Iterating over several nested iterators

Question

I have to implement an Iterator<E> that needs to internally iterate through multiple private iterators at different levels. For example, an Iterator<A>, an Iterator<B>, and an Iterator<C>. The Iterator<A> exists for the life of my Iterator, but the Iterator<B> and Iterator<C> instances are made from the current values for A and B, respectively.

At any given iteration, the Iterator<C> might be exhausted and so I'll have to advance to the next() of Iterator<B>, from which I will establish a new Iterator<C>. I hope this is clear.

My question is: is there any better structure than this (below)? My real case has 5-6 levels of nested iterators and I'm finding this ugly and unsatisfying.

if ( this.cIterator == null || ! this.cIterator.hasNext() ) {
    // We do not have a C iterator established, or the one we had established is exhausted
    // We need a new one, which will come from the next B
    if ( this.bIterator = null || !this.bIterator.hasNext() ) {
        // We do not have a B iterator established or the one we had is exhausted.
        // We need a new one, which will come from the next A
        if ( this.aIterator == null || !this.aIterator.hasNext() ) {
            // Our aIterator is non-existent or exhausted.  We are done.
            throw new NoSuchElementException();
    }
        // At this point, we can move on to the next A.
        this.currentA = this.aIterator.next();
        this.bIterator = this.currentA.getIteratorSourceCollection().iterator();
        // assert this.bIterator.hasNext(); // will be true in my case
    }
    // At this point, we are guaranteed to have a B iterator with at least one element left
    this.currentB = bIterator.next();
    this.cIterator = this.currentB.getIteratorSourceCollection().iterator();
    // assert this.cIterator.hasNext();  // will be true in my case
}
// At this point, we are guaranteed to have a C iterator with a next element.
return new E(this.currentA, this.currentB, this.cIterator.next());

Answer 1 · 2025-11-11 01:25:39Z

user207421

• Nov 11 at 1:25

I wonder whether this is really an occasion to use iterators at all, given the requirements to refresh those for B and C.

Answer 2 · 2025-11-11 02:46:40Z

Matthew McPeak

• Nov 11 at 2:46

I am using a 3rd party library that requires an Iterator. It is important for performance that I be honest about the Iterator implementation rather than, say, pre-creating all the entries, storing them in a collection, and then just returning an iterator on that collection.

Answer 3 · 2025-11-11 04:35:12Z

Louis Wasserman

• Nov 11 at 4:35

Your approach looks entirely reasonable, but you might implement a more general iterator-of-iterators that does one level of nesting, perhaps with a Function<A, Iterator<B>>, and then use it several levels deeply. You might profitably look at operators like Guava's Iterators.concat(Iterator<Iterator<T>>).

Answer 4 · 2025-11-11 13:31:45Z

Another option could be to use streams as intermediate:

Spliterator<A> spliteratorA = Spliterators.spliteratorUnknownSize(iteratorA, 0);
Iterator<E> iterator = StreamSupport.stream(spliteratorA, false)
        .flatMap(a -> {
            Iterator<B> iteratorB = a.getIteratorSourceCollection().iterator();
            Spliterator<B> spliteratorB = Spliterators.spliteratorUnknownSize(iteratorB, 0);
            return StreamSupport.stream(spliteratorB, false)
                    .flatMap(b -> {
                        Iterator<C> iteratorC = b.getIteratorSourceCollection().iterator();
                        Spliterator<C> spliteratorC = Spliterators.spliteratorUnknownSize(iteratorC, 0);
                        return StreamSupport.stream(spliteratorC, false)
                                .map(c -> new E(a, b, c));
                    });
        })
        .iterator();

By adding a utility method for the conversion of iterator to stream you can simplify it a bit.

As for performance, the returned iterator is just as lazy as its backing stream.

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Iterating over several nested iterators

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Iterating over several nested iterators

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