There is a default map function supplied by Hadoop Framework,which is known as Identity Mapper. If this mapper is used, nothing is specially executed and the data is transfer to reduce phase without any mapping work.
Purpose of Identity Mapper
An identity mapper can be used if you would only want to sort your input.
There is a default reducer method, called Identity Reducer.It doesn’t do any thing other than sending the output of map function to its output .
Purpose of Identity Reducer
An identity reducer can be used for example to implement embarrassingly parallel algorithms where you just use the mappers to perform the parallel tasks but you want the output key value pairs to be sorted.