gistfile1.txt

// The MR contract says we're guaranteed to get:
// 1. each key exactly once.
// 2. keys in sorted order.
// 3. all values for each key.
//
// We are not guaranteed to get values in any particular order, however.
// Let's fix that.

void reduce(Text key, Iterator<LongWritable> values, Context<Text, LongWritable> context) {
  // Create a copy buffer and slurp all values out of the iterator.

  List<Long> bufferedValues = new ArrayList<Long>();
  for (LongWritable value : values) {
    bufferedValues.add(value.get());
  }

  // Sort the values (if you want) and iterator over them.
  for (Long value : Collections.sort(bufferedValues)) {
    context.output(key, value);
  }
}

Assuming the input is:

reduce("a", [ 4, 3, 2 ]);
reduce("b", [ 1, 2 ]);

The output is:

"a", 2
"a", 3
"a", 4
"b", 1
"b", 2