Newbie InputFormat Question

[coneybeare]

I want to alter the default <"key", "line"> input format to be <"key", "line number:" + "line"> so that my mapper can have a reference to the line num.  It seems like this should be easy by overwriting either inputformat or inputsplit... but after reading some of the docs, I am still unsure of where to begin.  Any help is much appreciated.

-Matt