I tested the patch and it looks like it's working.
I had to change the config parameter to add .getName() to SOURCE_VERTEX in RandomWalkWithRestartVertex.java:35
/** Configuration parameter for the source vertex */
static final String SOURCE_VERTEX =
RandomWalkWithRestartVertex.class.getName() + ".sourceVertex";
Otherwise the String reads as "class blah..."
I compared some toy output with a reference implementation and it looks good!
I think the next step would be to support weighted graphs.
The graph can be made stochastic on the fly, at loading time.
I will try to hack some code.