Question

我是android新手，我尝试了很多解决方案，但没有任何作用！

请问我的代码有什么问题：

我在NullPointerException

中得到holder.CardContent.setText(card.getString("content"));

我尝试将layoutInflater从null更改为parent，false ids不是假的！

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;
    }
}

logcat的：

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

问题是holder至少在开始时是null。 card也可以null，但目前还不清楚。

虽然适配器回收逻辑是正确的，但您忘记了setTag View。

回收的逻辑是，您滚动的方式是将已读取/看过的数据保存（使用setTag）到视图（此处为convertView）。

然后，每次您传递相同的位置时，使用setTag检索它们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;
}

Answer 2

请参阅此代码：

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holder对象而不是cardContent。
mCards.get(position);返回null

Answer 3

您需要将convertView的标记设置为holder - 块中的if。

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);
}

您可以在else - 块中看到，您正在检索标记并将其投射到ViewHolder。如果尚未设置标记，getTag()将返回null，从而导致异常。

ArrayAdapter getView返回NullPointerException -

3 个答案: