在Android的空指针异常由FindViewById [英] Nullpointer Exception by FindViewById in Android

问题描述

今天，我开始开发自己的小闹钟应用程序。但现在我有一个问题，我搜索了三个小时，不可能解决这个问题。

today I started to develop my own little alarmclock app. But now I've got a problem, and I'm searching for three hours and couldn't solve it.

当我试图实现与定制设计的每一行的ListView，我得到一个NullPointerException异常，当我尝试设置在布局一个TextView的文本。这样做的原因是例外，那findViewById呼叫返回null，而不是我想有TextView的。

When I try to implement a listview with a custom design for each row, i get a NullPointerException, when I try to set the text from a textview in the layout. The reason for this exception is, that the findViewById-Call returns null and not the TextView I want to have.

在我的搜索，我发现这两个职位，但遗憾的是他们没有帮助非常好...

On my search I found these two posts, but unfortunately they didn't help very well...

POST1 - ＃1

<一个href=\"http://stackoverflow.com/questions/10699422/android-listview-custom-items-nullpointerexception-findviewbyid-returns-null\">Post2 - ＃1

我使用的是相同的基本结构在上述两个职位，但我无法发现其中的错误...
下面是我的应用程序的一些code：

I'm using the same basic structure as in the two post above, but I'm unable to find the mistake... Here is some code from my app:

该listitem_row.xml：

The listitem_row.xml:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent"
android:layout_height="?android:attr/listPreferredItemHeight"
android:orientation="vertical"
android:padding="6dip" >

 <TextView
    android:name="@+id/toptext"
    android:layout_width="fill_parent"
    android:layout_height="0dip"
    android:layout_weight="1"
    android:layout_gravity="center_vertical"/>

  <TextView
    android:name="@+id/bottomtext"
    android:layout_width="fill_parent"
    android:layout_height="0dip"
    android:layout_weight="1"
    android:layout_gravity="center_vertical"/>

 </LinearLayout>

我的适配器：

public class AlarmClockArrayAdapter extends ArrayAdapter<RowItem> {    
    private class ViewHolder {
    TextView headText;
    TextView subText;
   }


public AlarmClockArrayAdapter(Context context, int layoutId, List<RowItem> objects) {
    super(context, layoutId, objects);
    this.mContext    = context;
    this.layoutResourceId = layoutId;
    this.mListItems  = objects;
}


 /**    getView()
 * 
 *  ListView asks the Adapter for a view for each list item element 
 *  Override the getView()-Method to customize the ListView
 */
@Override
public View getView(int position, View convertView, ViewGroup parent) {

    ViewHolder holder = null;

    LayoutInflater inflater = (LayoutInflater) mContext.getSystemService(Activity.LAYOUT_INFLATER_SERVICE);
    if(convertView == null) {
        convertView = inflater.inflate(layoutResourceId, parent, false);
        holder = new ViewHolder();
                    // findViewById(...) returns Null 
        holder.headText = (TextView) convertView.findViewById(R.id.toptext);
        holder.subText  = (TextView) convertView.findViewById(R.id.bottomtext);
        convertView.setTag(holder);     
    }
    else {
        holder = (ViewHolder) convertView.getTag();
    }

    RowItem rowItem = (RowItem) getItem(position);

            // HERES THE NULLPOINTEREXCEPTION
    holder.headText.setText(rowItem.getHeadText());
    holder.subText.setText(rowItem.getSubText());

    return convertView;
}

和至少在MainActivity，从那里我实例化的适配器

And at least the MainActivity, from where I instantiate the Adapter

public class MainActivity extends Activity {
 public void initList() {

    //Adapter for the listview
    AlarmClockArrayAdapter mAdapter = new AlarmClockArrayAdapter(this, R.layout.listitem_row, this.mListItems);

    final ListView mListView = (ListView) findViewById(R.id.listView1);
    mListView.setAdapter(mAdapter);
}

任何帮助将是巨大的...
谢谢！ ：D

Any help would be great... Thanks! :D

推荐答案

1）检查

holder.headText.setText(rowItem.getHeadText());

正好holder.headText返回null

exactly holder.headText return null

2）检查

if(convertView == null) {}
else {
    holder = (ViewHolder) convertView.getTag();
}

执行的第一家分行。

executed first branch

3）通常我这样做：

// in constructor
inflater = LayoutInflater.from(context); // context from Activity

public View getView(int position, View convertView, ViewGroup parent) {
  if (convertView == null) {
    convertView = inflater.inflate(R.layout.listitem_row, null);
  }

  headText = (TextView) convertView.findViewById(R.id.toptext);
  subText  = (TextView) convertView.findViewById(R.id.bottomtext);

这篇关于在Android的空指针异常由FindViewById的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！