fragement用fragmentmanger,fragmenttransaction添加到activity碰到问题

1.初学fragment，写了一小段代码，用fragmentmanger,fragmenttransaction添加到activity碰到问题

2.代码如下

activity的layout

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

    <FrameLayout
        android:id="@+id/flNewItem"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_weight="1"/>

    <FrameLayout
        android:id="@+id/flItemList"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_weight="3"/>


</LinearLayout>

两个fragment的代码：

NewItemFragment：

public class NewItemFragment extends Fragment {
    // TODO: Rename parameter arguments, choose names that match
    // the fragment initialization parameters, e.g. ARG_ITEM_NUMBER
    private static final String ARG_PARAM1 = "param1";
    private static final String ARG_PARAM2 = "param2";

    // TODO: Rename and change types of parameters
    private String mParam1;
    private String mParam2;



    private OnAddNewItemListener mListener;

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

    /**
     * Use this factory method to create a new instance of
     * this fragment using the provided parameters.
     *
     * @param param1 Parameter 1.
     * @param param2 Parameter 2.
     * @return A new instance of fragment NewItemFragment.
     */
    // TODO: Rename and change types and number of parameters
    public static NewItemFragment newInstance(String param1, String param2) {
        NewItemFragment fragment = new NewItemFragment();
        Bundle args = new Bundle();
        args.putString(ARG_PARAM1, param1);
        args.putString(ARG_PARAM2, param2);
        fragment.setArguments(args);
        return fragment;
    }

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        if (getArguments() != null) {
            mParam1 = getArguments().getString(ARG_PARAM1);
            mParam2 = getArguments().getString(ARG_PARAM2);
        }
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        View view=inflater.inflate(R.layout.new_item_fragment, container, false);
        final EditText etNewItem;
        etNewItem= (EditText) view.findViewById(R.id.etNewItem);
        etNewItem.setOnKeyListener(new View.OnKeyListener() {
            @Override
            public boolean onKey(View view, int i, KeyEvent keyEvent) {
                if(keyEvent.getAction()==KeyEvent.ACTION_DOWN){
                    if((i==KeyEvent.KEYCODE_DPAD_CENTER) ||
                    (i==KeyEvent.KEYCODE_ENTER))  {
                        if (mListener != null) {
                            mListener.onAddItem(etNewItem.getText().toString());
                        }
                        etNewItem.setText("");
                        return true;
                    }
                }
                return false;
            }
        });
        return view;
    }


    @Override
    public void onAttach(Activity activity) {
        super.onAttach(activity);
        if (activity instanceof OnAddNewItemListener) {
            mListener = (OnAddNewItemListener) activity;
        } else {
            throw new RuntimeException(activity.toString()
                    + " must implement OnFragmentInteractionListener");
        }
    }

    @Override
    public void onDetach() {
        super.onDetach();
        mListener = null;
    }


    public interface OnAddNewItemListener {
        // TODO: Update argument type and name
        void onAddItem(String newItem);
    }
}

ToDoListFragment:

public class ToDoListFragment extends ListFragment {

    public ToDoListFragment() {

    }
}

activity的代码

public class MainActivity extends AppCompatActivity implements NewItemFragment.OnAddNewItemListener {

    ArrayList<String> newItems;
    ArrayAdapter<String> aa;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        newItems=new ArrayList<>();
        aa=new ArrayAdapter<String>(this,android.R.layout.simple_list_item_1,newItems);
        textView= (TextView) findViewById(R.id.tvSample);


        FragmentManager fm=getSupportFragmentManager();
        NewItemFragment newItemFragment= (NewItemFragment) fm.findFragmentById(R.id.flNewItem);
        if(newItemFragment==null){
            FragmentTransaction ft=fm.beginTransaction();
            ft.replace(R.id.flNewItem,new ToDoListFragment());
            ft.commit();
        }
        ToDoListFragment toDoListFragment= (ToDoListFragment) fm.findFragmentById(R.id.flItemList);
        if(toDoListFragment==null){
            textView.setText("todolistfragment");
            FragmentTransaction ft=fm.beginTransaction();
            ft.replace(R.id.flItemList,new ToDoListFragment());
            ft.commit();
        }
        toDoListFragment.setListAdapter(aa);
    }

    @Override
    public void onAddItem(String newItem) {
        newItems.add(0,newItem);
        aa.notifyDataSetChanged();
    }
}

当程序运行toDoListFragment.setListAdapter(aa);，就跳出，就报空指针错误
Caused by: java.lang.NullPointerException
at com.athand.master4.todolistfragment.MainActivity.onCreate(MainActivity.java:45)

如果layout不用frameLayout，直接用fragment,程序正常

<fragment
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:name="com.athand.master4.todolistfragment.NewItemFragment"
        android:id="@+id/NewItemFragment"/>
    <fragment
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:name="com.athand.master4.todolistfragment.ToDoListFragment"
        android:id="@+id/ToDoListFragment"/>
        
    

这个是怎么一回事呢

---

首先，可以记住一个观点：findFragmentById更多适应于xml布局中声明的fragment。

虽然android注释中说也适用于transaction中添加的fragment，但是commit方法不是一个同步的过程，而是异步的。所以你没法直接获取到fragment句柄，从而导致了空指针。

---

fm.findFragmentById(R.id.flItemList)通过寻找id返回一个fragment实例，但是你的这个R.ud.flItemList本身就不是一个fragment，自然toDoListFragment为空。所以你的toDoListFragment.setListAdapter(aa);会报java.lang.NullPointerException异常。