In the reducer, the MapReduce API provides you with an iterator over Writable values. Calling the
next () method:

A.
Returns a reference to a different Writable object each time.

B.
Returns a reference to a Writable object from an object pool.

C.
Returns a reference to the same writable object each time, but populated with different data.

D.
Returns a reference to a Writable object. The API leaves unspecified whether this is a reused
object or a new object.

E.
Returns a reference to the same writable object if the next value is the same as the previous
value, or a new writable object otherwise.

Explanation:
Calling Iterator.next() will always return the SAME EXACT instance of IntWritable,
with the contents of that instance replaced with the next value.
Reference:manupulating iterator in mapreduce