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 we are trying to iterate twice in Reducer as below

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());

We will only output as

Using ListIterator 1st pass
5.3
4.9
5.3
4.6
4.6
Using ListIterator 2nd pass
5.3
5.3
5.3
5.3
5.3

Inorder to get it in the right way we should loop like this:

ArrayList cache = new ArrayList();
 for (DoubleWritable aNum : values) {
    System.out.println("first iteration: " + aNum);
    DoubleWritable writable = new DoubleWritable();
    writable.set(aNum.get());
    cache.add(writable);
 }
 int size = cache.size();
 for (int i = 0; i < size; ++i) {
     System.out.println("second iteration: " + cache.get(i));
  }

Output

first iteration: 5.3
first iteration: 4.9
first iteration: 5.3
first iteration: 4.6
first iteration: 4.6
second iteration: 5.3
second iteration: 4.9
second iteration: 5.3
second iteration: 4.6
second iteration: 4.6