为什么在调用notifyDataSetChanged时发生NullPointerException？

Question

I am trying to update the ListView when the Data is update in the Android.

I use mLeDeviceListAdapter.addRSSIandAddress("20"); to set the value to addRSSIandAddress function.

And in the addRSSIandAddress , it receive the value and call mLeDeviceListAdapter.notifyDataSetChanged(); . But the NullPointerException happened at the viewHolder.deviceRssi.setText("RSSI:" + String.valueOf(mrssi));

It seems the viewHolder.deviceRssi is null or the viewHolder is null .

The code of ListView and the Adapter is like the following:

private LeDeviceListAdapter mLeDeviceListAdapter;
private static ListView device_list;
mLeDeviceListAdapter = new LeDeviceListAdapter(getActivity());
device_list.setAdapter(mLeDeviceListAdapter);

    public class LeDeviceListAdapter extends BaseAdapter{

                private LayoutInflater mInflator;

    ArrayList<BluetoothDevice> mLeDevices;
            private String mrssi;

            public LeDeviceListAdapter(Context context) {
                // TODO Auto-generated constructor stub
                mInflator = LayoutInflater.from(context);
            }

            private void addRSSIandAddress(String rssi) {
                // TODO Auto-generated method stub
                mrssi = rssi;
                mLeDeviceListAdapter.notifyDataSetChanged();
            }

            @Override
            public View getView(final int position, View view, ViewGroup parent) {
                // TODO Auto-generated method stub

                final ViewHolder viewHolder;
                Log.d(TAG, "getView");
                if(view == null){
                    view = mInflator.inflate(R.layout.device_list, null);
                    viewHolder = new ViewHolder();

                    viewHolder.deviceRssi = (TextView) view.findViewById(R.id.device_rssi);

                }else {
                    viewHolder = (ViewHolder) view.getTag();
                    viewHolder.deviceRssi.setText("RSSI:" + String.valueOf(mrssi));
                }

                return view;
            }

        }
        static class ViewHolder {
            TextView deviceRssi;
        }

How to solve this problem ? Thanks in advance.

Answer 1

As the others have said. I think you forgot to set

view.setTag(viewHolder).

Try Put this line here In your code

if(view == null)
{
    view = mInflator.inflate(R.layout.device_list, null);
    viewHolder = new ViewHolder();

    viewHolder.deviceRssi = (TextView) view.findViewById(R.id.device_rssi);

    view.setTag(viewHolder); //THE CODE

}
else 
{
     viewHolder = (ViewHolder) view.getTag();
     viewHolder.deviceRssi.setText("RSSI:" + String.valueOf(mrssi));

}

                    return view;

Answer 2

You use:

viewHolder = (ViewHolder) view.getTag();

But do you setTag somewhere? I don't think so.

Hope it helps

Answer 3

Try with below code:

if(view == null){
   view = mInflator.inflate(R.layout.device_list, null);
   viewHolder = new ViewHolder();
   viewHolder.deviceRssi = (TextView) view.findViewById(R.id.device_rssi);
   view.setTag(viewHolder);

}else {
    viewHolder = (ViewHolder) view.getTag();                 
}
viewHolder.deviceRssi.setText("RSSI:" + String.valueOf(mrssi));