Java™ Application Development on Linux - Dator

102Chapter 4Where Am I? Execution Contextclass. Even if CLine used other classes (it does—String is a class) it doesn’tmatter if those other classes have main() methods or not.4.2.2 Unit Testing Made EasyWhy 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 becontrolled by the command-line parameters. By testing each class you can reducethe time to integrate the parts of an application. Furthermore, a set of unittests can be built up (e.g., as shell scripts) to provide automated regression testsfor the entire project. As a more rigorous and systematic approach to unittesting, we discuss junit in Chapter 13.4.3THE System CLASSThe Java System class provides some of the answers to questions about ourenvironment. What follows is not an exhaustive discussion of all the methodsin 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. Thereforeyou never need to (and you can’t) call a constructor on System. You justuse the “class name, dot, method name” syntax to call the method (e.g.,System.getProperties()). Similarly, the accessible fields in System are allstatic, 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 ofJava 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 and Standard I/OJava adopted the UNIX concept of standard I/O (see Section 1.3.1.1). TheLinux file descriptors are available to a running Java program as I/O streamsvia the System class. The System class contains three public static fields