片段中的TextView在Android Studio中返回Null

Question

我是安卓工作室的新手。。我在片段内的TextView上遇到空指针异常错误，请帮助。。这是我的密码。这不过是两个片段之间的一个简单接口。当我点击ListView Fragment中的一个项目时，我得到了Null指针异常错误。确保DetailsFragment中的TextView没有分配或指向它的来源。请帮助解决此问题。

public class MainActivity extends AppCompatActivity implements list_Fragment.ItemSelected {
ArrayList<String> Descriptions = new ArrayList<String>();;
TextView tvDescription;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Descriptions.add("Description for Item 1");
Descriptions.add("Description for Item 2");
Descriptions.add("Description for Item 3");
Descriptions.add("Description for Item 4");
tvDescription = findViewById(R.id.tvDescription);
}
@Override
public void onItemSelected(int index) {
tvDescription.setText(Descriptions.get(index));
}}

列表碎片

public class list_Fragment extends ListFragment {
ArrayList<String> Data = new ArrayList<String>();

ItemSelected activity;
public interface ItemSelected{
void onItemSelected(int index);
}
public list_Fragment() {
// Required empty public constructor
}
@Override
public void onAttach(@NonNull Context context) {
super.onAttach(context);
activity=(ItemSelected) context;
}
@Override
public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) {
super.onViewCreated(view, savedInstanceState);
Data.add("Item 1");
Data.add("Item 2");
Data.add("Item 3");
Data.add("Item 4");
setListAdapter(new ArrayAdapter<String>(getActivity(), android.R.layout.simple_list_item_1, Data));
}
@Override
public void onListItemClick(@NonNull ListView l, @NonNull View v, int position, long id) {
activity.onItemSelected(position);
}
}

详细信息片段代码，我认为我得到错误的地方，是

public class DetailsFragment extends Fragment {
public DetailsFragment() {
// Required empty public constructor
}
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
// Inflate the layout for this fragment
return (LinearLayout) inflater.inflate(R.layout.fragment_details, container, false);
}
}

主XML文件

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/ll_Horizontal"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="horizontal"
tools:context=".MainActivity">

<androidx.fragment.app.FragmentContainerView
android:id="@+id/listfragmentView"
android:name="com.example.fragmentcheck.list_Fragment"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:layout_weight="3"
android:background="@android:color/holo_blue_dark"
tools:layout="@layout/fragment_list_" />
<androidx.fragment.app.FragmentContainerView
android:id="@+id/detailsfragmentView"
android:name="com.example.fragmentcheck.DetailsFragment"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:layout_weight="1"
android:background="@color/purple_200"
tools:layout="@layout/fragment_details" />
</LinearLayout>

ListFragment XML

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/linearLayout2"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="@color/design_default_color_primary_variant"
android:orientation="vertical"
tools:context=".list_Fragment">

<ListView
android:id="@+id/list"
android:layout_width="match_parent"
android:layout_height="match_parent" />
</LinearLayout>

细节碎片XML

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/linearLayout"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="@color/design_default_color_on_secondary"
android:orientation="vertical"
tools:context=".DetailsFragment">

<TextView
android:id="@+id/tvDescription"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@string/textview"
android:textColor="#FFFFFF"
android:textSize="20sp" />
</LinearLayout>

错误

E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.fragmentcheck, PID: 28112
java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object r*emphasized text*eference
at com.example.fragmentcheck.MainActivity.onItemSelected(MainActivity.java:34)
at com.example.fragmentcheck.list_Fragment.onListItemClick(list_Fragment.java:54)

Answer 1

tvDescription在DetailsFragment布局中，您需要在该类的代码中进行任何布局配置

即

class DetailsFragment extends Fragment {
...
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View view = inflater.inflate(R.layout.fragment_details, container, false)
tvDescription = view.findViewById(R.id.tvDescription);
tvDescription.setText("some text");

return view
}
}

Answer 2

您在DetailsFragmentxml中有tvDescription，而在MainAcitvity中找到它的id。MainActivity只有FragmentContainerView，里面没有任何文本视图。

您需要扩展Fragment的视图，并在它返回的View上调用findViewById()。

更新的细节碎片

public class DetailsFragment extends Fragment {
TextView tvDescription;
public DetailsFragment() {
// Required empty public constructor
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View view = inflater.inflate(R.layout.fragment_details, container, false)
tvDescription = view.findViewById(R.id.tvDescription);
tvDescription.setText("some text");

return view
}
}

Answer 3

同时更改：androidx.fragment.app.FragmentContainerView

收件人：碎片

在main_activity xml 中