我希望在ListView的选项卡布局中显示数据库中的用户列表,但是尝试在空对象引用上调用虚方法'java.lang.String java.lang.Object.toString()'
时出错。经过多次搜索后,我没有成功删除它。任何人都可以帮助我吗?
错误
03-28 23:26:06.652 25767-25767 / com.example.avinash.tablayoutcheck E / AndroidRuntime:致命异常:主要 处理:com.example.avinash.tablayoutcheck,PID:25767 java.lang.NullPointerException:尝试调用虚方法 空对象上的'java.lang.String java.lang.Object.toString()' 参考 在 android.widget.ArrayAdapter.createViewFromResource(ArrayAdapter.java:401) 在android.widget.ArrayAdapter.getView(ArrayAdapter.java:369) 在android.widget.AbsListView.obtainView(AbsListView.java:2474) 在android.widget.ListView.measureHeightOfChildren(ListView.java:1301) 在android.widget.ListView.onMeasure(ListView.java:1203) 在android.view.View.measure(View.java:19161) 在android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6118) 在 android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1723) 在android.widget.LinearLayout.measureVertical(LinearLayout.java:788) 在android.widget.LinearLayout.onMeasure(LinearLayout.java:648) 在android.view.View.measure(View.java:19161) 在android.support.v4.view.ViewPager.onMeasure(ViewPager.java:1630) 在android.view.View.measure(View.java:19161) 在android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6118) 在 android.support.design.widget.CoordinatorLayout.onMeasureChild(CoordinatorLayout.java:714) 在 android.support.design.widget.HeaderScrollingViewBehavior.onMeasureChild(HeaderScrollingViewBehavior.java:91) 在 android.support.design.widget.AppBarLayout $ ScrollingViewBehavior.onMeasureChild(AppBarLayout.java:1361) 在 android.support.design.widget.CoordinatorLayout.onMeasure(CoordinatorLayout.java:784) 在android.view.View.measure(View.java:19161) 在android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6118) 在android.widget.FrameLayout.onMeasure(FrameLayout.java:223) 在 android.support.v7.widget.ContentFrameLayout.onMeasure(ContentFrameLayout.java:139) 在android.view.View.measure(View.java:19161)
UserlistFragment.java
public class UserlistFragment extends Fragment {
private static final String TAG = "UserList";
ListView usersList;
private DatabaseReference mDatabaseUser;
ArrayList<String> list=new ArrayList<>();
private DatabaseReference root = FirebaseDatabase.getInstance().getReference().getRoot();
@Nullable
@Override
public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
View view= inflater.inflate(R.layout.userlist_fragment,container,false);
mDatabaseUser = FirebaseDatabase.getInstance().getReference("users");
return view;
}
@Override
public void onActivityCreated(@Nullable Bundle savedInstanceState) {
super.onActivityCreated(savedInstanceState);
usersList = (ListView)getActivity().findViewById(R.id.usersList);
final ArrayAdapter<String> adapter=new ArrayAdapter<String>(getActivity(),android.R.layout.simple_dropdown_item_1line,list);
usersList.setAdapter(adapter);
mDatabaseUser.addChildEventListener(new ChildEventListener() {
@Override
public void onChildAdded(DataSnapshot dataSnapshot, String s) {
/* list.add((String) dataSnapshot.getValue(String.class));*/
User user = dataSnapshot.getValue(User.class);
// Check for null
if (user == null) {
Log.e(TAG, "User data is null!");
return;
}
list.add(user.username);
adapter.notifyDataSetChanged();
}
@Override
public void onChildChanged(DataSnapshot dataSnapshot, String s) {
}
@Override
public void onChildRemoved(DataSnapshot dataSnapshot) {
}
@Override
public void onChildMoved(DataSnapshot dataSnapshot, String s) {
}
@Override
public void onCancelled(DatabaseError databaseError) {
}
});
}
}
userlist_fragment.xml
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical" android:layout_width="match_parent"
xmlns:tools="http://schemas.android.com/tools"
tools:context=".MainActivity"
android:layout_height="match_parent">
<ListView
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:id="@+id/usersList"/>
</LinearLayout>