Generating Random Long value in Java

I am trying to generate two 9 digit long random long value in Java using the below code:

for (int i =0;i<2;i++) {                
    String axisIdStr = Long.toString((long)(System.nanoTime() * (Math.random() * 1000)));
    System.out.println("@@@@@@@@ axisIdStr "+axisIdStr);
    String axId = axisIdStr.substring((axisIdStr.length() -9), axisIdStr.length()) ;
}

But when I run this in windows, i get two different numbers where as when run in mac, I get same two numbers. Why is this happening ? Can you suggest a better way to generate the long values?