怎么在Android应用中利用ViewHolder优化Adapter

public class MarkerItemAdapter extends BaseAdapter{  private Context mContext = null;  private List<MarkerItem> mMarkerData = null;  public MarkerItemAdapter(Context context, List<MarkerItem> markerItems)  {    mContext = context;    mMarkerData = markerItems;  }  public void setMarkerData(List<MarkerItem> markerItems)  {    mMarkerData = markerItems;  }  @Override  public int getCount()  {    int count = 0;    if (null != mMarkerData)    {      count = mMarkerData.size();    }    return count;  }  @Override  public MarkerItem getItem(int position)  {    MarkerItem item = null;    if (null != mMarkerData)    {      item = mMarkerData.get(position);    }    return item;  }  @Override  public long getItemId(int position)  {    return position;  }  @Override  public View getView(int position, View convertView, ViewGroup parent)  {    ViewHolder viewHolder = null;    if (null == convertView)    {      viewHolder = new ViewHolder();      LayoutInflater mInflater = LayoutInflater.from(mContext);      convertView = mInflater.inflate(R.layout.item_marker_item, null);      viewHolder.name = (TextView) convertView.findViewById(R.id.name);      viewHolder.description = (TextView) convertView          .findViewById(R.id.description);      viewHolder.createTime = (TextView) convertView          .findViewById(R.id.createTime);      convertView.setTag(viewHolder);    }    else    {      viewHolder = (ViewHolder) convertView.getTag();    }    // set item values to the viewHolder:    MarkerItem markerItem = getItem(position);    if (null != markerItem)    {      viewHolder.name.setText(markerItem.getName());      viewHolder.description.setText(markerItem.getDescription());      viewHolder.createTime.setText(markerItem.getCreateDate());    }    return convertView;  }  private static class ViewHolder  {    TextView name;    TextView description;    TextView createTime;  }}