Question

I have as a task to simulate how queues evolve in a supermarket and I have to use timers to output whenever a client enters or exists the store. In the supermarket I have 3 queues that should process customers. At each client, they should output in my GUI the arrival time and the leaving time. Can I use the same timer for each queue? (they should not start outputting simultaneously or for the same period of time). I tried to pass the timer as a parameter in the event class but I get a Null Pointer exception. Help pls.

  public void simulationPrep (Queue q,JPanel p,int time,Timer t, int selection)

  {

    TimeClass tc;
    int queueCapacity=0;
    float queueProcTime=0;
    float tempSTMin, float tempSTMax;
    tempSTMin=getServiceTimeMin();
    tempSTMax=getServiceTimeMax();
    queueCapacity = 10 + (int)( Math.random()*20);
    queueProcTime = tempSTMin + (float)(Math.random()*tempSTMax);
    q1.setCapacity(queueCapacity);
    q1.setProcessingTime(queueProcTime);
    tc = new TimeClass((int)(queueProcTime/queueCapacity),p,selection);
    t = new Timer(time, tc);
    t.start();
}
public void simulate()
{
    if(getSelection()>=1)
    {
        simulationPrep(q1,p21,1000,timer1,1);
        if(getSelection()>=2)
        {
            simulationPrep(q2,p22,1500,timer2,2);
            if(getSelection()==3)
            {
                simulationPrep(q3,p23,1700,timer3,3);
            }
        }
    }

}

public class TimeClass implements ActionListener
{
    int counter;
    JPanel p;
    int selection;
    public TimeClass(int counter,JPanel p,int selection)
    {
        this.counter = counter;
        this.p = p;
        this.selection = selection;
    }
    public void actionPerformed(ActionEvent e)
    {
        counter--;
        if(counter>=1)
        {
            p.add(new JLabel("Hi "+counter));
            p.updateUI();
        }
        if(counter<1)
        {
            p.add(new JLabel("Done"));
            p.updateUI();
            timer1.stop();
            timer2.stop();
            timer3.stop();
        }
    }
}

If one does not understand the code please ask, I’ll be more specific, but this is my first post here and I’m kind of new to Java.

This is what I get in the output window :

    Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at simulation.Simulation$TimeClass.actionPerformed(Simulation.java:229)
at javax.swing.Timer.fireActionPerformed(Unknown Source)
at javax.swing.Timer$DoPostEvent.run(Unknown Source)
at java.awt.event.InvocationEvent.dispatch(Unknown Source)
at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
at java.awt.EventQueue.access$000(Unknown Source)
at java.awt.EventQueue$1.run(Unknown Source)
at java.awt.EventQueue$1.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.AccessControlContext$1.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue.dispatchEvent(Unknown Source)
at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.run(Unknown Source)
    Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at simulation.Simulation$TimeClass.actionPerformed(Simulation.java:229)
at javax.swing.Timer.fireActionPerformed(Unknown Source)
at javax.swing.Timer$DoPostEvent.run(Unknown Source)
at java.awt.event.InvocationEvent.dispatch(Unknown Source)
at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
at java.awt.EventQueue.access$000(Unknown Source)
at java.awt.EventQueue$1.run(Unknown Source)
at java.awt.EventQueue$1.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.AccessControlContext$1.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue.dispatchEvent(Unknown Source)
at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.run(Unknown Source)

Answer 1

Line 229 of your Simulation.java file is causing a NullPointerException. Specifically it is in the actionPerformed(…) method of your TimeClass inner class.

Go to that line, and there you’ll have your problem.

I suspect it is one of these 3 lines:

        timer1.stop();
        timer2.stop();
        timer3.stop();

One of these timer variables is probably null.

A quick fix (although probably not addressing the real problem) is this:

if (timer1 != null) {
    timer1.stop();
}

and so on with the other two timer variables.