Iterate twice on values (MapReduce)

Question

I receive an iterator as argument and I would like to iterate on values twice.

public void reduce(Pair key, Iterator          


        

Answer 1

If method signature cannot be changed then I would suggest using Apache Commons IteratorUtils to convert Iterator to ListIterator. Consider this example method for iterating twice on values:

void iterateTwice(Iterator it) {
    ListIterator lit = IteratorUtils.toListIterator(it);
    System.out.println("Using ListIterator 1st pass");
    while(lit.hasNext())
        System.out.println(lit.next());

    // move the list iterator back to start
    while(lit.hasPrevious())
        lit.previous();

    System.out.println("Using ListIterator 2nd pass");
    while(lit.hasNext())
        System.out.println(lit.next());
}

Using code like above I was able to iterate over the list of values without saving a copy of List elements in my code.