reduce a data stream to other type

[Soheil Pourbafrani]

Hi, I have a keyed datastream in the type of Tuple2<Long, byte[]>. I want
to reduce it and merge all of the byte[] for a key. (the first filed (Long)
is the key). So I need reduce function return the type Tuple2<Long,
ArrayList<byte[]>>, but reduce function didn't allow that! How can I do
such job in Flink?