Get position item of list view error

Question

listContent.setOnItemClickListener(new AdapterView.OnItemClickListener()
    {

        @Override
        public void onItemClick(AdapterView<?> parent, View view, int position, long id) {

            for(int i=0;i<showLists.size();i++){
              //
                TextView v=(TextView)listContent.getChildAt(i).findViewById(R.id.txtDes);
                v.setTextColor(Color.BLACK);

            }

            TextView v=(TextView)listContent.getChildAt(position).findViewById(R.id.txtDes);
            v.setTextColor(Color.RED);
            Toast.makeText(context,"POS "+showLists.get(position).getDes(),Toast.LENGTH_SHORT).show();

        }
    });

I have been the problem with get position item of listview. Android just has shown about 12 row on devide's creen, when I click another item on listview (my listview have 30 item ) which shown the error. And this is error: "Attempt to invoke virtual method 'android.view.View android.view.View.findViewById(int)' on a null object reference" . Thanks for read.

Possible duplicate of [What is a NullPointerException, and how do I fix it?](http://stackoverflow.com/questions/218384/what-is-a-nullpointerexception-and-how-do-i-fix-it) — 1615903, May 05 '17 at 10:10

score 0 · Answer 1 · edited May 23 '17 at 11:47

Your question is indeed about Null Pointer Exception, but is harder to identify why this is happening. The problem can be found here:

for(int i=0;i<showLists.size();i++){ <-- this line actually causes the crash
//
    the crash is in the next line, at the findViewById
    TextView v=(TextView)listContent.getChildAt(i).findViewById(R.id.txtDes); 
    v.setTextColor(Color.BLACK);
}

Your for has the wrong upper limit because of a mechanism known as recycling, and because of this mechanism your list view will never have the same number of rows as the amount of data that needs to be displayed (read about recycling to understand this). Given this fact, we know for sure that the number of views found in list view (listContent.getChildCount()) will be smaller than showLists.size() and thus making the call listContent.getChildAt(i) to return a NULL value when the index equals listContent.getChildCount() creating the crash.

Now you might be tempted to change showLists.size() with listContent.getChildCount() and see that the app doesn't crash anymore, but if you click a row, then other rows are coloured also as you scroll the list (the recycling is again the problem). To really fix the problem you should save the index of the selected row and call notifyDatasetChanged, so when getView is called in adapter you simply check the current position to be displayed with the selected position. In case of equality you change the color of text to red, otherwise to black. Below, you will find some parts of an example:

int currentPosition = -1;

// Just a basic adapter. The getView method is the key here
final ArrayAdapter<String> adapter = new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1) {
        @NonNull
        @Override
        public View getView(int position, @Nullable View convertView, @NonNull ViewGroup parent) {
            View view = super.getView(position, convertView, parent);
            if (position == currentPosition) {
                ((TextView) view).setTextColor(Color.RED);
            } else {
                ((TextView) view).setTextColor(Color.BLACK);
            }
            return  view;
        }
    };

// and here is the onItemClick
listView.setOnItemClickListener(new AdapterView.OnItemClickListener()
    {

        @Override
        public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
            currentPosition = position;
            adapter.notifyDataSetChanged();
        }
    });

Thanks, I have known that the list view had mechanism as recyling. When I click a item on listview(30 items) example item have index=0, hmm, even 2 items below this marker RED too. Can you fix it? — Đỗ Ngọc Khánh, May 05 '17 at 11:16
sorry, I read this comment so late. Thanks and I'll try it now. — Đỗ Ngọc Khánh, May 05 '17 at 11:23
I tried this code, and all of item listview set color red or my code was wrong. @@ — Đỗ Ngọc Khánh, May 05 '17 at 12:00
Check the `if` where you set the color. On each branch you are setting the color to red. Change the second one to black. — Iulian Popescu, May 05 '17 at 12:35
ah,I fixed it, but not working. It didn't call back method getView(...), and don't change currentposition. The object "final customListMoiNhat customListMoiNhat" I set final, this is my problem. — Đỗ Ngọc Khánh, May 05 '17 at 12:56
Can you post the adapter code here: https://ideone.com ? After post a comment with the link. — Iulian Popescu, May 05 '17 at 13:11

Bhadresh · Answer 2 · 2017-05-05T11:10:17.770

listContent.setOnItemClickListener(new AdapterView.OnItemClickListener() {
        @Override
        public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
            for (int i = 0; i < showLists.size(); i++) {

                TextView v = (TextView) view.findViewById(R.id.txtDes);
                v.setTextColor(Color.BLACK);

            }

            TextView v = (TextView) view.findViewById(R.id.txtDes);
            v.setTextColor(Color.RED);
            Toast.makeText(context, "POS " + showLists.get(position).getDes(), Toast.LENGTH_SHORT).show();

        }
    });

This will still crash. – Iulian Popescu May 05 '17 at 11:12 — Iulian Popescu, May 05 '17 at 11:12

score 0 · Answer 3 · answered May 05 '17 at 12:16

public View getView(int position, View convertView, ViewGroup parent)
{
    LayoutInflater inflater= (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
    View viewRow=convertView;
    if(viewRow==null){
        viewRow=inflater.inflate(layout,parent,false);
        viewHolder viewHolder=new viewHolder();
        viewHolder.imgIcon    = (ImageView) viewRow.findViewById(R.id.imgIcon);
        viewHolder.txtDes     = (TextView) viewRow.findViewById(R.id.txtDes);

        viewRow.setTag(viewHolder);
    }

    viewHolder  holder= (viewHolder) viewRow.getTag();
    holder.imgIcon.setImageResource(listMoiNhat.get(position).getIcon());
    holder.txtDes.setText(listMoiNhat.get(position).getDes());
    if(position==currentpos){
        holder.txtDes.setTextColor(Color.RED);
    }
    else {
        holder.txtDes.setTextColor(Color.RED);

    }
    return viewRow;
}

//and here is create customListMoiNhat object final customListMoiNhat customListMoiNhat=new customListMoiNhat(context,R.layout.moinhat_customlistview,showLists,currentpos); customListMoiNhat.notifyDataSetChanged(); listContent.setAdapter(customListMoiNhat);

    listContent.setOnItemClickListener(new AdapterView.OnItemClickListener()
    {

        @Override
        public void onItemClick(AdapterView<?> parent, View view, int position, long id) {



            currentpos=position;
            customListMoiNhat.notifyDataSetChanged();


        }
    });

Get position item of list view error

3 Answers3