Is it possible to merge iterators in Java?

Question

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) rathe

Answer 1

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 implements Iterator {

    private final List> iterables;
    private Iterator current;

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

Simple JUnit test:

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