带有自定义适配器的 ListView 给出空指针异常答案

【问题标题】：ListView with CustomAdapter giving NullPointer Exception带有自定义适配器的 ListView 给出空指针异常
【发布时间】：2013-03-12 12:55:19
【问题描述】：

我正在使用listview 来显示地点的详细信息。为此我创建了一个CustomAdapter，但我不知道为什么它给了我Null Pointer Exception，请帮我解决这个问题。

这是我的 xml 文件。

activity_main.xml

    <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity" >

    <ListView
        android:id="@+id/list"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:scrollbars="vertical" />

</LinearLayout>

list_item.xml

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent" >

    <LinearLayout
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:orientation="horizontal" >

        <ImageView
            android:id="@+id/img_hotel"
            android:layout_width="100dp"
            android:layout_height="match_parent" />

        <LinearLayout
            android:layout_width="fill_parent"
            android:layout_height="wrap_content"
            android:orientation="vertical" >

            <TextView
                android:id="@+id/txt_hotelName"
                android:layout_width="fill_parent"
                android:layout_height="20dp" />

            <TextView
                android:id="@+id/txt_hotelAddress"
                android:layout_width="fill_parent"
                android:layout_height="20dp" />

            <RatingBar
                android:id="@+id/ratingBar"
                style="?android:attr/ratingBarStyleIndicator"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:numStars="5"
                android:rating="2.0"
                android:stepSize="0.5" />
        </LinearLayout>
    </LinearLayout>

</RelativeLayout>

这是我更新的自定义适配器

public class CustomListViewAdapter extends BaseAdapter {

Context context;

public CustomListViewAdapter(Context context, int resource,
        List<Result> results) {

    // super(context, resource, results);
    this.context = context;
    Log.v("Result Data in Adapter", results.toString());
    // TODO Auto-generated constructor stub
}

@Override
public View getView(int position, View convertView, ViewGroup parent) {
    // TODO Auto-generated method stub
    ViewHolder holder = null;
    Result result = (Result) getItem(position);
    LayoutInflater mInflater = (LayoutInflater) context
            .getSystemService(Activity.LAYOUT_INFLATER_SERVICE);

    // LayoutInflater mInflater = (LayoutInflater)
    // context.getSystemService(Activity.LAYOUT_INFLATER_SERVICE);
    if (convertView == null) {
        convertView = mInflater.inflate(R.layout.list_item, null);

        holder = new ViewHolder();
        holder.imgView = (ImageView) convertView
                .findViewById(R.id.img_hotel);
        holder.txtName = (TextView) convertView
                .findViewById(R.id.txt_hotelName);
        holder.txtAddress = (TextView) convertView
                .findViewById(R.id.txt_hotelAddress);
        holder.rate = (RatingBar) convertView
                .findViewById(R.id.ratingBar);
        convertView.setTag(holder);
    } else {
        /*
         * holder = (ViewHolder) convertView.getTag();
         * 
         * String url = result.icon; Log.e("URL is ", url); if (url !=
         * null) { Bitmap bitmap = fetchBitmapFromCache(url); if (bitmap
         * == null) { new
         * BitmapDownloaderTask(holder.imgView).execute(url); } else {
         * holder.imgView.setImageBitmap(bitmap); } } else {
         * holder.imgView.setImageBitmap(null); }
         */
        holder.imgView.setImageDrawable(getResources().getDrawable(
                R.drawable.ic_launcher));
        holder.txtName.setText(result.name);
        holder.txtAddress.setText(result.formatted_address);
        holder.rate.setRating(result.rating);
    }
    return convertView; // super.getView(position, convertView, parent);
}

private class ViewHolder {
    ImageView imgView;
    TextView txtName;
    TextView txtAddress;
    RatingBar rate;
}

@Override
public int getCount() {
    // TODO Auto-generated method stub
    return results.size();
}

@Override
public Object getItem(int position) {
    // TODO Auto-generated method stub
    return results.get(position);
}

@Override
public long getItemId(int position) {
    // TODO Auto-generated method stub
    return results.indexOf(getItem(position));
}

}

这里我将适配器设置为 listView

list.setAdapter(new CustomListViewAdapter(MainActivity.this,
                R.layout.list_item, results));

