Question

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

public void reduce(Pair<String,String> key, Iterator<IntWritable> values,
                   Context context)

Is it possible ? How ?
The signature is imposed by the framework I am using (namely Hadoop).

— edit —
Finally the real signature of the reduce method is with an iterable. I was misled by this wiki page (which is actually the only non-deprecated (but wrong) example of wordcount I found).

Answer 1

We have to cache the values from the iterator if you want to iterate again. At least we can combine the first iteration and the caching:

Iterator<IntWritable> it = getIterator();
List<IntWritable> cache = new ArrayList<IntWritable>();

// first loop and caching
while (it.hasNext()) {
   IntWritable value = it.next();
   doSomethingWithValue();
   cache.add(value);
}

// second loop
for(IntWritable value:cache) {
   doSomethingElseThatCantBeDoneInFirstLoop(value);
}

(just to add an answer with code, knowing that you mentioned this solution in your own comment 😉 )

---

why it’s impossible without caching: an Iterator is something that implements an interface and there is not a single requirement, that the Iterator object actually stores values. Do iterate twice you either have to reset the iterator (not possible) or clone it (again: not possible).

To give an example for an iterator where cloning/resetting wouldn’t make any sense:

public class Randoms implements Iterator<Double> {

  private int counter = 10;

  @Override 
  public boolean hasNext() { 
     return counter > 0; 
  }

  @Override 
  public boolean next() { 
     count--;
     return Math.random();        
  }      

  @Override 
  public boolean remove() { 
     throw new UnsupportedOperationException("delete not supported"); 
  }
}