Android中Fragment列表点击触发Null Pointer Exception的解决求助

开发基于Fragment的Android应用时，点击List Fragment中的条目，应用崩溃并抛出空指针异常：

Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference

尝试过同类问题的解决方案但无效，相关代码及布局如下：

Main Activity 代码 #

public class MainActivity extends AppCompatActivity implements ListFrag.ItemSelected {

    TextView tvDescription;
    ArrayList<String> descriptions;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

       tvDescription = (TextView) findViewById(R.id.tvDescription);

        descriptions = new ArrayList<>();
        descriptions.add("Description for item 1");
        descriptions.add("Description for item 2");
        descriptions.add("Description for item 3");


    }

    @Override
    public void onItemSelected(int index) {

        tvDescription.setText(descriptions.get(index));
    }
}

List Fragment 代码 #

public class ListFrag extends ListFragment {

    ItemSelected activity;

    public  interface ItemSelected
    {
        void onItemSelected(int index);
    }


    public ListFrag() {
        // Required empty public constructor
    }

    @Override
    public void onAttach(@NonNull Context context) {
        super.onAttach(context);

        activity = (ItemSelected) context;
    }

    @Override
    public void onActivityCreated(@Nullable Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);


        ArrayList<String> data = new ArrayList<>();
        data.add("1.This is item 1");
        data.add("2.This is item 2");
        data.add("3.This is item 3");

        setListAdapter(new ArrayAdapter<>( 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);
    } }

Detail Fragment 代码 #

public class DetailFrag extends Fragment {



    public DetailFrag() {
        // Required empty public constructor
    }



    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        return inflater.inflate(R.layout.fragment_detail, container, false);
    }
}

activity_main 布局 #

<?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:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="horizontal"
    tools:context=".MainActivity">


    <androidx.fragment.app.FragmentContainerView
        android:id="@+id/listFrag"
        android:name="com.example.fragmentspart1.ListFrag"
        android:layout_width="8dp"
        android:layout_height="match_parent"
        android:layout_weight="1"
        tools:layout="@layout/fragment_list" />

    <androidx.fragment.app.FragmentContainerView
        android:id="@+id/detailFrag"
        android:name="com.example.fragmentspart1.DetailFrag"
        android:layout_width="wrap_content"
        android:layout_height="match_parent"
        android:layout_weight="2"
        tools:layout="@layout/fragment_detail" />


</LinearLayout>

fragment_list 布局 #

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:background="#FFFFFF"
    tools:context=".ListFrag">


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

fragment_detail 布局 #

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:background="@color/design_default_color_secondary"
    tools:context=".DetailFrag">


    <TextView
        android:id="@+id/tvDescription"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginLeft="20dp"
        android:layout_marginTop="20dp"
        android:layout_marginRight="20dp"
        android:text="@string/textview"
        android:textColor="#FFFFFF"
        android:textSize="20sp"
        android:textStyle="bold" />
</LinearLayout>

问题根源 #

tvDescription 这个TextView存在于DetailFrag的布局文件fragment_detail.xml中，但你在MainActivity里调用findViewById(R.id.tvDescription)去查找——而MainActivity的布局activity_main.xml里并没有这个控件，所以tvDescription会被赋值为null，调用setText时自然触发空指针异常。

修复步骤 #

1. 修改DetailFrag，添加TextView成员变量并初始化，同时提供更新文本的方法：

public class DetailFrag extends Fragment {

    private TextView tvDescription;

    public DetailFrag() {
        // Required empty public constructor
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.fragment_detail, container, false);
        // 从Fragment自身的布局中查找TextView
        tvDescription = view.findViewById(R.id.tvDescription);
        return view;
    }

    // 提供外部调用的更新文本方法
    public void updateDescription(String text) {
        if (tvDescription != null) {
            tvDescription.setText(text);
        }
    }
}

2. 修改MainActivity，获取DetailFrag实例，在回调中调用其更新方法：

public class MainActivity extends AppCompatActivity implements ListFrag.ItemSelected {

    ArrayList<String> descriptions;
    private DetailFrag detailFrag;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        // 获取布局中已加载的DetailFrag实例
        detailFrag = (DetailFrag) getSupportFragmentManager().findFragmentById(R.id.detailFrag);

        descriptions = new ArrayList<>();
        descriptions.add("Description for item 1");
        descriptions.add("Description for item 2");
        descriptions.add("Description for item 3");
    }

    @Override
    public void onItemSelected(int index) {
        // 确保Fragment已添加到Activity中再调用方法
        if (detailFrag != null && detailFrag.isAdded()) {
            detailFrag.updateDescription(descriptions.get(index));
        }
    }
}

这样修改后，点击列表条目时，MainActivity会通知DetailFrag更新自身的TextView内容，避免了空指针问题。

内容的提问来源于stack exchange，提问作者Usama Ali