Question

I’m inflating two layout in one listview. The data are in json format. What I do is I concat the json together, and check the appropriate position for each layout (the first layout only appear from position 0 to first-json’s length – 1).

The problem arise when the list view get longer, and I can scroll down through it. It came out with a Null Pointer exception. So I commented some of the code, and the error disappear. But what I got is not really what I’m expecting:
It interchange the format randomly. (Assume the length of the first json is x, so 0 to x-1 should be the first layout. But when I scroll up & down, sometimes somewhere between 0 and x changed to second layout. As well as other row > x, sometimes changed to first layout)

Here’s the code of the adapter

public class CustomAdapter extends BaseAdapter {

private Activity activity;
private JSONArray data;
private static LayoutInflater inflater=null;
private SecondItem second_item;
private FirstItem first_item;
private int firstLength;
private boolean hasFirst = false;

public CustomAdapter(Activity a, JSONArray firstArray, JSONArray secondArray) {
    activity = a;
    first_item = new FirstItem (firstArray);
    second_item = new SecondItem (secondArray);
    firstLength = first_item.getLength();
    if (!firstArray.isNull(0)) {
        data=CustomUtils.concatJsonArray(firstArray, secondArray);
        hasFirst = true;
    }
    else
        data = secondArray;
    inflater = (LayoutInflater)activity.getSystemService(Context.LAYOUT_INFLATER_SERVICE);

}

public int getCount() {
    // TODO Auto-generated method stub
    return data.length();
}

public Object getItem(int position) {
    // TODO Auto-generated method stub
    return position;
}

public long getItemId(int position) {
    // TODO Auto-generated method stub
    return position;
}

public static class ViewHolder{
    public TextView txt_both;
    public TextView txt_first_only;
    ...
}

public View getView(final int position, View convertView, ViewGroup parent) {
    // TODO Auto-generated method stub
    View vi=convertView;
    ViewHolder holder;
    if(convertView==null){
        if (position < firstLength && hasFirst) {
            vi = inflater.inflate(R.layout.first_item, null);
        }
        else {
            vi = inflater.inflate(R.layout.second_item, null);
        }


        holder=new ViewHolder();
        holder.txt_both=(TextView)vi.findViewById(R.id.txt_both);
        ...

        if (position < firstLength && hasFirst) {
            holder.txt_firstOnly=(TextView)vi.findViewById(R.id.txt_firstOnly);
            ...

        }
        vi.setTag(holder);
    }
    else
        holder=(ViewHolder)vi.getTag();

    if (position < firstLength && hasFirst) {
            holder.txt_both.setText(first_item.getContent(position));
            ...

here is the commented block, which causes null pointer exception before (The error point the line holder.txt_firstOnly.setText) :

        /*if (first_item.getStatus(position)==0) {
            holder.txt_firstOnly.setText(...);              

        }
        else if (first_item.getStatus(position)==1) {
            holder.txt_firstOnly.setText(...);
        }
        else if (first_item.getStatus(position)==2) {
            holder.txt_firstOnly.setText(...);

        } */

and here’s the rest of the code :

        }
    else {
        holder.txt_both.setText(second_item.getContent(position - firstLength));
        ...

    }

    return vi;
}


}

Answer 1

Your custom adapter should override the methods getViewTypeCount() and getItemViewType (int position). Right now, the OS thinks all the views have the same view type, so it’s arbitrarily passing in a previously created view. As you’ve seen, sometimes this isn’t the right view for the position.