NullPointerException在javax计时器上吗？

Question

I want to show a timer on a status bar.

How does I get rid of the " throwing NullPointerException below on the line " t = earthclock.getTimeStamp() ?

    public void startEarthTimer() {
    //final String t = null;
    earthTimer = new javax.swing.Timer(TIMEDELAY, 
        new ActionListener() {      
        //String t = null;
            public void actionPerformed(ActionEvent evt) {
                String t = null;
                //leftLabel.setText(statusText); 
                try {
                    // TODO: investigate why the line below is causing NullPointerException
                    EarthClock earthclock = Simulation.instance().getMasterClock().getEarthClock();
                    t = earthclock.getTimeStamp();
                } catch (Exception ee) {
                    ee.printStackTrace(System.err);
                }
                timeLabel.setText("Earth Time: " + t);
            }
        });

    earthTimer.start();
}

The trace stack said this:

 java.lang.NullPointerException
at org.mars_sim.msp.ui.swing.MainWindow$4.actionPerformed(MainWindow.java:277)
at javax.swing.Timer.fireActionPerformed(Unknown Source)
at javax.swing.Timer$DoPostEvent.run(Unknown Source)
at java.awt.event.InvocationEvent.dispatch(Unknown Source)
at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
at java.awt.EventQueue.access$400(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$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)

My getTimeStamp() is nothing complicated:

    public synchronized String getTimeStamp() {
    String result = formatter.format(cal.getTime());// + " UT";
    if (result == null) result = "0";
    return result;
}

Also my EarthClock is just this:

public class EarthClock {
    private final GregorianCalendar cal;


private final SimpleDateFormat formatter;

  public EarthClock(String dateString) {

    // Use GregorianCalendar constructor
    cal = new GregorianCalendar();

    // Set GMT timezone for calendar
    SimpleTimeZone zone = new SimpleTimeZone(0, "GMT");
    cal.setTimeZone(zone);

    // Initialize formatter
    formatter = new SimpleDateFormat("yyyy-MM-dd  HH:mm:ss 'UT'");
    formatter.setTimeZone(zone);

    // Set Earth clock to Martian Zero-orbit date-time. 
    // This date may need to be adjusted if it is inaccurate.
    cal.clear();
    DateFormat tempFormatter = new SimpleDateFormat("MM/dd/yyyy hh:mm:ss");
    tempFormatter.setTimeZone(zone);
    try {
        cal.setTime(tempFormatter.parse(dateString));
    } catch (ParseException ex) {
        throw new IllegalStateException(ex);
    }
}

I've tried 3 locations for placing "String t=null" inside the method startEarthTimer() and none works. I also have String t=null in the main class and in all cases it's throwing NullPointerException.

Can anyone point out what went wrong? Thanks much!

EDIT: In MasterClock, I started an instance of EarthClock as follows:

   public MasterClock() {
    // Initialize data members
    SimulationConfig config = SimulationConfig.instance();

    // Create an Earth clock
    earthTime = new EarthClock(config.getEarthStartDateTime());

    ....

    }

Answer 1

The method getEarthClock() is returning null . Try breaking up your code like so, and adding some sanity checks to gain certainty of which method is returning null:

MasterClock master = Simulation.instance().getMasterClock();
if (master == null) {
  throw new IllegalStateException("master clock is null");
}
EarthClock earthclock = master.getEarthClock();
if (earthclock == null) {
  throw new IllegalStateException("earthclock is null");
}
t = earthclock.getTimeStamp();

You may be mis-configuring the earth clock, or possibly missing some configuration required for the library you're using.