The code you have is nicely formatted, and well documented, etc. ... but, as a singleton, it has a number of problems....

The two most glaring are:

Earth real = Earth.getInstance();
Earth.setEarth(null);
Earth alternate = Earth.getInstance();
if (real != alternate) {
   System.out.println("Oops...");
}

The second most glaring issue is the synchronization.

You suggest that the getInstance() needs to be synchronized to avoid thread problems... but your other setter/getter methods are not synchronized.... as a result, threads all over the place can be getting stale, wrong, and otherwise incomplete populations, ages, etc.

As an example of a singleton 'best use case', this one has some problems... ;-)

But, that's sort of OK, since Earth has problems anyway!

The code you have is nicely formatted, and well documented, etc. ... but, as a singleton, it has a number of problems....

The two most glaring are:

1. It is not a singleton

   Earth real = Earth.getInstance();
   Earth.setEarth(null);
   Earth alternate = Earth.getInstance();
   if (real != alternate) {
      System.out.println("Oops...");
   }
   

2. The synchronization.

   You suggest in your comments that the getInstance() needs to be synchronized to avoid thread problems... but your other setter/getter methods are not synchronized.... as a result, threads all over the place can be getting stale, wrong, and otherwise incomplete populations, ages, etc.

As an example of a singleton 'best use case', this one has some problems... ;-)

But, that's sort of OK, since Earth has problems anyway!