这是我到现在为止的更新代码，但它仍然给我空指针异常。

这是我的堆栈跟踪。

03-13 11:13:20.355: E/AndroidRuntime(413): FATAL EXCEPTION: main
03-13 11:13:20.355: E/AndroidRuntime(413): java.lang.NullPointerException
03-13 11:13:20.355: E/AndroidRuntime(413):  at com.th.hotelreservationsystem.MainActivity$CustomListViewAdapter.getView(MainActivity.java:183)
03-13 11:13:20.355: E/AndroidRuntime(413):  at android.widget.AbsListView.obtainView(AbsListView.java:1294)
03-13 11:13:20.355: E/AndroidRuntime(413):  at android.widget.ListView.measureHeightOfChildren(ListView.java:1198)
03-13 11:13:20.355: E/AndroidRuntime(413):  at android.widget.ListView.onMeasure(ListView.java:1109)
03-13 11:13:20.355: E/AndroidRuntime(413):  at android.view.View.measure(View.java:8171)
03-13 11:13:20.355: E/AndroidRuntime(413):  at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:3132)
03-13 11:13:20.355: E/AndroidRuntime(413):  at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1012)
03-13 11:13:20.355: E/AndroidRuntime(413):  at android.widget.LinearLayout.measureHorizontal(LinearLayout.java:696)
03-13 11:13:20.355: E/AndroidRuntime(413):  at android.widget.LinearLayout.onMeasure(LinearLayout.java:306)
03-13 11:13:20.355: E/AndroidRuntime(413):  at android.view.View.measure(View.java:8171)
03-13 11:13:20.355: E/AndroidRuntime(413):  at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:3132)
03-13 11:13:20.355: E/AndroidRuntime(413):  at android.widget.FrameLayout.onMeasure(FrameLayout.java:245)
03-13 11:13:20.355: E/AndroidRuntime(413):  at android.view.View.measure(View.java:8171)
03-13 11:13:20.355: E/AndroidRuntime(413):  at android.widget.LinearLayout.measureVertical(LinearLayout.java:526)
03-13 11:13:20.355: E/AndroidRuntime(413):  at android.widget.LinearLayout.onMeasure(LinearLayout.java:304)
03-13 11:13:20.355: E/AndroidRuntime(413):  at android.view.View.measure(View.java:8171)
03-13 11:13:20.355: E/AndroidRuntime(413):  at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:3132)
03-13 11:13:20.355: E/AndroidRuntime(413):  at android.widget.FrameLayout.onMeasure(FrameLayout.java:245)
03-13 11:13:20.355: E/AndroidRuntime(413):  at android.view.View.measure(View.java:8171)
03-13 11:13:20.355: E/AndroidRuntime(413):  at android.view.ViewRoot.performTraversals(ViewRoot.java:801)
03-13 11:13:20.355: E/AndroidRuntime(413):  at android.view.ViewRoot.handleMessage(ViewRoot.java:1727)
03-13 11:13:20.355: E/AndroidRuntime(413):  at android.os.Handler.dispatchMessage(Handler.java:99)
03-13 11:13:20.355: E/AndroidRuntime(413):  at android.os.Looper.loop(Looper.java:123)
03-13 11:13:20.355: E/AndroidRuntime(413):  at android.app.ActivityThread.main(ActivityThread.java:4627)
03-13 11:13:20.355: E/AndroidRuntime(413):  at java.lang.reflect.Method.invokeNative(Native Method)
03-13 11:13:20.355: E/AndroidRuntime(413):  at java.lang.reflect.Method.invoke(Method.java:521)
03-13 11:13:20.355: E/AndroidRuntime(413):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868)
03-13 11:13:20.355: E/AndroidRuntime(413):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626)
03-13 11:13:20.355: E/AndroidRuntime(413):  at dalvik.system.NativeStart.main(Native Method)

请帮助我的朋友。

【问题讨论】：

首先，永远不要为 ListView 设置 layout_width 或 layout_height 为 wrap_content。

标签： android android-listview custom-adapter

【解决方案1】：

您在 getView 的开头将 viewHolder 设置为 null。如果 convertView 为空，则初始化它。但是如果convertView不为null，你就直接使用了。如果 convertView 不为空，请不要忘记 viewHolder 为空。

 在 else 块的开头添加如下代码：

holder = (ViewHolder) convertView.getTag();

【讨论】：

【解决方案2】：

感谢大家的帮助。

我得到了解决方案，这是我使用 cmets 更新的代码。

我的代码有3个错误。感谢帮助过我的人

public class CustomListViewAdapter extends BaseAdapter {

    Context context;

    public CustomListViewAdapter(Context context, int resource,
            List<Result> results) {

         super(); // 1) here i was trying to call parameterised Constructor which          
                      // was wrong.
        this.context = context;

    }

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        // TODO Auto-generated method stub
        ViewHolder holder = null;
        Result result = (Result) getItem(position);
        LayoutInflater mInflater = (LayoutInflater) context
                .getSystemService(Activity.LAYOUT_INFLATER_SERVICE);


        if (convertView == null) {
            convertView = mInflater.inflate(R.layout.list_item, null);

            holder = new ViewHolder(); // 2) i had not instantiated View holder 
                                               //  here i did.
            holder.imgView = (ImageView) convertView
                    .findViewById(R.id.img_hotel);
            holder.txtName = (TextView) convertView
                    .findViewById(R.id.txt_hotelName);
            holder.txtAddress = (TextView) convertView
                    .findViewById(R.id.txt_hotelAddress);
            holder.rate = (RatingBar) convertView
                    .findViewById(R.id.ratingBar);
            convertView.setTag(holder);
        } else {

            holder = (ViewHolder) convertView.getTag();// 3) here by mistake i
   //had commited holde with other unwanted code. here removed it.
            holder.imgView.setImageDrawable(getResources().getDrawable(
                    R.drawable.ic_launcher));
            holder.txtName.setText(result.name);
            holder.txtAddress.setText(result.formatted_address);
            holder.rate.setRating(result.rating);
        }
        return convertView; // super.getView(position, convertView, parent);
    }

    private class ViewHolder {
        ImageView imgView;
        TextView txtName;
        TextView txtAddress;
        RatingBar rate;
    }

