Can an empty Iterator be used to concat with another iterator in Java

Question

I have an API which connects to DB and returns an Iterator. I have to call this API in loop to get records for multiple inputs. My requirement is to concatenate all iterators for which I am using com.google.common.collect.Iterators class.

My code snippet is shown below:

Iterator<Record> iterator = Iterators.emptyIterator();
for (QueryHolder<String> queryHolder : queries) {
    Iterators.concat(iterator, fetchAnotherRecordIterator(queryHolder));
}
return iterator;

But at the end I am getting empty iterator. On debugging, I am getting records from fetchAnotherRecordIterator method.

Don't know if we can append iterators to an empty iterator.

Answer 1

com.google.common.collect.Iteratorsis returning a new Iterator when you call the method concat. This means you have to update a reference with the new Iterator:

Iterator<Record> iterator = Iterators.emptyIterator();
for (QueryHolder<String> queryHolder : queries) {
    iterator = Iterators.concat(iterator, fetchAnotherRecordIterator(queryHolder));
}
return iterator;