Android：我的自定义viewHolder出了什么问题？

Question

我使用自定义ViewHolder类来实例化视图。 我在这段代码上得到了NullPointException。如果我在bindView函数中注释掉 iconView.setImageResource 和 descriptionView.setText（）行，应用程序运行正常。

如果其中两个显示NullPointExcepttion，为什么不显示其余部分？

ADAPTER CLASS：

 private final int VIEW_TYPE_TODAY = 0;
private final int VIEW_TYPE_FUTURE_DATE = 1;

public static class ViewHolder {
    public final ImageView iconView;
    public final TextView dateView;
    public final TextView descriptionView;
    public final TextView highTempView;
    public final TextView lowTempView;

    public ViewHolder(View view) {
        iconView = (ImageView) view.findViewById(R.id.list_item_icon);
        dateView = (TextView) view.findViewById(R.id.list_item_date_textview);
        descriptionView = (TextView) view.findViewById(R.id.list_item_forecast_textview);
        highTempView = (TextView) view.findViewById(R.id.list_item_high_textview);
        lowTempView = (TextView) view.findViewById(R.id.list_item_low_textview);
    }
}

@Override
public View newView(Context context, Cursor cursor, ViewGroup parent) {

    int viewType = getItemViewType(cursor.getPosition());
    int locationid = -1;
    if(viewType == VIEW_TYPE_TODAY)
        locationid = R.layout.list_item_forecast_today;

    else if(viewType == VIEW_TYPE_FUTURE_DATE)
        locationid = R.layout.list_item_forecast;

    View view = LayoutInflater.from(context).inflate(locationid, parent, false);
    ViewHolder viewHolder = new ViewHolder(view);
    view.setTag(viewHolder);
    return view;
}

/*
    This is where we fill-in the views with the contents of the cursor.
 */
@Override
public void bindView(View view, Context context, Cursor cursor) {
    // our view is pretty simple here --- just a text view
    // we'll keep the UI functional with a simple (and slow!) binding.
    ViewHolder viewHolder = (ViewHolder) view.getTag();

    viewHolder.iconView.setImageResource(R.drawable.ic_launcher);

    long dateInMillis = cursor.getLong(ForecastFragment.COL_WEATHER_DATE);
    viewHolder.dateView.setText(Utility.getFriendlyDayString(context, dateInMillis));

    String description = cursor.getString(ForecastFragment.COL_WEATHER_DESC);
    viewHolder.descriptionView.setText(description);

    boolean isMetric = Utility.isMetric(context);
    double high = cursor.getDouble(ForecastFragment.COL_WEATHER_MAX_TEMP);
    viewHolder.highTempView.setText(Utility.formatTemperature(mContext, high, isMetric));

    double low = cursor.getDouble(ForecastFragment.COL_WEATHER_MIN_TEMP);
    viewHolder.lowTempView.setText(Utility.formatTemperature(mContext, low, isMetric));
}

XML list_item_forecast

    <?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:gravity="center_vertical"
    android:minHeight="?android:attr/listPreferredItemHeight"
    android:orientation="horizontal"
    android:padding="16dp" >

    <ImageView
        android:id="@+id/list_item_icon"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"/>
    <LinearLayout
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:orientation="vertical"
        android:layout_weight="1"
        android:paddingLeft="16dp">
        <TextView
            android:id="@+id/list_item_date_textview"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"/>
        <TextView
            android:id="@+id/list_item_forecast_textview"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"/>

    </LinearLayout>
    <LinearLayout
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:orientation="vertical">
        <TextView
            android:id="@+id/list_item_high_textview"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"/>
        <TextView
            android:id="@+id/list_item_low_textview"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"/>
    </LinearLayout>


    </LinearLayout>

XML list_item_forecast_today

    <?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="horizontal" android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:gravity="center_vertical"
    android:minHeight="?android:attr/listPreferredItemHeight"
    android:padding="16dp">

    <LinearLayout
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_weight="1"
        android:orientation="vertical"
        android:gravity="center_horizontal">

