Conocer Sistema Operativo con Java

package com.mkyong.core;
 
public class OSValidator {
 
 private static String OS = System.getProperty("os.name").toLowerCase();
 
 public static void main(String[] args) {
 
  System.out.println(OS);
 
  if (isWindows()) {
   System.out.println("This is Windows");
  } else if (isMac()) {
   System.out.println("This is Mac");
  } else if (isUnix()) {
   System.out.println("This is Unix or Linux");
  } else if (isSolaris()) {
   System.out.println("This is Solaris");
  } else {
   System.out.println("Your OS is not support!!");
  }
 }
 
 public static boolean isWindows() {
 
  return (OS.indexOf("win") >= 0);
 
 }
 
 public static boolean isMac() {
 
  return (OS.indexOf("mac") >= 0);
 
 }
 
 public static boolean isUnix() {
 
  return (OS.indexOf("nix") >= 0 || OS.indexOf("nux") >= 0 || OS.indexOf("aix") > 0 );
 
 }
 
 public static boolean isSolaris() {
 
  return (OS.indexOf("sunos") >= 0);
 
 }
 
}

FUENTE: http://www.mkyong.com/java/how-to-detect-os-in-java-systemgetpropertyosname/