Android尝试从空对象引用中的字段读取

Question

I have an Activity with some Fragments used in a ViewPager , sometimes user sends me some crash report that i don't understand.

I can not produce this crash and most of the user have no problem at all.

The crash Log says:

java.lang.NullPointerException: Attempt to read from field 'com.mehdok.views.treeview.TreeNode$TreeNodeClickListener com.mehdok.mafatih.MainActivity.nodeClickListener2' on a null object reference

The nodeClickListener2 is in my main activity:

public TreeNode.TreeNodeClickListener nodeClickListener2 = new TreeNode.TreeNodeClickListener()
{
    @Override
    public void onClick(TreeNode node, Object value)
    {
        TreeViewHolderNoBorder.TreeViewItem item = (TreeViewHolderNoBorder.TreeViewItem) value;
        if(node.isLeaf() && !item.nodeNavUri.equals(""))
        {
            openNodePage2(item);
        }
    }
};

And in fragmets i use it like this:

tView.setDefaultNodeClickListener(((MainActivity) getActivity()).nodeClickListener2);

What can i do to prevent this crash?

tnx in advance.

Answer 1

That is possible, sometimes Activity will be recycled, when you call getActivity() in Fragment, it will return null. so before you call getActivity() you can write code as below:

if (isAdded()) {
        tView.setDefaultNodeClickListener(((MainActivity) getActivity()).nodeClickListener2);
        ...
    }

Answer 2

Where do you access your activity field from your fragment? You should do it starting from onAttach() up until onDetach().

If you try to access it too soon like in your constructor or in your fragment's onCreate(), this would explain the crash. Same if you try to access it after the fragment is detached from the Activity.