    @Override
    public int getCount() {
        // TODO Auto-generated method stub
        return results.size();
    }

    @Override
    public Object getItem(int position) {
        // TODO Auto-generated method stub
        return results.get(position);
    }

    @Override
    public long getItemId(int position) {
        // TODO Auto-generated method stub
        return results.indexOf(getItem(position));
    }
}

【讨论】：

【解决方案3】：

您的context 为空，因为您尚未在构造函数中对其进行初始化，

尝试像这样初始化它

 public CustomListViewAdapter(Context context, int resource,
                List<Result> results) {
            super(context, resource, results);
           this.context=context;

        }

而且你的viewholder是空的

尝试初始化您的视图持有者

 holder = new viewHolder();

【讨论】：

构造函数的第一行应该是super()
@Pragnani：感谢 helo，但是当我添加这一行时，它又给了我一些错误 java.lang.IllegalStateException: ArrayAdapter requires the resource ID to be a TextView
@MahaveerMuttha 您的自定义适配器应该扩展类 BaseAdapter 而不是 ArrayAdapter
即公共类 CustomListViewAdapter 扩展 BaseAdapter
@Pragnani：是的，我也这样做了。现在它没有给出任何错误，但它没有在 Activity 上显示任何内容，它显示白屏..请帮助我

【解决方案4】：

您应该在 getView 方法中返回 convertView 而不是调用：

return super.getView(position, convertView, parent); // replace this line by return convertView;

编辑：所以完整的答案是这样的：

public class CustomListViewAdapter extends ArrayAdapter<Result>{

        Context context;

        public CustomListViewAdapter(Context context, int resource,
                List<Result> results) {

            super(context, resource, results);
            this.context = context; // 1
            Log.v("Result Data in Adapter", results.toString());
            // TODO Auto-generated constructor stub
        }

        private class ViewHolder {
            ImageView imgView;
            TextView txtName;
            TextView txtAddress;
            RatingBar rate; 
        }
        @Override
        public View getView(int position, View convertView, ViewGroup parent) {
            // TODO Auto-generated method stub
            ViewHolder holder = null;
            Result result = getItem(position);
            LayoutInflater mInflater = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
            if (convertView == null ){
                convertView = mInflater.inflate(R.layout.list_item, null);
                holder = new ViewHolder(); //2
            //  holder.imgView = (ImageView)convertView.findViewById(R.id.img_hotel);
                holder.txtName = (TextView) convertView.findViewById(R.id.txt_hotelName);
                holder.txtAddress = (TextView) convertView.findViewById(R.id.txt_hotelAddress);
                holder.rate = (RatingBar)convertView.findViewById(R.id.ratingBar);
                convertView.setTag(holder);
            }else {
                holder = (ViewHolder) convertView.getTag();

                //holder.imgView.setImageResource();
                holder.txtName.setText(result.name);
                holder.txtAddress.setText(result.formatted_address);
                holder.rate.setRating(result.rating);
            }
            return convertView; //3
        }
    }

【讨论】：

@michel Z：感谢您的帮助..我纠正了我的错误，但现在它没有给我任何错误，但它没有在我的屏幕上显示任何内容，只是出现了一个白屏。请帮帮我
Z.: 我已经更新了我的代码请看一下，我已经在代码中编辑了我的自定义适配器
您正在调用 Result result = (Result) getItem(position); 但您的 getItem() 方法始终返回 null。
@Michal Z。请检查最新代码。我做了所有的事情，但它仍然给我错误。我没有找到解决方案，请帮助我，我正在努力解决 2 天.....
我不知道你为什么评论这行holder = (ViewHolder) convertView.getTag();。应该取消注释。关于如何创建 ArrayAdapter 的教程有很多，非常简单。 LogCat 可以告诉你错误的确切位置（MainActivity 类的 183 行）。

【解决方案5】：

@Mahaveer 你能做到吗

@Override
public int getCount() {
    return <Size of list items>;
}

希望这能解决您的问题。

【讨论】：

【解决方案6】：

您将ViewHolder 设置为null，但从未对其进行初始化。

所以让你的getView() 像这样：

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        // TODO Auto-generated method stub
        ViewHolder holder = null;
        Result result = getItem(position);
        LayoutInflater mInflater = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
        if (convertView == null ){
            holder = new viewHolder(); //initialize it
            convertView = mInflater.inflate(R.layout.list_item, null);
            ....
            convertView.setTag(holder);
        }...

【讨论】：