55. Null Iterator

• Null Iterator is a degenerate iterator that's helpful for handling container boundary conditions.

• By definition, a null iterator's IsDone( ) operation always evaluates to true.

• Null Iterator makes traversing the aggregates easier:

  1. At any point in the traversal, a Concrete Iterator can be compared to the Null Iterator;

  2. If the result is true, the traversal is complete;

  3. If the result is false, the traversal should continue.