which would be more memory efficient?

Question

Consider this value object Myclass which has following members. There would be hundred-thousands of MyClass instances.

Optional fields would have about 50% population.

All optional would either be populated or they will not.

which is more efficient for memory?

1.

    class MyClass{
      String mandatory1;
      int mandatory2;
      double mandatory3;

      String optional1;
      int optional2;
      double optional3;
      //lets say there are 10+ like these
}

2. Wrap optional fields in a DS and set it as null when optional value are not present.

    class MyClass{
      String mandatory1;
      int mandatory2;
      double mandatory3;

     OptionalParams params;
}

Question - Does wrapping up in DS wont set the null references for the members within ? So effectively one null reference vs 10+ null references ?

I understand that with null values it doesn't matter much but i do care about the memory used by references ?