Using RunOnUIThread causing null pointer Exception After Switching to another Fragment

Question

public class ViewPagerTask extends TimerTask {
    @Override
    public void run() {
        getActivity().runOnUiThread(new Runnable() {
            @Override
            public void run() {
                if (viewPager.getCurrentItem() == 0){
                    viewPager.setCurrentItem(1);
                } else if (viewPager.getCurrentItem() == 1){
                    viewPager.setCurrentItem(2);
                } else  viewPager.setCurrentItem(0);
            }
        });
    }
}

I'm using this code to change images in a view. But the problem is I'm using it in fragment and when I change the fragment and app runs for few seconds and then suddenly pop's null pointer error. Now What I understand the reason is that it tries to change the image but doesn't find the view and create this error I have no clue what to do. Please Help :)

Error

E/AndroidRuntime: FATAL EXCEPTION: Timer-0
Process: com.example.android.indianmetro, PID: 5150
java.lang.NullPointerException: Attempt to invoke virtual method 'void 
android.support.v4.app.FragmentActivity.runOnUiThread(java.lang.Runnable)' 
on a null object reference
    at com.example.android.indianmetro.HomeFragment$ViewPagerTask.run(HomeFragment.java:258)
at java.util.TimerThread.mainLoop(Timer.java:555)
at java.util.TimerThread.run(Timer.java:505)
E/AbstractTracker: Can't create handler inside thread that has not called 
Looper.prepare()

it seems `getActivity()` is returning null in your case. Are you sure that you are not finishing the Actiivty when your `TimerTask` executes? — Michael Lam, Dec 18 '18 at 02:28
yes I think the problem is with the view activity is same I'm just replacing my fragments. — Vishva Sharma, Dec 21 '18 at 08:16

score 5 · Accepted Answer · answered Dec 18 '18 at 05:32

The first, you should understand about threads in Android. In Android, we have a thread called MainUIThread. I'm not sure that's the main thread or not. However, It has an instance of Looper class.

Your fragment source basically runs on MainUIThread by default. When you create the sub-class of TimerTask, you are creating a new Runnable for run your source in another thread, is that right?

Your error message java.lang.NullPointerException means that the return value of getActivity() method are null. A good place to start is the JavaDocs. They have this covered:

Thrown when an application attempts to use null in a case where an object is required. These include:

Calling the instance method of a null object.
Accessing or modifying the field of a null object.
Taking the length of null as if it were an array.
Accessing or modifying the slots of null as if it were an array.
Throwing null as if it were a Throwable value.

Applications should throw instances of this class to indicate other illegal uses of the null object.

The solution for you:

Let check and make sure you are not finishing the Activity. (Another case that's you detach this Fragment of called Activity. Because you are using ViewPager, I guess that you are swipe two times to left or right. It automatically detach your fragment by default) Try to abandon it.

If you have to finish your Activity which contains your Fragment or has to detach your fragment. The easy way is checked null before call runOnUiThread() method.

public class ViewPagerTask extends TimerTask {
    @Override
    public void run() {
        Activity activity = getActivity();
        if (activity != null) {
            activity.runOnUiThread(new Runnable() {
                @Override
                public void run() {
                    if (viewPager.getCurrentItem() == 0) {
                        viewPager.setCurrentItem(1);
                    } else if (viewPager.getCurrentItem() == 1) {
                        viewPager.setCurrentItem(2);
                    } else {
                        viewPager.setCurrentItem(0);
                    }
                }
            });
        }
    }
}

works perfectly fine, thanks for the help and also for explaining everything. Really appreciated. — Vishva Sharma, Dec 21 '18 at 08:26

score 2 · Answer 2 · answered Dec 18 '18 at 02:37

2

Your TimerTask is fired after the fragment is removed. In that case, getActivity() will return null and hence the exception.

I can suggest two choices:

assign getActivity() output to a local var when TimeTask is fired, check and use if not null.
cancel your TimerTask in onDetach(). This is a recommended approach.

answered Dec 18 '18 at 02:37

mesibo

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I used the first one and it's working fine, but can you explain 2nd point a lil bit more. – Vishva Sharma Dec 21 '18 at 08:50
1

onDetach() will be called when Fragment is detached. You should override that and cancel any pending timer task from there. – mesibo Dec 22 '18 at 09:05

score 1 · Answer 3 · answered Apr 12 '23 at 05:17

public class The_slide_timer extends TimerTask {
        @Override
        public void run() {
            Activity activity = getActivity();

            if (activity != null) {
                activity.runOnUiThread(new Runnable() {
                    @Override
                    public void run() {
                        if (page.getCurrentItem() < listItems.size() - 1) {
                            page.setCurrentItem(page.getCurrentItem() + 1);
                        } else
                            page.setCurrentItem(0);
                    }
                });

            }
        }
    }

just checking whither the activity null or not just solve the problems.

Using RunOnUIThread causing null pointer Exception After Switching to another Fragment

3 Answers3