Crash on ListView.removeFooterView(View)

Question

I am getting crash reports on

 android.widget.ListView  lv;  lv.removeFooterView(v)

The error is null pointer exception. I check that listView itself is not null. What causes this? Is it necessary to make sure the view to be removed is not null? Is that enough or do I first need to also check that the footer view actually has been added?

java.lang.NullPointerException
at android.widget.ListView.removeFooterView(ListView.java:374)

It seems to me this method should be robust enough not to crash! Why does it not just return false if it cannot remove the view?

PS. I would like to know if anyone else has seen this?

Answer 1

Unfortunately you don't mention what Android version the error reports are coming from. However, looking at the source code, Android 2.1-update1 seems like a good candidate.

I'll just copy in the whole method to make things clear:

public boolean removeFooterView(View v) {
    if (mFooterViewInfos.size() > 0) {
        boolean result = false;
        if (((HeaderViewListAdapter) mAdapter).removeFooter(v)) { // <- line 274
            mDataSetObserver.onChanged();
            result = true;
        }
        removeFixedViewInfo(v, mFooterViewInfos);
        return result;
    }
    return false;
}

Now compare above removeFooterView(...) method with the implementation of a more recent platform:

public boolean removeFooterView(View v) {
    if (mFooterViewInfos.size() > 0) {
        boolean result = false;
        if (mAdapter != null && ((HeaderViewListAdapter) mAdapter).removeFooter(v)) {
            if (mDataSetObserver != null) {
                mDataSetObserver.onChanged();
            }
            result = true;
        }
        removeFixedViewInfo(v, mFooterViewInfos);
        return result;
    }
    return false;
}

As you can see, the've added in a couple of extra checks for certain members not being null. That would suggest that the first method will indeed fail on line 274 if mAdapter == null, whereas that wouldn't cause a crash with the newer implementation.

To work around it, all you probably need to do is add something like lv.getAdapter() != null before trying to remove the footer view.