Explain the core methods of the Reducer?

 The API of Reducer is very similar to that of Mapper, there's a run() method 
that receives a Context containing the job's configuration as well as interfacing 
methods that return data from the reducer itself back to the framework. The run() 
method calls setup() once, reduce() once for each key associated with the 
reduce task, and cleanup() once at the end. Each of these methods can access
the job's configuration data by using Context.getConfiguration().
As in Mapper, any or all of these methods can be overridden with custom
implementations. If none of these methods are overridden, the default reducer
operation is the identity function; values are passed through without further
processing.
The heart of Reducer is its reduce() method. This is called once per key; the
second argument is an Iterable which returns all the values associated with that
key.