        <TextView
            android:id="@+id/list_item_date_textview"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content" />
        <TextView
            android:id="@+id/list_item_high_textview"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content" />
        <TextView
            android:id="@+id/list_item_low_textview"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content" />
    </LinearLayout>

</LinearLayout>

错误是：

java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ImageView.setImageResource(int)' on a null object reference
        at com.example.abhishek.sunshine.ForecastAdapter.bindView(ForecastAdapter.java:78)
        at android.support.v4.widget.CursorAdapter.getView(CursorAdapter.java:256)
        at android.widget.AbsListView.obtainView(AbsListView.java:2344)
        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:1659)
        at android.widget.AbsListView.onLayout(AbsListView.java:2148)
        at android.view.View.layout(View.java:15614)
        at android.view.ViewGroup.layout(ViewGroup.java:4968)
        at android.widget.FrameLayout.layoutChildren(FrameLayout.java:573)
        at android.widget.FrameLayout.onLayout(FrameLayout.java:508)
        at android.view.View.layout(View.java:15614)
        at android.view.ViewGroup.layout(ViewGroup.java:4968)
        at android.widget.FrameLayout.layoutChildren(FrameLayout.java:573)
        at android.widget.FrameLayout.onLayout(FrameLayout.java:508)
        at android.view.View.layout(View.java:15614)
        at android.view.ViewGroup.layout(ViewGroup.java:4968)
        at android.widget.FrameLayout.layoutChildren(FrameLayout.java:573)
        at android.widget.FrameLayout.onLayout(FrameLayout.java:508)
        at android.view.View.layout(View.java:15614)
        at android.view.ViewGroup.layout(ViewGroup.java:4968)
        at android.support.v7.internal.widget.ActionBarOverlayLayout.onLayout(ActionBarOverlayLayout.java:502)
        at android.view.View.layout(View.java:15614)
        at android.view.ViewGroup.layout(ViewGroup.java:4968)
        at android.widget.FrameLayout.layoutChildren(FrameLayout.java:573)
        at android.widget.FrameLayout.onLayout(FrameLayout.java:508)
        at android.view.View.layout(View.java:15614)
        at android.view.ViewGroup.layout(ViewGroup.java:4968)
        at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1703)
        at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1557)
        at android.widget.LinearLayout.onLayout(LinearLayout.java:1466)
        at android.view.View.layout(View.java:15614)
        at android.view.ViewGroup.layout(ViewGroup.java:4968)
        at android.widget.FrameLayout.layoutChildren(FrameLayout.java:573)
        at android.widget.FrameLayout.onLayout(FrameLayout.java:508)
        at android.view.View.layout(View.java:15614)
        at android.view.ViewGroup.layout(ViewGroup.java:4968)
        at android.view.ViewRootImpl.performLayout(ViewRootImpl.java:2102)
        at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1859)
        at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1077)
        at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:5884)
        at android.view.Choreographer$CallbackRecord.run(Choreographer.java:767)
        at android.view.Choreographer.doCallbacks(Choreographer.java:580)
        at android.view.Choreographer.doFrame(Choreographer.java:550)
        at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:753)
        at android.os.Handler.handleCallback(Handler.java:739)
        at android.os.Handler.dispatchMessage(Handler.java:95)
        at android.os.Looper.loop(Looper.java:135)
        at android.app.ActivityThread.main(ActivityThread.java:5312)
        at java.lang.reflect.Method.invoke(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:372)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:901)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:696)

Answer 1

您的第二个布局（list_item_forecast）缺少ImageView个list_item_icon个。{ 在ViewHolder构造函数中，您可以使用此id设置iconView。它将在ViewHolder中以null进行第二次布局。然后在bindView方法中访问此null对象的方法 - setDrawable 您可以在第二个布局中添加图像视图或在bindView中检查视图类型，并仅使用相关字段 第二种布局中也缺少list_item_forecast_textview。

Answer 2

您没有使用视图实例化ViewHolder类（View视图），因此从不初始化iconView。

尝试这样做：

ViewHolder viewHolder = new ViewHolder(view);