Question

I have the code below:

public class PatientAgent extends Agent {

  private final String HYPHEN = "-";

  private ArrayList<HashSet<Integer>> prefs;

  private AID provider ;

  private boolean hasAppointment;

  private int appointmentNo;

  @Override
  protected void setup() {

      hasAppointment = false;

      appointmentNo = 0; // A value of zero means agent does not have any 
                         // allocated appointments (yet)

      initPrefs(getArguments());

      System.out.println(prefs.toString());

      // Build the description used as template for the subscription
      DFAgentDescription template = new DFAgentDescription();
      ServiceDescription templateSd = new ServiceDescription();
      templateSd.setType("allocate-appointments");
      template.addServices(templateSd);


      SearchConstraints sc = new SearchConstraints();
      // We want to receive 10 results at most
      sc.setMaxResults(new Long(10));

      addBehaviour(new SubscriptionInitiator(this, DFService.createSubscriptionMessage(this, getDefaultDF(), template, sc)) {
          protected void handleInform(ACLMessage inform) {
              System.out.println("Agent "+getLocalName()+": Notification received from DF");
              try {
                  DFAgentDescription[] results = DFService.decodeNotification(inform.getContent());

                  if (results.length > 0) {

                      // Assume there is only one hospital agent
                      assert(results.length == 1);
                      DFAgentDescription dfd = results[0];


                      Iterator it = dfd.getAllServices();
                      while (it.hasNext()) {
                          ServiceDescription sd = (ServiceDescription) it.next();
                          if (sd.getType().equals("allocate-appointments")) {
                              provider = dfd.getName(); 
                              System.out.println("Allocate-appointments service found:");
                              System.out.println("- Service \""+sd.getName()+"\" provided by agent "+provider.getName());

                          }
                      }



                  }

              }
              catch (FIPAException fe) {
                  fe.printStackTrace();
              }
          }
      } );


public AID getProvider() { return provider; }

I clearly init provider with the line

   provider = dfd.getName();

However the line

       public AID getProvider() { return provider; }

returns null for it which I dont get why. Anyone has an idea about whats going on ?

Answer 1

Assuming you are hitting your assignment statement, here are some things to consider:

• Is getProvider() being called called before handleInform?

• Are handleInform and getProvider() called on separate threads, creating a race or memory visibility issue?

• Is the provider variable being overwritten some time between assignment in handleInform and retrieval with getProvider()?

I can’t think of anything else. Your variable scoping looks OK. You should add a log statement to the getProvider() method to see when it’s being called with respect to handleInform, and log any other places in the program that may overwrite the variable’s value.