The default MapReduce output format, TextOutputFormat, writes records as lines of text. Its keys and values may be of any type, since TextOutputFormat turns them to strings by calling toString() on them.
Each key-value pair is separated by a tab character. We can change this separator to some character of our choice using themapreduce.output.textoutputformat.separator (In the older MapReduce API this was mapred.textoutputformat.separator).
To do this you have to add this line in your driver function -
Configuration.set("mapreduce.output.key.field.separator", ",");
HOW TO CHANGE THE DEFAULT KEY-VALUE SEPARATOR OF A MAPREDUCE JOB
原文：http://www.cnblogs.com/feelgood/p/4704203.html