Enforcing order in Java Iterator

Question

I am providing a library for a different team. One of the methods I provide receives as argument an Iterator. I would like to somehow require a certain order of iteration. Is there any way to do this in code by extending Iterator?

Answer 1

Not directly. The iterator is made just to give you an item at time, avoiding to storing in memory and pass a whole list of values, which could be unfeasible at times.

Unless you have more knowledge on how the values are generated and which bounds have to be applied to the sorting of data, the only way is to get all elements from the iterator, store them in some list/vector/database, sort them and return another iterator using the sorted list.

Answer 2

You're being passed, as an argument, an instance of some concrete class which implements Iterator. You can't extend its class because its class is decided upon instantiation, which is done by the code that calls your method.

If you want to fail fast when the required order is not respected, try Guava's Ordering.isOrdered() method.

NB This will only work if your argument is an Iterable, rather than Iterator. You need it to be Iterable (an interface which allows you to retrieve the Iterator) so that you can iterate twice: once to check order, once to process.