing produces a much more general and
powerful system.
More than one programmer has attempted to recreate printf() and scanf() in Java. This
task is difficult, since those functions are designed around variable length argument lists,
which Java does not support. However, overloading the + signs for string concatenation is
easily as effective, probably more so, since it doesn't share the problems of mismatched
argument lists. For example, which is clearer to you? This:
printf("%s worked %d hours at $%d per/hour for a total of %d dollars.\n",
hours, salary, hours*salary);
or this:
System.out.println(employee + " worked " + hours + " hours at $" + salary
+ "per/hour for a total of $%d.");
I'd argue that the second is clearer. Among other advantages, it avoids problems with
mismatched format strings and argument lists. (Did you notice that an argument is missing
from the previous printf() statement?) On the flip side, the format string approach is a little
less prone to missing spaces. (Did you notice that the println() statement would print pay
scales as "$5.35per/hour" rather than "$5.35 per/hour"?) However, this is only a cosmetic
problem and is easily fixed. A mismatched argument list in a printf() or scanf() statement
may crash the computer, especially if pointers are involved.
The real advantage of the printf()/scanf() family of functions is not the format string. It's
number formatting:
printf(
"%s worked %4.1d hours at $%6.2d per/hour for a total of %8.2d dollars.\n",
employee, hours, salary, hours*salary);
Java 1.0 did not provide classes for specifying the width, precision, and alignment of numeric
strings. Java 1.1 and later make these available as subclasses of java.text.NumberFormat.
As well as handling the traditional formatting achieved by languages like C and Fortran,
NumberFormat also internationalizes numbers with different character sets, thousands
separators, decimal points, and digit characters.
16.1 The Old Way
Traditional computer languages have combined input of text with the parsing of numeric
strings. For example, to read a decimal number into the variable x, programmers are
accustomed to writing C code like this:
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