ArrayAdapter getView返回NullPointerException -

Question

我是新来的android，我试过很多解决方案，但没有任何作品！

能否请你看看有什么错我的代码：

我holder.CardContent.setText(card.getString("content"));

得到NullPointerException我试图从null改变layoutInflater到parent，false ID是不假的！

CardAdapter.java

public class CardAdapter extends ArrayAdapter<ParseObject> { 
    protected Context mContext; 
    protected List<ParseObject> mCards; 

    public CardAdapter(Context context, List<ParseObject> cards) { 
     super(context, R.layout.card_item, cards); 

     mContext = context; 
     mCards = cards; 
    } 

    @Override 
    public View getView(int position, View convertView, ViewGroup parent) { 
     ViewHolder holder; 
     if(convertView == null) { 
      convertView = LayoutInflater.from(mContext).inflate(R.layout.card_item, null); 
      holder = new ViewHolder(); 

      holder.CardContent = (TextView) convertView.findViewById(R.id.cardText); 
     } else{ 
      holder = (ViewHolder) convertView.getTag(); 
     } 

     ParseObject card = mCards.get(position); 

     holder.CardContent.setText(card.getString("content")); // <-- NPE here. 

     return convertView; 
    } 

    private static class ViewHolder{ 
     TextView CardContent; 
    } 
} 

的logcat：

at com.kardapps.lifehacks.activities.CardAdapter.getView(CardAdapter.java:64) 
at android.widget.AbsListView.obtainView(AbsListView.java:2255) 
at android.widget.ListView.measureHeightOfChildren(ListView.java:1263) 
at android.widget.ListView.onMeasure(ListView.java:1175) 
....... 

Answer 1

您需要设置convertView的标签到holder在if - 块。

if (convertView == null) 
{ 
    convertView = LayoutInflater.from(mContext).inflate(R.layout.card_item, null); 
    holder = new ViewHolder(); 

    holder.CardContent = (TextView) convertView.findViewById(R.id.cardText); 

    // Add this line 
    convertView.setTag(holder); 
} 

你可以看到，在else - 块，你检索标签，并将其强制转换为ViewHolder。如果你没有设置标签，getTag()返回null，导致异常。

Answer 2

看到这个代码：

if(convertView == null){ 
     convertView = LayoutInflater.from(mContext).inflate(R.layout.card_item, null); 
     holder = new ViewHolder(); 
     holder.CardContent = (TextView) convertView.findViewById(R.id.cardText); 
    } 
    else{ 
     holder = (ViewHolder) convertView.getTag(); 
    } 
    ParseObject card = mCards.get(position); 
    holder.CardContent.setText(card.getString("content")); 

两种可能性：

• 如果convertView不为null，则只是初始化持有者对象而不是cardContent。
• mCards.get(position);返回null

Answer 3

问题是holder是null至少在一开始。 card也可能是null，但目前尚不清楚。

尽管适配器回收逻辑正确，但您已将setTag遗忘到View。

回收的逻辑是，您使用滚动方式将已读取/看到的数据保存到（使用setTag）视图中（这里是convertView）。

然后每次传递相同的位置时，您使用setTag对它们进行检索而不是重新创建它们（LayoutInflater.from...）

@Override 
public View getView(int position, View convertView, ViewGroup parent) { 
    ViewHolder holder; 
    if(convertView == null) { 
     //      This is not an error but using parent you avoid Lint warnings 
     convertView = LayoutInflater.from(mContext).inflate(R.layout.card_item, parent); 
     holder = new ViewHolder(); 

     holder.CardContent = (TextView) convertView.findViewById(R.id.cardText); 
     convertView.setTag(holder); // <-- As suggested for improvement 
    } else{ 
     holder = (ViewHolder) convertView.getTag(); 
    } 

    ParseObject card = mCards.get(position); 

    holder.CardContent.setText(card.getString("content")); // <-- NPE here. 

    // convertView.setTag(holder); // <-- This line is missing 
    return convertView; 
}