Does anybody know how to join multiple iterators in Java? The solution I found iterate through one iterator first, and then move on to the next one. However, what I want is when next() gets called, it first returns the first element from the first iterator. Next time when next() gets called, it returns the first element from the second iterator, and so on.
Thanks
If you are given two Iterators in Java, how are you supposed to merge them into one list if both iterators are sorted? The Java's iterator has two important methods you can use: hasNext() and next(). The hasNext() returns a boolean telling if this iterator reaches the end.
One way to merge multiple lists is by using addAll() method of java. util. Collection class, which allows you to add the content of one List into another List. By using the addAll() method you can add contents from as many List as you want, it's the best way to combine multiple List.
Iterator and for-each loop are faster than simple for loop for collections with no random access, while in collections which allows random access there is no performance change with for-each loop/for loop/iterator.
Using Guava's AbstractIterator
for simplicity:
final List<Iterator<E>> theIterators;
return new AbstractIterator<E>() {
private Queue<Iterator<E>> queue = new LinkedList<Iterator<E>>(theIterators);
@Override protected E computeNext() {
while(!queue.isEmpty()) {
Iterator<E> topIter = queue.poll();
if(topIter.hasNext()) {
E result = topIter.next();
queue.offer(topIter);
return result;
}
}
return endOfData();
}
};
This will give you the desired "interleaved" order, it's smart enough to deal with the collections having different sizes, and it's quite compact. (You may wish to use ArrayDeque
in place of LinkedList
for speed, assuming you're on Java 6+.)
If you really, really can't tolerate another third-party library, you can more or less do the same thing with some additional work, like so:
return new Iterator<E>() {
private Queue<Iterator<E>> queue = new LinkedList<Iterator<E>>(theIterators);
public boolean hasNext() {
// If this returns true, the head of the queue will have a next element
while(!queue.isEmpty()) {
if(queue.peek().hasNext()) {
return true;
}
queue.poll();
}
return false;
}
public E next() {
if(!hasNext()) throw new NoSuchElementException();
Iterator<E> iter = queue.poll();
E result = iter.next();
queue.offer(iter);
return result;
}
public void remove() { throw new UnsupportedOperationException(); }
};
For reference, the "all of iter1, all of iter2, etc" behavior can also be obtained using Iterators.concat(Iterator<Iterator>)
and its overloads.
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