Question

我的任务是模拟超市中队列的演变方式，每当客户进入或存在商店时，我都必须使用计时器进行输出。在超市，我有3个队列，应该处理客户。在每个客户端，他们应该在我的GUI中输出到达时间和离开时间。我可以为每个队列使用相同的计时器吗？（他们不应该同时或同一时间开始输出）。我试图将计时器作为参数传递给事件类，但我得到一个空指针异常。请帮助。

  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();
        }
    }
}

如果一个人不理解这些代码请问，我会更具体，但这是我在这里的第一篇文章，我对Java有点新意。

这是我在输出窗口中得到的结果：

    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

Simulation.java文件的第229行导致NullPointerException。具体来说，它位于TimeClass内部类的actionPerformed（...）方法中。

转到那条线，你就会遇到问题。

我怀疑它是这3行之一：

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

其中一个计时器变量可能为空。

快速修复（尽管可能无法解决实际问题）是：

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

以及其他两个计时器变量。

Java队列计时器

1 个答案: