All the distance measure classes are in org.apache.mahout.common.distance package.

They all implements the DistanceMeasure interface which also extends the Parametered interface.

There are two methods in the DistanceMeasure interface:

double distance(Vector v1, Vector v2)

double distance(double centroidLengthSquare, Vector centroid, Vector v)

You can create a new distance measure by implementing the DistanceMeasure interface 🙂