Android：初学者适配器问题。如何解决 setAdapter 导致 NullPointerException

Question

我收到以下错误：

java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.test/com.example.test.ListActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null object reference
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null object reference

我被困住了，希望你能帮忙。

ThingsDB 是 Thing(String what, String where)

的列表

列表片段：

import ... 

public class ListFragment extends Fragment{
    private static ThingsDB thingsDB;
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        thingsDB = thingsDB.get(this.getActivity());
        ListView listView= (ListView) getActivity().findViewById(R.id.thing_list_view);
        ArrayAdapter<Thing> adapter = new ArrayAdapter<Thing>(this.getActivity(), R.layout.list_item, R.id.text1, thingsDB.getThingsDB());
        listView.setAdapter(adapter);
    }
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstance) {
        View v = inflater.inflate(R.layout.fragment_list, container, false);
        return v;
    }
}

xml

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
              android:id="@+id/fragment_list"
              android:layout_width="match_parent"
              android:layout_height="match_parent"
              android:background="@color/darkGrey"
              android:orientation="vertical">
   <ListView
        android:id="@+id/thing_list_view"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:textSize="12dp">
    </ListView>
</LinearLayout>

Answer 1

您在视图尚未膨胀时调用 getview，请检查片段生命周期，onCreate() 在 onCreateView() 之前调用 https://developer.android.com/reference/android/app/Fragment.html#onCreateView(android.view.LayoutInflater, android.view.ViewGroup, android.os.Bundle)

onCreate: Added in API level 11 void onCreate (Bundle savedInstanceState) Called to do initial creation of a fragment. This is called after onAttach(Activity) and before onCreateView(LayoutInflater, ViewGroup, Bundle), but is not called if the fragment instance is retained across Activity re-creation (see setRetainInstance(boolean)).

onCreateView Called to have the fragment instantiate its user interface view. This is optional, and non-graphical fragments can return null (which is the default implementation). This will be called between onCreate(Bundle) and onActivityCreated(Bundle).

在这种情况下，您将拥有

import ... 

public class ListFragment extends Fragment{
    private static ThingsDB thingsDB;
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        thingsDB = thingsDB.get(this.getActivity());
    }
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstance) {
        View v = inflater.inflate(R.layout.fragment_list, container, false);
        ListView listView= (ListView) v.findViewById(R.id.thing_list_view);
        ArrayAdapter<Thing> adapter = new ArrayAdapter<Thing>(this.getActivity(), R.layout.list_item, R.id.text1, thingsDB.getThingsDB());
        listView.setAdapter(adapter);
        return v;
    }
}

Android：初学者适配器问题。如何解决 setAdapter 导致 NullPointerException

Android: Beginner Adapter Trouble. How to solve setAdapter causing NullPointerException

android

android-arrayadapter