自定义列表视图抛出空指针异常

问题描述：

我在android中创建自定义ListView。我面临的问题是在我的适配器代码中。它抛出一个NullPointerException，即使其他一切似乎都是按顺序的。下面的示例代码：自定义列表视图抛出空指针异常

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)

出了什么问题？

答

你会得到异常，因为你没有使用视图，而你正在充气。更改getview（）方法的代码按如下：

@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; 
}

感谢它指向了 –

@JosephJoey：你可以接受的答案！ :) – Jai

答

只是改变这一行：

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

这样：

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

编辑

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; 
}

执行此更改后屏幕只是空白，但未显示错误。什么可能导致这个问题？ –

@JosephJoey看看我编辑的答案 –

答

只是通过下面更改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

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