动态计时器JAVA setTitle

Question

我想使用在主窗体USERGUI中执行的计时器动态更改标题

计时器的第一次执行如下所示：

public void startTimer(GUI gui, String staticTitle) {
    //this.gui = gui; 
    stayingTitle = staticTitle;
    timer = new Timer();
    myTimerTask = new MyTimerTask();
    int millisek = 1000; 
    timer.schedule(myTimerTask, millisek, millisek); 
}

在计时器的运行功能中，其定义为：

long hours = minuntilrecon / 3600;
long minutes = (minuntilrecon % 3600) / 60;
long seconds = minuntilrecon % 60;

String timeString = String.format("%02d:%02d:%02d", hours, minutes, seconds);
String setthisTitle = stayingTitle+" | Time left: "+ timeString;
UserGUI.newTitle(setthisTitle);
minuntilrecon = Math.abs(minuntilrecon-1000);

计时器应每秒在USERGUI中执行以下功能：

public void newTitle(String gottitle){
    this.setTitle(gottitle);
}

但是我在控制台中得到的只是带有UserGUI.newTitle(setthisTitle);的行的空指针异常

Exception in thread "Timer-0" java.lang.NullPointerException
at gui.TimerTitle$MyTimerTask.run(TimerTitle.java:74)
at java.util.TimerThread.mainLoop(Unknown Source)
at java.util.TimerThread.run(Unknown Source)

有什么想法吗