hi Vijaykanth,
Debugging code of mapreduce is similar to how we debug the java code in eclipse.
Select a line and click on the left border, eclipse will set a small marker.
Now, launch the program in eclipse in Debug mode from Run -> Debug.

once the debug mode starts, use the "stepover" icon to step through the values , while stepping over , mouse over on the line where you have setup the debug point. here eclipse will display actual values just below the mouse