First, you should really read the handout for information about how the contrast enhancement works.

The main() that runs is in driver/Driver.java.
expand(), reduce(), etc is in transform/PGMTransform.java.
reading, writing and getting the pixels of .pgm files is in image/PGMImage.java.
A high level overview of the process is in contrast/PGMContrast.java

When you implement your hadoop version, you will probably want to reuse PGMImage.java.
If not, feel free to reinvent the wheel.
