NullPointerException由findViewById引起

Question

I'm having trouble finding why this line throws a NullPointerException exception:

ImageView imageView1 =  (ImageView) rootView.findViewById(R.id.imageView1);

It's all happening within a fragment This is the fragment:

public static class MyFragment extends Fragment {

    private View rootView;

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {

        rootView = inflater.inflate(R.layout.fragment_layout,
                container, false);

        return rootView;
    }

    public void setImage(Uri imageUri) {
        ImageView imageView1 =  (ImageView) rootView.findViewById(R.id.imageView1);
        imageView1.setImageURI(imageUri);
    }
}

And this is the fragment layout:

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="fill_parent"
android:layout_height="fill_parent">

<ScrollView
      android:layout_width="fill_parent"
      android:layout_height="wrap_content">

    <ImageView
        android:id="@+id/imageView1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" />

</ScrollView>
</RelativeLayout>

The method setImage() is called from outside the fragment.

Answer 1

So, it looks like your rootView is null ;

Are you sure you are calling setImage() on the instance of your fragment that has had onCreateView called? You could test this by adding a println to both methods and checking the console/LogCat for their order.

You could post the code where you call setImage() .

If it is indeed that, which is the problem, there are several solutions, some better than others. For starters, you could try passing either the rootView or the imageView1 to the activity where you call setImage() and move setImage() to that activity. Not best practice, but try it and see if it works.

(Would have commented, but I don't have enough reputation. Comment back and I'll try to edit the answer according to your comments.)

Answer 2

This is happening because of this line private View rootView; which declares rootView but does not set it equal to anything so by default it has a value of NULL or rootView == NULL is true. Although you set it in onCreateView() what happens if onCreateView() is never called? rootView would still be NULL even though setImage() relies on it being set to something and this will cause a NULLPounterException . There is no requirement that says onCreateView() is called before setImage() or even called at all meaning that rootView could potentially still be null and thus not have the .findViewById() method call. While it is technically true that onCreateView() is called first the Java compilier doesn't know this. This has to do with how Android itself works not with the actual Java language. In actuality onCreateView() is indeed run before you get a chance to run setImage() but the compilier will still complain. This should be fixed if you just wrap the function with an if statement to make sure that rootView is never NULL before you do anything to it.

Change this:

public void setImage(Uri imageUri) {
        ImageView imageView1 =  (ImageView) rootView.findViewById(R.id.imageView1);
        imageView1.setImageURI(imageUri);
    }
}

To this:

public void setImage(Uri imageUri) {
        //Will only run if rootView isn't NULL so you won't be able to get a NULLPointer
        if(rootView != NULL){
        ImageView imageView1 =  (ImageView) rootView.findViewById(R.id.imageView1);
        imageView1.setImageURI(imageUri);
        }
    }
}