System-independent machine shutdown in Java

Possible Duplicate:
Shutting down a computer using Java

I am making a personal program that will shut down my computer after a certain amount of time or at a certain time/date. However, I am running multiple operating systems and want to do this with one simple Java program. Is there any way to send a system-independent machine shutdown request in Java without any using any external libraries? I know you can use java.awt.Desktop.getDesktop().browse(new URI("shutdown /s")); in Windows, but, again, I want system independence.

@robjb gave me the best solution. Though it is a little too inflexible for my tastes, I will be suing it until I run into a problem.

  String shutdownCommand;
  StringPP operatingSystem = new StringPP(System.getProperty("os.name"));

  if (operatingSystem.containsIgnoreCase("linux") ||
      operatingSystem.containsIgnoreCase("mac") ||
      operatingSystem.containsIgnoreCase("unix"))
  {
    shutdownCommand = "sudo shutdown -h -t 30";
  }
  else if (operatingSystem.containsIgnoreCase("windows"))
  {
    shutdownCommand = "shutdown /s /d P:0:0 /t 30 /c \"Blue Husky Timer 2 is shutting down your system, as you requested. \n"
        + "You have 30 seconds to save and close programs\"";
  }
  else
  {
    throw new UnsupportedOperationException("Unsupported operating system.");
  }

  try
  {
    Runtime.getRuntime().exec(shutdownCommand);
  }
  catch (Throwable t)
  {
    Main.LOG.logThrowable(t);
  }
  System.exit(0);

In the above example, StringPP is a custom class that augments the capabilities of a String with methods such as the above used #containsIgnoreCase. Main.LOG is a logging utility that I made and use.