moveNext method Null safety
Advances the iterator to the next element of the iteration.
Should be called before reading current.
It the call to
then current will contain the next element of the iteration
moveNext is called again.
If the call returns
false, there are no further elements
and current should not be used any more.
It is safe to call moveNext after it has already returned
but it must keep returning
false and not have any other effect.
A call to
moveNext may throw for various reasons,
including a concurrent change to an underlying collection.
If that happens, the iterator may be in an inconsistent
state, and any further behavior of the iterator is unspecified,
including the effect of reading current.