Is it possible to merge iterators in Java?

Is it possible to merge iterators in Java? I have two iterators and I want to combine/merge them so that I could iterate though their elements in one go (in same loop) rather than two steps. Is that possible?

Note that the number of elements in the two lists can be different therefore one loop over both lists is not the solution.

Iterator<User> pUsers = userService.getPrimaryUsersInGroup(group.getId());
Iterator<User> sUsers = userService.getSecondaryUsersInGroup(group.getId());

while(pUsers.hasNext()) {
  User user = pUsers.next();
  .....
}

while(sUsers.hasNext()) {
  User user = sUsers.next();
  .....
}

Guava (formerly Google Collections) has Iterators.concat.

Also the Apache Commons Collection have several classes for manipulating Iterators, like the IteratorChain, that wraps a number of Iterators.

You could create your own implementation of the Iterator interface which iterates over the iterators:

public class IteratorOfIterators implements Iterator {
    private final List<Iterator> iterators;

    public IteratorOfIterators(List<Iterator> iterators) {
        this.iterators = iterators;
    }

    public IteratorOfIterators(Iterator... iterators) {
        this.iterators = Arrays.asList(iterators);
    }


    public boolean hasNext() { /* implementation */ }

    public Object next() { /* implementation */ }

    public void remove() { /* implementation */ }
}

(I've not added generics to the Iterator for brevity.) The implementation is not too hard, but isn't the most trivial, you need to keep track of which Iterator you are currently iterating over, and calling next() you'll need to iterate as far as you can through the iterators until you find a hasNext() that returns true, or you may hit the end of the last iterator.

I'm not aware of any implementation that already exists for this.

Update:
I've up-voted Andrew Duffy's answer - no need to re-invent the wheel. I really need to look into Guava in more depth.

I've added another constructor for a variable number of arguments - almost getting off topic, as how the class is constructed here isn't really of interest, just the concept of how it works.

I haven't written Java code in a while, and this got me curious to whether I've still "got it".

First try:

import java.util.Iterator;
import java.util.Arrays; /* For sample code */

public class IteratorIterator<T> implements Iterator<T> {
    private final Iterator<T> is[];
    private int current;

    public IteratorIterator(Iterator<T>... iterators)
    {
            is = iterators;
            current = 0;
    }

    public boolean hasNext() {
            while ( current < is.length && !is[current].hasNext() )
                    current++;

            return current < is.length;
    }

    public T next() {
            while ( current < is.length && !is[current].hasNext() )
                    current++;

            return is[current].next();
    }

    public void remove() { /* not implemented */ }

    /* Sample use */
    public static void main(String... args)
    {
            Iterator<Integer> a = Arrays.asList(1,2,3,4).iterator();
            Iterator<Integer> b = Arrays.asList(10,11,12).iterator();
            Iterator<Integer> c = Arrays.asList(99, 98, 97).iterator();

            Iterator<Integer> ii = new IteratorIterator<Integer>(a,b,c);

            while ( ii.hasNext() )
                    System.out.println(ii.next());
    }
}

You could of course use more Collection classes rather than a pure array + index counter, but this actually feels a bit cleaner than the alternative. Or am I just biased from writing mostly C these days?

Anyway, there you go. The answer to you question is "yes, probably".

public class IteratorJoin<T> implements Iterator<T> {
	private final Iterator<T> first, next;
	
	public IteratorJoin(Iterator<T> first, Iterator<T> next) {
		this.first = first;
		this.next = next;
	}

	@Override
	public boolean hasNext() {
		return first.hasNext() || next.hasNext();
	}

	@Override
	public T next() {
		if (first.hasNext())
			return first.next();
		return next.next();
	}
}

Starting with Java 8 and later this can be done without external dependencies using Stream API. This also allows concatenation of iterator with other types of streams.

Streams.concat(StreamSupport.stream(<iter1>, false), StreamSupport.stream(<iter2>, false));

move your loop to a method and pass the iterator to method.

void methodX(Iterator x) {
    while (x.hasNext()) {
        ....
    }
}

an iterator comes FROM a collection or a set.
why not use the method already available
Collection.addAll(Collection c);
and then create your iterator from the last object.
this way, your iterator will iterate all the contents of both collection.

