以下代码为适用于ListView的Adapter的部分代码，请根据代码解释下
1）getView方法中对convertView是否为空做了不同处理，目的是什么？
2）viewholder有什么作用？
private List<String> mData = new ArrayList<>();
…
private static class ViewHolder {
TextView text;
public ViewHolder(View rootView) {
text = (TextView)rootView.findViewById(R.id.text);
}
public void bind(String item) {
...
}
}
@Override
public View getView(int position, View convertView, ViewGroup parent) {
String item = mData.get(position);
ViewHolder viewHolder;
if (null == convertView) {
convertView = LayoutInflater.from(mContext).inflate(R.layout.list_item, parent, false);
viewHolder = new ViewHolder(convertView);
convertView.setTag(viewHolder);
} else {
viewHolder = (ViewHolder) convertView.getTag();
}
viewHolder.bind(item);
return convertView;
}
