How can I make an iterator that never ends?

Question

I was just wondering what the easiest way to iterate over a set indefinitely, i.e. when it reaches the end it next(); calls the first object. I'm assuming that this is not an already predefined function in Java, so just looking for the easiest way to implement this in Java.

What could be requirement for this.If you just want to loop forever some elements then why to use Iterator? — Khangharoth, Jun 19 '09 at 08:34
If I were only using the iterator to access the objects via a while loop then there would be no point, but I'm not so I use an iterator as it is the best fit for what I want to do. — Bobby, Jun 20 '09 at 00:27

score 42 · Accepted Answer · edited Apr 10 '18 at 10:24

42

There's a method in the excellent Google Collections library which does this:

Set<String> names = ...;
Iterable<String> infinite = Iterables.cycle(names);

(I can't recommend the Google Collections library strongly enough. It rocks very hard. I'm biased as I work for Google, but I think pretty much every Googler writing Java would tell you how useful the collections are.)

edited Apr 10 '18 at 10:24

kdb

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answered Jun 19 '09 at 06:47

Jon Skeet

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The same functionality using [Apache Commons Collections](https://commons.apache.org/proper/commons-collections/): `Iterator infinite = new LoopingIterator(names);` – Omid Jan 15 '22 at 15:40
or as `Iterables.cycle` Javadoc states `Stream.generate(() -> collection).flatMap(Collection::stream).iterator()` – Edgar Asatryan Jan 11 '23 at 15:16

score 6 · Answer 2 · answered Jun 19 '09 at 11:55

6

Iterator it = mylist.iterator();
while (it.hasNext())
{
  MyType t = (MyType)it.next();

  // do something

  if (!it.hasNext())
    it = mylist.iterator();
}

answered Jun 19 '09 at 11:55

AlexC

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Isn't this an infinite loop? – Michael Hewson Sep 25 '16 at 02:08
No, .next advances the iterator. – AlexC Sep 25 '16 at 02:11
1

Ya, but at the bottom, you replace `it` with a new iterator if `it` is exausted, which means you will never come out of the while loop... which I guess is the whole point, but I misread the question and thought the OP was asking how to *create* an iterator that loops infinitely, which this doesn't do – Michael Hewson Sep 26 '16 at 02:05

yegor256 · Answer 3 · 2017-07-10T09:07:44.847

2

Try EndlessIterator from Cactoos:

Iterator<String> names = new EndlessIterator<>("John");

It will always return "John" and will never end.

Also, check EndlessIterable, which implements Iterable and does the same.

edited Jul 10 '17 at 09:07

answered Jun 19 '17 at 09:04

yegor256

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score 1 · Answer 4 · answered Jun 19 '09 at 06:31

1

If you're making the iterator, in the next method you can have an if condition that checks if there's another object in the list. If there is, then you return that object, if there isn't then you go back to the start of the list and return that object.

answered Jun 19 '09 at 06:31

mnuzzo

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I understand you posted your answer before mine, but as seen in my answer that is what I am basically doing right now, I was just wondering if there was a better way than instantiating a new Iterator object every time I get to the end – Bobby Jun 19 '09 at 06:39

score 1 · Answer 5 · answered Jun 19 '09 at 06:32

1

This is what I can think of...

iterator = set.getIterator
//other code
if (iterator.hasNext())
    //do code here
else
    iterator = set.getIterator();

answered Jun 19 '09 at 06:32

Bobby

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score 1 · Answer 6 · answered May 25 '16 at 05:55

1

How about ?

List<String> list = // ArraysList
Interator<String> it = null;

while(true) {
 it = list.iterator();
 while(it.hasNext()) {
   System.out.println(it.next());
 }
}

answered May 25 '16 at 05:55

bluelurker

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score 1 · Answer 7 · answered Jun 13 '17 at 18:45

If you don't want to use Guava but still want a reusable solution:

public static class CyclicIterator<E, C extends Collection<E>> implements Iterator<E> {
    final private C mElements;
    private Iterator<E> mIterator;

    public CyclicIterator(C elements) {
        mElements = elements;
        mIterator = elements.iterator();
    }

    @Override
    public boolean hasNext() {
        if (! mIterator.hasNext()) {
            mIterator = mElements.iterator();
        }
        return mIterator.hasNext();
    }

    @Override
    public E next() {
        if (! mIterator.hasNext()) {
            mIterator = mElements.iterator();
        }
        return mIterator.next();
    }
}

Note: this doesn't support the remove() method but it could easily be added if needed. Also it's not thread safe.

score 0 · Answer 8 · answered Apr 21 '10 at 21:51

I think what you want never help You can do anything with your iterator that's easy but you must be carefull with any new thing you add im not used with this style but this is what you want though :

if (! It.hasNext() ) { while ( It.hasPrevious() ) { It = It.Previous(); } } else { It = It.Next(); }

This way is nothing if your really interested you should instead make next pointer of the last to the first always when pushing a new list.

score 0 · Answer 9 · answered Mar 14 '22 at 17:00

0

std jdk:

Iterable<String> infinite = Stream.generate(names.stream()).flatMap(e -> e).iterator()

answered Mar 14 '22 at 17:00

bernstein

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Actually, it's a compile error because generate accepts `Supplier`. See my comment here: https://stackoverflow.com/questions/1016605/how-can-i-make-an-iterator-that-never-ends#comment132502479_1016650 – Edgar Asatryan Jan 11 '23 at 15:22

How can I make an iterator that never ends?

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