Java™ Application Development on Linux - Dator

100Chapter 4Where Am I? Execution Context4.1WHAT YOU WILL LEARNWe’ll show you how Java provides access to the command-line parameters andenvironment variables. We’ll also discuss the Java RunTime and Propertyclasses. Java’s use of the standard input/output/error streams is also brieflycovered, along with an introduction to those concepts. We’ll end with a shortword on portability concerns.4.2A SIMPLE STARTThe most basic external information that a program may use is the informationsupplied on its invocation—simple parameters or arguments, such as filenamesor options, that can direct its running and make it a more flexible tool. Let’sstart with getting at that information from a Java program.4.2.1 Command-Line ArgumentsWhen a program is run from the command line, more than just the programname can be supplied. Here are some examples:$ javac Hi.java$ mv Acct.java core/Account.java$ ls -lIn the first example, we invoked a program called javac and gave it theparameter Hi.java, the name of the file containing the Java program that wewant javac to compile to Java byte code. (We’ve got a whole chapter on howto set up and run the Java compiler, see Chapter 5.) The mv got two commandlinearguments, Acct.java and core/Account.java, which look a lot likepathnames. The ls command has one argument, -l, which in Linux usuallyindicates, by its leading minus sign, that it is an option for altering the behaviorof the command. (In this case it produces the “long” version of the directorylisting.)Even point-and-click GUIs allow such parameters to be supplied, thoughoften not visible to the user. In KDE, one can create a new desktop icon thatis a link to an application. Such an icon has a property sheet that lists, on theExecute tab, the command to be run, including any parameters.In Java, the parameters supplied on the command line are available to themain() method of a Java class. The signature for this method is: