The Java command-line allows you to specify system properties with the
-D<name>=<value> option.  You can also programmatically set system
properties using the System.setProperty method.

I'm afraid I can't make out what you're asking beyond that, though.

It's used at run time, not compile time.

public class Demo {
   public static void main(String[] args) {
       System.out.println(System.getProperty("foo"));
   }
}

java -Dfoo="meaningful value" Demo

Will print "meaningful value" on stdout.

(If you have a property you want to set at compile time,
you can just write it directly into the program you're
compiling.)