What is the Hadoop MapReduce API contract for a key and value Class?
answered By  0
This new API, sometimes referred to as Context Objects, was designed to make the API easier to evolve in the future. It is type-incompatible with the old, how-ever, so applications need to be rewritten to take advantage of it.
The new API is in the org.apache.hadoop.mapreduce package (and subpackages). The old API can still be found in org.apache.hadoop.mapred.
In both APIs, key-value record pairs are pushed to the mapper and reducer, but in addition, the new API allows both mappers and reducers to control the execution flow by overriding the run() method.
The new API makes extensive use of context objects that allow the user code to communicate with the MapReduce system. The new Context, for example, essentially unifies the role of the JobConf, the OutputCollector, and the Reporter from the old API.