When the reducer task receives the output from the various mappers, it sorts the
incoming data on the key of the (key/value) pair and groups together all values of
the same key.
当reducer task接受到从不同r的mappers接受到输出时,他进行排序,并且进行累加。
Table 3.3 Some useful Reducer implementations predefined by Hadoop
Class Description
IdentityReducer<K,V> Implements Reducer<K,V,K,V> and maps inputs directly to outputs
LongSumReducer<K> Implements Reducer<K,LongWritable,K,LongWritable> and
determines the sum of all values corresponding to the given key
