java - 在数组适配器类中设置 TextView 的值时出现空指针异常 - Android

Question

我在尝试启动包含 ListView 的 Activity 时收到 NullPointerException。

适配器类的getView方法中，调用textView的setText函数时出现异常。

下面的代码是我的适配器类:

public class QuestionsListAdapter extends ArrayAdapter<Question> {

    Context context;
    List<Question> questions;

    public QuestionsListAdapter(Context context, List<Question> questions){

        super(context, R.layout.list_item_question, questions);
        this.context = context;
        this.questions = questions;
    }

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {

        LayoutInflater vi = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
        View view = vi.inflate(R.layout.list_item_question, null);

        Question question = questions.get(position);

        TextView tv = (TextView) view.findViewById(R.id.question_list_item_string);

        Log.i(TableCreator.LOG_TAG, question.toString()); //this works fine and the string is not null .

        tv.setText(question.toString()+""); //NULL POINTER EXCEPTION .

        return view;
    }
}

如您所见，我已将字符串记录在 logcat 中并且工作正常，但下一行出错了。这是 logcat 输出:

05-27 13:24:02.979    5325-5325/org.kabiri.operationcheklist I/Operation Checklist﹕ |-Question-> id:1 summary:mySummary comment:myComment solution:mySolution ownerList:dummyOwner
05-27 13:24:02.979    5325-5325/org.kabiri.operationcheklist D/AndroidRuntime﹕ Shutting down VM
05-27 13:24:02.979    5325-5325/org.kabiri.operationcheklist W/dalvikvm﹕ threadid=1: thread exiting with uncaught exception (group=0xb0f5f648)
05-27 13:24:02.979    5325-5325/org.kabiri.operationcheklist E/AndroidRuntime﹕ FATAL EXCEPTION: main
    java.lang.NullPointerException
            at org.kabiri.operationchecklist.adapter.QuestionsListAdapter.getView(QuestionsListAdapter.java:43)
            at android.widget.AbsListView.obtainView(AbsListView.java:2177)
            at android.widget.ListView.makeAndAddView(ListView.java:1840)
            at android.widget.ListView.fillDown(ListView.java:675)
            at android.widget.ListView.fillFromTop(ListView.java:736)
            at android.widget.ListView.layoutChildren(ListView.java:1655)
            at android.widget.AbsListView.onLayout(AbsListView.java:2012)
            at android.view.View.layout(View.java:14289)
            at android.view.ViewGroup.layout(ViewGroup.java:4562)
            at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1671)
            at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1525)
            at android.widget.LinearLayout.onLayout(LinearLayout.java:1434)
            at android.view.View.layout(View.java:14289)
            at android.view.ViewGroup.layout(ViewGroup.java:4562)
            at android.widget.FrameLayout.onLayout(FrameLayout.java:448)
            at android.view.View.layout(View.java:14289)
            at android.view.ViewGroup.layout(ViewGroup.java:4562)
            at android.support.v7.internal.widget.ActionBarOverlayLayout.onLayout(ActionBarOverlayLayout.java:502)
            at android.view.View.layout(View.java:14289)
            at android.view.ViewGroup.layout(ViewGroup.java:4562)
            at android.widget.FrameLayout.onLayout(FrameLayout.java:448)
            at android.view.View.layout(View.java:14289)
            at android.view.ViewGroup.layout(ViewGroup.java:4562)
            at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1671)
            at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1525)
            at android.widget.LinearLayout.onLayout(LinearLayout.java:1434)
            at android.view.View.layout(View.java:14289)
            at android.view.ViewGroup.layout(ViewGroup.java:4562)
            at android.widget.FrameLayout.onLayout(FrameLayout.java:448)
            at android.view.View.layout(View.java:14289)
            at android.view.ViewGroup.layout(ViewGroup.java:4562)
            at android.view.ViewRootImpl.performLayout(ViewRootImpl.java:1976)
            at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1730)
            at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1004)
            at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:5481)
            at android.view.Choreographer$CallbackRecord.run(Choreographer.java:749)
            at android.view.Choreographer.doCallbacks(Choreographer.java:562)
            at android.view.Choreographer.doFrame(Choreographer.java:532)
            at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:735)
            at android.os.Handler.handleCallback(Handler.java:730)
            at android.os.Handler.dispatchMessage(Handler.java:92)
            at android.os.Looper.loop(Looper.java:137)
            at android.app.ActivityThread.main(ActivityThread.java:5103)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:525)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
            at dalvik.system.NativeStart.main(Native Method)

logcat 显示错误发生在我的适配器类的这一行:

tv.setText(question.toString()+""); 

非常感谢您的帮助。

Answer 1

你已经知道问题出在哪里了!

tv.setText(question.toString()+"");

导致 NPE，这意味着 TextView tv 为 null。这意味着这条线

 TextView tv = (TextView) view.findViewById(R.id.question_list_item_string);

无法找到 TextView。检查 question_list_item_string id 并确保它与 list_item_question.xml 文件中的 id 匹配