How can I make my ArrayAdapter follow the ViewHolder pattern?

Question

Here is my ArrayAdapter. I would like to make this more efficient by following the ViewHolder pattern:

http://developer.android.com/resources/samples/ApiDemos/src/com/example/android/apis/view/List14.html

but am not sure how to accomplish this.

UPDATE: ViewHolder Pattern

private class QuoteAdapter extends ArrayAdapter<Quote> {      private ArrayList<Quote> items;     // used to keep selected position in ListView     private int selectedPos = -1; // init value for not-selected      public QuoteAdapter(Context context, int textViewResourceId, ArrayList<Quote> items) {         super(context, textViewResourceId, items);         this.items = items;     }      public void setSelectedPosition(int pos) {         selectedPos = pos;         // inform the view of this change         notifyDataSetChanged();     }      @Override     public View getView(int position, View convertView, ViewGroup parent) {         View v = convertView;         ViewHolder holder; // to reference the child views for later actions          if (v == null) {             LayoutInflater vi = (LayoutInflater)getSystemService(Context.LAYOUT_INFLATER_SERVICE);             v = vi.inflate(R.layout.mainrow, null);              // cache view fields into the holder             holder = new ViewHolder();             holder.nameText = (TextView) v.findViewById(R.id.nameText);             holder.priceText = (TextView) v.findViewById(R.id.priceText);             holder.changeText = (TextView) v.findViewById(R.id.changeText);              // associate the holder with the view for later lookup             v.setTag(holder);         }         else {             // view already exists, get the holder instance from the view             holder = (ViewHolder)v.getTag();         }          // change the row color based on selected state         if (selectedPos == position) {             v.setBackgroundResource(R.drawable.stocks_selected_gradient);             holder.nameText.setTextColor(Color.WHITE);             holder.priceText.setTextColor(Color.WHITE);             holder.changeText.setTextColor(Color.WHITE);         } else {             v.setBackgroundResource(R.drawable.stocks_gradient);             holder.nameText.setTextAppearance(getApplicationContext(), R.style.BlueText);             holder.priceText.setTextAppearance(getApplicationContext(), R.style.BlueText);             holder.changeText.setTextAppearance(getApplicationContext(), R.style.BlueText);         }          Quote q = items.get(position);         if (q != null) {             if (holder.nameText != null) {                 holder.nameText.setText(q.getSymbol());             }             if (holder.priceText != null) {                 holder.priceText.setText(q.getLastTradePriceOnly());             }             if (holder.changeText != null) {                 try {                     float floatedChange = Float.valueOf(q.getChange());                     if (floatedChange < 0) {                         if (selectedPos != position)                             holder.changeText.setTextAppearance(getApplicationContext(), R.style.RedText); // red                     } else {                         if (selectedPos != position)                             holder.changeText.setTextAppearance(getApplicationContext(), R.style.GreenText); // green                     }                 } catch (NumberFormatException e) {                     System.out.println("not a number");                 } catch (NullPointerException e) {                     System.out.println("null number");                 }                 holder.changeText.setText(q.getChange() + " (" + q.getPercentChange() + ")");             }         }         return v;     } } 

Answer 1

The ViewHolder is basically a static class instance that you associate with a view when it's created, caching the child views you're looking up at runtime. If the view already exists, retrieve the holder instance and use its fields instead of calling findViewById.

In your case:

@Override public View getView(int position, View convertView, ViewGroup parent) {     View v = convertView;     ViewHolder holder; // to reference the child views for later actions      if (v == null) {         LayoutInflater vi =              (LayoutInflater) getSystemService(Context.LAYOUT_INFLATER_SERVICE);         v = vi.inflate(R.layout.mainrow, null);         // cache view fields into the holder         holder = new ViewHolder();         holder.nameText = (TextView) v.findViewById(R.id.nameText);         holder.priceText = (TextView) v.findViewById(R.id.priceText);         holder.changeText = (TextView) v.findViewById(R.id.changeText);         // associate the holder with the view for later lookup         v.setTag(holder);     }     else {         // view already exists, get the holder instance from the view         holder = (ViewHolder) v.getTag();     }     // no local variables with findViewById here      // use holder.nameText where you were      // using the local variable nameText before      return v; }  // somewhere else in your class definition static class ViewHolder {     TextView nameText;     TextView priceText;     TextView changeText; } 

caveat: I didn't try to compile this, so take with a grain of salt.