Question

What is a simple and fast way to get an iterator that returns at most N elements from the start of a List?

The simplest versions I could come up with are:

#1:

import com.google.common.collect.Iterators;

// ...

public static <E> Iterator<E> lengthLimitedIterator(Iterable<E> source, int maxLen) {
    return Iterators.partition(source.iterator(), maxLen).next().iterator();
}

#2:

public static <E> Iterator<E> lengthLimitedIterator(List<E> source, int maxLen) {
    return source.subList(0, Math.min(source.size(), maxLen)).iterator();
}

Unfortunately both versions create a temporary List which significantly affects performance as I am calling this method millions of times in a tight loop.

Are there any other library functions I could use for this?

---

Note: I cannot avoid iterating over the list as I am passing it to a method which takes an iterator as its argument and I cannot modify that class.

Answer 1

Seems as if the feature will be added to guava, currently (as of r06) in beta:

public static <T> Iterator<T> limit(Iterator<T> iterator, int limitSize)