Question

I’ve a ListView and when the user clicks on one of its items I want that item to become blue. In order to do this, in the onCreate() method of the ListView activity, I’ve set a listener for the user clicks.

m_listFile=(ListView)findViewById(R.id.ListView01);  
      m_listFile.setOnItemClickListener(new OnItemClickListener() {  

            public void onItemClick(AdapterView<?> arg0, View arg1,int arg2, long arg3) {  
                arg0.getChildAt(arg2).setBackgroundColor(Color.BLUE);  
            }
});

Everything works fine for the first visible items, but when I scroll the list, I’ve a
NullPointerException at arg0.getChildAt(arg2).setBackgroundColor(...), even if the arg2 value has the right item index position.

My ListView has a two lines item structure, when I load the ListView I use this adapter:

 SimpleAdapter sa = new SimpleAdapter(
            getApplicationContext(), 
            expsList, 
            R.layout.listelement, 
            new String[] { "screen_name","text" },
            new int[] { R.id.Name, R.id.Value}) {

      };

      m_listFile.setAdapter(sa);

I don’t understand how to solve this problem. Can I get some help?

Answer 1

You could extend SimpleAdapter like this:

private class MyAdapter extends SimpleAdapter {

        public MyAdapter(Context context, List<? extends Map<String, ?>> data,
                int resource, String[] from, int[] to) {
            super(context, data, resource, from, to);
        }

        @Override
        public View getView(int position, View convertView, ViewGroup parent) {
            View v = super.getView(position, convertView,   parent);
            v.setBackgroundColor(Color.BLACK); //or whatever is your default color
              //if the position exists in that list the you must set the background to BLUE
          if(pos!=null){
            if (pos.contains(position)) {
                v.setBackgroundColor(Color.BLUE);
            }
          }
            return v;
        }

    }

Then in your activity add a field like this:

//this will hold the cliked position of the ListView
ArrayList<Integer> pos = new ArrayList<Integer>();

and set the adapter:

sa = new MyAdapter(
            getApplicationContext(), 
            expsList, 
            R.layout.listelement, 
            new String[] { "screen_name","text" },
            new int[] { R.id.Name, R.id.Value}) {

      };
m_listFile.setAdapter(sa);

When you click the row:

    public void onItemClick(AdapterView<?> arg0, View arg1,int arg2, long arg3) {  
                    // check before we add the position to the list of clicked positions if it isn't already set
                if (!pos.contains(position)) {
                pos.add(position); //add the position of the clicked row
            }
        sa.notifyDataSetChanged(); //notify the adapter of the change       
}