Java™ Application Development on Linux - Dator

4.5 The Runtime Class111screen with the list() method. The argument to list() is either aPrintStream or a PrintWriter. System.out is a PrintStream, so thatwill work.The format of the properties file is name=value. But it is also possible toput comments in a properties file. Any line beginning with a “#” is ignored.Try it.It’s also easy to (re)write a file of properties with the store() method.The parameters are an OutputStream and a String; the latter will serve as alabel for the parameters, written to an opening comment in the properties file.If your program needs to examine the list of property names, you can getan Enumerator of the entire list via the propertyNames() method. ModifyExample 4.3 to replace the list() call with a do-it-yourself version that usesthe Enumerator returned from propertyNames() to list all the names andvalues. Hint: Use getProperty() on each name retrieved via the enumeration.The Java Properties class extends the java.util.Hashtable class.This means, in part, that all the other Hashtable methods are available to aProperties class. Methods such as containsKey() or containsValue()can be helpful, as can isEmpty(). One caution, though. You should usesetProperty() if you want to add values to Properties, rather thanthe Hashtable’s put() method. They do largely the same thing, butsetProperty() enforces that its parameters are Strings. This is important ifyou want to write out the properties to a file, as it’s meant for Strings only.4.5THE Runtime CLASSLet’s discuss one last way to get to the underlying (Linux) system information.Be warned, though, that this is the least portable approach of all we havementioned.4.5.1 exec()Familiar to C/C++ programmers, the exec() call in the Java Runtime classdoes much the same thing. It gives you a way to start another program outsideof the current Java Virtual Machine. In doing so, you can connect to its standardin/out/err and either drive it by writing to its standard in, or read itsresults from its standard out. (Yes, that’s correct—we write to its input andread from its output. If that sounds wrong, think it through. Our Java code is