You can use my version of an extendable iterator. It uses a double-ended queue of iterators which to me makes sense:

import java.util.Deque;
import java.util.Iterator;
import java.util.concurrent.ConcurrentLinkedDeque;

public class ExtendableIterator<T> implements Iterator<T> {
	
	public Deque<Iterator<T>> its = new ConcurrentLinkedDeque<Iterator<T>>();

	public ExtendableIterator() {
	
	}

	public ExtendableIterator(Iterator<T> it) {
		this();
		this.extend(it);
	}
	
	@Override
	public boolean hasNext() {
		// this is true since we never hold empty iterators
		return !its.isEmpty() && its.peekLast().hasNext();
	}
	
	@Override
	public T next() {
		T next = its.peekFirst().next();
		if (!its.peekFirst().hasNext()) {
			its.removeFirst();
		}
		return next;
	}
	
	public void extend(Iterator<T> it) {
		if (it.hasNext()) {
			its.addLast(it);
		}
	}
}

I would refactor the original design from:

Iterator<User> pUsers = userService.getPrimaryUsersInGroup(group.getId());
Iterator<User> sUsers = userService.getSecondaryUsersInGroup(group.getId());

To something like:

Iterator<User> users = userService.getUsersInGroup(group.getId(), User.PRIMARY, User.SECONDARY, ...);

The Merged Iterator:

import static java.util.Arrays.asList;

import java.util.Iterator;
import java.util.LinkedList;
import java.util.List;
import java.util.NoSuchElementException;


public class ConcatIterator<T> implements Iterator<T> {

    private final List<Iterable<T>> iterables;
    private Iterator<T> current;

    @SafeVarargs
    public ConcatIterator(final Iterable<T>... iterables) {
        this.iterables = new LinkedList<>(asList(iterables));
    }

    @Override
    public boolean hasNext() {
        checkNext();
        return current != null && current.hasNext();
    }

    @Override
    public T next() {
        checkNext();
        if (current == null || !current.hasNext()) throw new NoSuchElementException();
        return current.next();
    }

    @Override
    public void remove() {
        if (current == null) throw new IllegalStateException();
        current.remove();
    }

    private void checkNext() {
        while ((current == null || !current.hasNext()) && !iterables.isEmpty()) {
            current = iterables.remove(0).iterator();
        }
    }

}

The concat method to create an Iterable:

@SafeVarargs
public static <T> Iterable<T> concat(final Iterable<T>... iterables) {
    return () -> new ConcatIterator<>(iterables);
}

Simple JUnit test:

@Test
public void testConcat() throws Exception {
    final Iterable<Integer> it1 = asList(1, 2, 3);
    final Iterable<Integer> it2 = asList(4, 5);
    int j = 1;
    for (final int i : concat(it1, it2)) {
        assertEquals(j, i);
        j++;
    }
}

You can try ConcatIterator from Cactoos:

Iterator<String> names = new ConcatIterator<>(
  Arrays.asList("Sarah", "Mary").iterator(),
  Arrays.asList("Jeff", "Johnny").iterator(),
);

Also check ConcatIterable, which concatenates Iterables.

In the Apache Commons Collections there is public static <E> Iterator<E> org.apache.commons.collections4.IteratorUtils.chainedIterator(Collection<Iterator<? extends E>> iterators) that says

>Gets an iterator that iterates through a collections of Iterators one after another.

which should be what you want.

import java.util.Arrays;
import java.util.Iterator;

import org.apache.commons.collections4.IteratorUtils;
//also works: import org.apache.commons.collections.IteratorUtils;

class Scratch {
	public static void main( String[] args ) {
		final Iterator<String> combinedIterator = IteratorUtils.chainedIterator(
				Arrays.asList( "a", "b", "c" ).iterator(),
				Arrays.asList( "1", "2", "3" ).iterator()
			);
		while( combinedIterator.hasNext() ){
			System.out.println( combinedIterator.next() );
		}
        // "abc123" will have been printed out
	}
}

every Iterator object holds own memory location (adress), so you can't simply "merge" them. except if you extend iterator class and write your own implementation there.

If you are dealing with the same number of objects in both iterators an alternative solution would be to process two iterators in one loop like this :

   while (iterator1.hasNext() && iterator2.hasNext()) {
      // code
    }