I can't speak to the actual rational behind SimState, but I can say that I've had similar frustrations in the past.  I do think that the "God object" approach is a bit easier for newcomers to learn.  It's easier to think in terms of an agent that "reaches out into the world and does stuff" than an agent that "is handed exactly what it needs and only what it needs as references at construction time, and works only with those references to affect the world" (which is the approach I naturally gravitate toward).

But if you aren't careful, boy can it make unit testing frustrating!  To say nothing of tracking down weird unintended side effects.

The solution I've settled on is to make sure that my "God object" can easily be partially initialized with only the components that are needed to provide indirect inputs and outputs to the agent under test.  This minimizes the need for stubbing and mocking, and arguably isn't any more difficult than testing an agent that takes references to data it needs at construction time.


On Thu, Mar 15, 2018 at 11:28 AM, Simon Sohrt <[log in to unmask]> wrote:
I don't have a question about how to use some part of mason, but I am rather curious about the reasoning behind the architecture of the interface Steppable.

Why was the method step(SimState state) designed to take a parameter?
If my agents directly implement the interface Steppable, my agents have access to all other objects in my simulation which makes my agents hard to unit-test (since I have to initialize the 'god-object' SimState to call the step-method).

I am aware that I can circumvent this by creating a Steppable as described in the manual on page 109:
Steppable even = new Steppable() { public void step(SimState state) {; }};
But isn't this a little bit ugly from a design perspective, because the parameter state is completely ignored by

I am also aware that I can use the sim.engine.MethodStep-method to avoid passing the SimState, but the manual does not recommend it for good reasons.

The manual also mentions that the Repast-method for stepping agents works like the sim.engine.MethodStep-method from Mason, but I recently looked into the current version of Repast Simphony 2.5 and Repast uses costume annotations @ScheduledMethod(start = 1, interval = 1), to add methods to the schedule, which seems to me that this is not a violation of Java contracts (as the mason manual claims on page 108). Is the mason manual maybe outdated or am I not correctly understanding the critique about the way Repast schedules methods?


Ph.D student in Computer Science, George Mason University
Web Director, Journal of Mason Graduate Research