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I am writing a method (Java) that will reset the position of e-puck in Webots. I have been following tutorial on Supervisor approach. I have two controllers in my project:

  • SupervisorController extends Supervisor - responsible for genetic algorithm and resetting e-puck's position
  • EpuckController extends Robot - drives the robot

Robots are communicating via Emitter and Receiver, and everything works fine but the position reset. This is what I'm doing in SupervisorController:

412 Node epuck = getFromDef("epuck");    
413 Field fldTranslation = epuck.getField("translation");

And as a result I get this exception:

[SupervisorController] Exception in thread "main" java.lang.NullPointerException
[SupervisorController]  at SupervisorController.initialise(SupervisorController.java:413)
[SupervisorController]  at SupervisorController.main(SupervisorController.java:497)

epuck variable is null. I tried calling different methods on epuck, and they all resulted in NullPointerException. The name of e-puck matches the world file.

DEF EPUCK DifferentialWheels {
  translation 0.134826 -0.000327529 0.107963
  rotation 0.0244439 0.999246 -0.0301538 1.95838
  children [
  (........)
  ]
  name "epuck"
  controller "EpuckController"
  axleLength 0.052
  wheelRadius 0.0205
  maxSpeed 6.28
  speedUnit 0.00628
}

I would appreciate any advice on how to get a handle to the robot or where to look for issues in simulation/code.

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The Solid nodes (including Robot and Supervisor nodes) have a DEF name, like any other node and it is usually written in upper-case. These nodes also have a field "name". Both may be used by a Supervisor, it depends on which function is called.

In my case, the function getFromDef() expects a DEF name as argument, and not the name from the "name" field. The DEF name of my e-puck is "EPUCK" - hence modifying controller to:

Node epuck = getFromDef("EPUCK");

solved the problem.

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