为什么我在Android 4.0.3中得到TimePicker NullPointerException？

Question

I have this class which is showing a time picker with a 10 minute interval:

public class TimePickerFragment extends DialogFragment
        implements TimePickerDialog.OnTimeSetListener {

    @Override
    public Dialog onCreateDialog(Bundle savedInstanceState) {
        final Calendar c = Calendar.getInstance();
        int hour = c.get(Calendar.HOUR_OF_DAY);
        // Convert current minutes to tens
        // 55 = 50, 56 = 00
        int minute = c.get(Calendar.MINUTE) / 10;
        minute = (minute > 5) ? 0 : minute;

        final TimePickerDialog tpd = new TimePickerDialog(getActivity(), this, hour, minute,
                DateFormat.is24HourFormat(getActivity()));

        tpd.setOnShowListener(new DialogInterface.OnShowListener() {
            @Override
            public void onShow(DialogInterface dialog) {
                int tpLayoutId = getResources().getIdentifier("timePickerLayout", "id", "android");

                ViewGroup tpLayout = (ViewGroup) tpd.findViewById(tpLayoutId);
                ViewGroup layout = (ViewGroup) tpLayout.getChildAt(0);

                // Customize minute NumberPicker
                NumberPicker minutePicker = (NumberPicker) layout.getChildAt(2);
                minutePicker.setDisplayedValues(new String[]{"00", "10", "20", "30", "40", "50"});
                minutePicker.setMinValue(0);
                minutePicker.setMaxValue(5);
            }
        });

        return tpd;

    }

    public void onTimeSet(TimePicker view, int hourOfDay, int minute) {
        minute = minute * 10;
        // Do something with the time chosen by the user
        final String selectedDate = String.valueOf(hourOfDay) + ":" + String.valueOf(minute);
        EditText selectedTimeTxt = (EditText) getActivity().findViewById(R.id.selectedTime);
        selectedTimeTxt.setText(selectedDate);
    }
}

The code is perfectly working on android Lollipop , but when I run it on emulator, or real device running android 4.0.3 it throws the following exception:

java.lang.NullPointerException regarding this line: ViewGroup layout = (ViewGroup) tpLayout.getChildAt(0);

The error is more than clear, but I can't see the layout component IDs, since the screen capturing in the Device monitor is not supported in API 15.

Can you give me a push? Why I'm getting this error? What's the layout ID im missing? I'm pretty sure that many developers faced the same issue. I know that I'm missing a really small part here, but I'm not able to spot it.

Answer 1

Actually the problem is here:

int tpLayoutId = getResources().getIdentifier("timePickerLayout", "id", "android");

There's no internal identifier with the name timePickerLayout available in API-15 .

Hence tpLayoutId is 0 which makes your tpLayout equals to null ; and calling a method on null object throws NullPointerException .

Recommendation: don't rely on android's internal resource declarations, as there's a reason that they have been made private.