@Override
public synchronized T next1() {
    ElementWrapper<T> ew = minHeap.remove();
    addNextElementFromCurrentIterator(ew);
    T currentElement = ew.getElement();
    while(nextElementHasTheSameKey(currentElement)) {
        ElementWrapper<T> nextEw = minHeap.remove();
        addNextElementFromCurrentIterator(nextEw);
        currentElement = merge.apply(currentElement, nextEw.getElement());
    }
    return currentElement;
}


@Override
public T next2() {
    T currentElement;
    List<ElementWrapper> elementsWithTheSameKey = new LinkedList<>();
    synchronized (this) {
        ElementWrapper<T> ew = minHeap.remove();
        addNextElementFromCurrentIterator(ew);
        currentElement = ew.getElement();
        while (nextElementHasTheSameKey(currentElement)) {
            ElementWrapper<T> nextEw = minHeap.remove();
            addNextElementFromCurrentIterator(nextEw);
            elementsWithTheSameKey.add(nextEw);
        }
    }
    for(ElementWrapper<T> ew : elementsWithTheSameKey) {
        currentElement = merge.apply(currentElement, ew.getElement());
    }
    return currentElement;
}

private boolean nextElementHasTheSameKey(T currentElement) {
    return minHeap.peek() != null && minHeap.peek().getElement().compareTo(currentElement) == 0;
}

private void addNextElementFromCurrentIterator(ElementWrapper<T> elementWrapper) {
    if(elementWrapper.getIterator().hasNext()) {
        T element = elementWrapper.getIterator().next();
        minHeap.add(new ElementWrapper<>(element, elementWrapper.getIterator(), elementWrapper.getIndex()));
    }
}