R> method Null safety
Provides a view of this queue as a queue of
R instances, if necessary.
If this queue contains only instances of
R, all read operations
will work correctly. If any operation tries to access an element
that is not an instance of
R, the access will throw instead.
Elements added to the queue (e.g., by using addFirst or addAll)
must be instance of
R to be valid arguments to the adding function,
and they must be instances of
E as well to be accepted by
this queue as well.