Java Reference
In-Depth Information
class. Even if
CLine
used other classes (it does—
String
is a class) it doesn't
matter if those other classes have
main()
methods or not.
4.2.2
Why all the fuss about
main()
and command-line parameters? Such
main()
methods are a handy way to provide unit tests for a class. The tests can be
controlled by the command-line parameters. By testing each class you can re-
duce the time to integrate the parts of an application. Furthermore, a set of unit
tests can be built up (e.g., as shell scripts) to provide automated regression tests
for the entire project. As a more rigorous and systematic approach to unit
testing, we discuss
junit
in Chapter 13.
Unit Testing Made Easy
T
HE
System
C
LASS
4.3
The Java
System
class provides some of the answers to questions about our
environment. What follows is not an exhaustive discussion of all the methods
in the
System
class, but only of those areas that touch on our specific focus—
input/output (I/O) and environment variables.
Be aware that all of the methods in the
System
class are
static
. There-
fore you never need to (and you can't) call a constructor on
System
. You just
use the “class name, dot, method name” syntax to call the method (e.g.,
System.getProperties()
). Similarly, the accessible fields in
System
are all
static
, so for some of the I/O-related methods you use the “class name, dot,
field name, dot, method name” syntax (e.g.,
System.out.println()
). As of
Java 5.0, you can shorten this, by using a
static import
, that is:
import static java.lang.System.*;
Then in your other references you can leave off
System
, for example,
getProperties()
and
out.println()
.
4.3.1
Java adopted the UNIX concept of standard I/O (see Section 1.3.1.1). The
Linux file descriptors are available to a running Java program as I/O streams
via the
System
class. The
System
class contains three
public static
fields
Java and Standard I/O
