Question

我让listview工作以在MainActivity上显示它，并且布局设置为具有新Person对象的3个TextView属性。

我以前的代码工作正常但滚动速度很慢，因此我研究了使用Holder模式进行优化。但是，在尝试实施此功能时，我会在NullPointerException

行上获得TextView personID=(TextView) view.findViewById(R.id.person_field1)

listView在MainActivity上设置为ListView，我需要在MainActivity中的此视图上显示person对象。

MainActivity类：

public void onButtonClick() 
{
   //put the persons objects on the listView within MainActivity
   listView.setAdapter(new personAdapter(MainActivity.this, R.layout.list_layout, 
                        personList));
}

class personAdapter extends ArrayAdapter<Person>
{
    private LayoutInflater layoutInflator;

    public personAdapter(Context context, int resource, List<Person> p) {
        super(context, resource, p);
        layoutInflator = LayoutInflater.from(context);
    }

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        View view = convertView;
        Holder holder = null;
        Person p = getItem(position);

        if (view == null) {
            layoutInflator.inflate(R.layout.list_layout, null);  

            //getting null pointer exceptions here
            TextView personID = (TextView) view.findViewById(R.id.person_field1);
            TextView personFName = (TextView) view.findViewById(R.id.person_field2);
            TextView personLName = (TextView) view.findViewById(R.id.person_field3);

            holder = new Holder(personID, personFName, personLName);

            view.setTag(holder);
        }
        else {
            holder = (Holder) view.getTag();
        }

        holder.stop.setText(person.getID());
        holder.location.setText(person.getFName());
        holder.time.setText(person.getLName()); 
        return view;
    }
}

static class Holder 
{
    public TextView id;
    public TextView FName;
    public TextView LName;

    public Holder(TextView id, TextView FName, TextView LName) {
        this.id = id;
        this.FName = FName;
        this.LName = LName;
    }
}

Answer 1

因为确实有空注意检查view == null，然后view从未初始化试试这个

view = layoutInflator.inflate(R.layout.list_layout, null);

Answer 2

只需添加对您视图的引用：

view = layoutInflator.inflate(R.layout.list_layout, null);

Answer 3

view为空，您甚至会检查它：

if (view == null) { ...
}

您缺少的是将夸大的布局分配给view

view = layoutInflator.inflate(R.layout.list_layout, null);

为MainActivity创建ListView持有者

3 个答案: