weird behaviour of java while using System.out and System.err

Question

I am printing following output on screen

     System.out.println(System.currentTimeMillis());
     System.err.println("Hello World");
     System.out.println(System.currentTimeMillis());
     System.out.println(System.currentTimeMillis());
     System.out.println(System.currentTimeMillis());

and when i try to run my program by pressing F11 rapidly i am getting mixed output as below:

   1447918537647
   Hello World
   1447918537648
   1447918537648
   1447918537648

second time

   1447918954719
   1447918954720
   1447918954720
   1447918954720Hello World

third time

   1447919057129
   1447919057130Hello World

   1447919057130
   1447919057131

I try to find documentation on this but failed to find answer.