Question

我在android中创建自定义ListView。面临的问题是我的适配器代码。即使其他所有内容都按顺序显示，它也会抛出NullPointerException。示例代码如下：

public class CustomAdapter extends BaseAdapter {

   private Context ctx;
   private List<LearnLangs> langsList;
   private LayoutInflater layoutInflater;

   public CustomAdapter(Context ctx, List<LearnLangs> langsList) {
       this.ctx = ctx;
       this.langsList = langsList;
   }

   @Override
   public int getCount() {
       return langsList.size();
   }

   @Override
   public Object getItem(int position) {
       return position;
   }

   @Override
   public long getItemId(int position) {
       return position;
   }

   public static class ViewHolder{

       public ImageView img;
       public TextView title;
       public TextView sub;
   }

   @Override
   public View getView(int position, View convertView, ViewGroup parent) {

       ViewHolder holder;
       LearnLangs langs = langsList.get(position);

       if (convertView == null) {
           layoutInflater = (LayoutInflater) ctx.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
           layoutInflater.inflate(R.layout.custom_list, parent, false);

           holder = new ViewHolder();
           //error occurs on this line below
           holder.img = (ImageView) convertView.findViewById(R.id.list_img);
           holder.title = (TextView) convertView.findViewById(R.id.list_title);
           holder.sub = (TextView) convertView.findViewById(R.id.list_subT);

           convertView.setTag(holder);
       }

       else
       {
           holder = (ViewHolder) convertView.getTag();

           holder.img.setImageResource(langs.getImgId());
           holder.title.setText(langs.getNameLang());
           holder.sub.setText(langs.getnTuts());
       }
       return convertView;
   }
}

抛出异常：

java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.view.View.findViewById(int)' on a null object reference
                                                                           at com.example.joey.sunshine.CustomAdapter.getView(CustomAdapter.java:62)
                                                                           at android.widget.AbsListView.obtainView(AbsListView.java:2347)
                                                                           at android.widget.ListView.makeAndAddView(ListView.java:1864)
                                                                           at android.widget.ListView.fillDown(ListView.java:698)
                                                                           at android.widget.ListView.fillFromTop(ListView.java:759)
                                                                           at android.widget.ListView.layoutChildren(ListView.java:1673)
                                                                           at android.widget.AbsListView.onLayout(AbsListView.java:2151)
                                                                           at android.view.View.layout(View.java:15761)

什么似乎是问题？

Answer 1

您正在获得异常，因为您根本没有使用正在膨胀的视图。按以下方式更改getview（）方法的代码：

@Override
public View getView(int position, View convertView, ViewGroup parent) {

    View mView = convertView;
    ViewHolder holder;
    LearnLangs langs = langsList.get(position);

    if (mView == null) {
        layoutInflater = (LayoutInflater) ctx.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
        mView = layoutInflater.inflate(R.layout.custom_list, parent, false);

        holder = new ViewHolder();
        //error occurs on this line below
        holder.img = (ImageView) mView.findViewById(R.id.list_img);
        holder.title = (TextView) mView.findViewById(R.id.list_title);
        holder.sub = (TextView) mView.findViewById(R.id.list_subT);

        mView.setTag(holder);
    }

    else
    {
        holder = (ViewHolder) mView.getTag();

        holder.img.setImageResource(langs.getImgId());
        holder.title.setText(langs.getNameLang());
        holder.sub.setText(langs.getnTuts());
    }
    return mView;
}

Answer 2

只需更改此行：

layoutInflater.inflate(R.layout.custom_list, parent, false);

到此：

convertView = layoutInflater.inflate(R.layout.custom_list, parent, false);

修改

public AdapterConstructor() { layoutInflater = (LayoutInflater) ctx.getSystemService(Context.LAYOUT_INFLATER_SERVICE); //move this line inside the constructor } @Override public View getView(int position, View convertView, ViewGroup parent) { ViewHolder holder; LearnLangs langs = langsList.get(position); if (convertView == null) { convertView = layoutInflater.inflate(R.layout.custom_list, parent, false); holder = new ViewHolder(); //error occurs on this line below holder.img = (ImageView) convertView.findViewById(R.id.list_img); holder.title = (TextView) convertView.findViewById(R.id.list_title); holder.sub = (TextView) convertView.findViewById(R.id.list_subT); convertView.setTag(holder); } else { holder = (ViewHolder) convertView.getTag(); } holder.img.setImageResource(langs.getImgId()); holder.title.setText(langs.getNameLang()); holder.sub.setText(langs.getnTuts()); return convertView; }

Answer 3

只需按以下方式更改getView()方法：

@Override
public View getView(int position, View convertView, ViewGroup parent) {

   ViewHolder holder;
   LearnLangs langs = langsList.get(position);

   if (convertView == null) {
       layoutInflater = (LayoutInflater) ctx.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
       convertView = layoutInflater.inflate(R.layout.custom_list, parent, false);

       holder = new ViewHolder();
       //error occurs on this line below
       holder.img = (ImageView) convertView.findViewById(R.id.list_img);
       holder.title = (TextView) convertView.findViewById(R.id.list_title);
       holder.sub = (TextView) convertView.findViewById(R.id.list_subT);

       convertView.setTag(holder);
   }

   else
   {
       holder = (ViewHolder) convertView.getTag();
   }

   holder.img.setImageResource(langs.getImgId());
   holder.title.setText(langs.getNameLang());
   holder.sub.setText(langs.getnTuts());

   return convertView;
}

自定义列表视图抛出空指针异常

3 个答案: