You can use the Core.retrieveByPath
method here, passing the context, starting object and the association name
See the apidocs page
(Note there's also an overloaded method with a boolean for when you're passing a child object of a self association, you probably won't need that)
The javadoc:
/**
* Retrieves objects using the given object and path.
*
* @param context the context.
* @param mxObject the start point of the path.
* @param path the path (association) to the objects to retrieve.
* @return the list of retrieved objects.
*/