Java隊列計時器

Question

我的任務是模擬超級市場中隊列的演變過程，每當客戶端進入或存在商店時，我都必須使用計時器來輸出。在超市裏我有3個應該處理顧客的隊列。在每個客戶端，他們應該在我的GUI中輸出到達時間和離開時間。我可以爲每個隊列使用相同的計時器嗎？ （他們不應該同時開始輸出或在相同的時間內開始輸出）。我試圖通過計時器作爲事件類中的參數，但我得到一個空指針異常。請幫助。

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(); 
     } 
    } 
} 

如果你不明白代碼請問，我會更具體，但這是我的第一篇文章，我對Java很陌生。

這是我得到的輸出窗口：

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

線229的Simulation.java文件引起一個NullPointerException。具體來說，它在TimeClass內部類的actionPerformed（...）方法中。

去那條線，那裏你會有你的問題。

我懷疑它是這3個行之一：

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

其中一個定時器變量的可能是零。

速戰速決（雖然可能沒有解決真正的問題）是這樣的：

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

等與其他兩個定時器變量。