How to create a Java 8 Stream from an iterator?

Question

Is it possible to create a Stream from an Iterator, in which the sequence of objects is the same as that generated by calling the iterator\'s next() method repeatedly? The s

Answer 1

For the particular example of NavigableSet.descendingIterator(), I think the simplest way is to use NavigableSet.descendingSet() instead.

But given you are probably interested in the more general case, the following seems to work:

import java.util.Iterator;
import java.util.Spliterator;
import java.util.Spliterators;
import java.util.TreeSet;
import java.util.stream.Stream;
import java.util.stream.StreamSupport;

public class Streams {
    public static void main(String... args) {
        TreeSet set = new TreeSet<>();
        set.add("C");
        set.add("A");
        set.add("B");

        Iterator iterator = set.descendingIterator();

        int characteristics = Spliterator.DISTINCT | Spliterator.SORTED | Spliterator.ORDERED;
        Spliterator spliterator = Spliterators.spliteratorUnknownSize(iterator, characteristics);

        boolean parallel = false;
        Stream stream = StreamSupport.stream(spliterator, parallel);

        stream.forEach(System.out::println); // prints C, then B, then A
    }
}

In short, you have to create a Spliterator from the Iterator first using one of the static methods in Spliterators. Then you can create a Stream using the static methods in StreamSupport.

I don't have that much experience with creating Spliterators and Streams by hand yet, so I can't really comment on what the characteristics should be or what effect they will have. In this particular simple example, it didn't seem to matter whether I defined the characteristics as above, or whether I set it to 0 (i.e. no characteristics). There is also a method in Spliterators for creating a Spliterator with an initial size estimate - I suppose in this particular example you could use set.size(), but if you want to handle arbitrary Iterators I guess this won't be the case. Again, I'm not quite sure what effect it has on performance.