尝试使用列表视图打开片段时应用程序崩溃

Question

当我尝试打开由列表视图组成的片段时，我的应用程序崩溃了。

场景：

1. 第一次打开fragmentA，listview不显示。

2. 单击fragmentA 上的按钮（转到fragmentB）。

3. 在 fragmentB 上单击返回按钮，在 fragmentA 上显示列表视图。

4. 退出应用，再次尝试打开fragmentA，应用崩溃。

我的片段类：

public class FamilyTreeFragment extends Fragment {

// Declare Variables
ListView listview;
List<ParseObject> ob;
ProgressDialog mProgressDialog;
FamilyTreeListViewAdapter adapter;
private List<FamilyTree> familytreelist = null;
View rootView;

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
    View rootView = inflater.inflate(R.layout.fragment_familytree, container, false);

    new RemoteDataTask().execute();
    // Locate the listview in listview_main.xml
    listview = (ListView) rootView.findViewById(R.id.list);
    // Pass the results into ListViewAdapter.java
    // adapter = new FamilyTreeListViewAdapter(getActivity(), familytreelist);
    // Binds the Adapter to the ListView
    // listview.setAdapter(adapter);
    // Close the progressdialog
    mProgressDialog.dismiss();

    // btn_add
    Button btn_add = (Button) rootView.findViewById(R.id.btn_add);
    btn_add.setOnClickListener(new OnClickListener() {
        @Override
        public void onClick(View view) {
            // Toast.makeText(SuggestionsFragment.this.getActivity(),
            // "You have selected Parks.", Toast.LENGTH_SHORT).show();
            getFragmentManager().beginTransaction().replace(R.id.container, new FamilyTreeFragment2()).addToBackStack(null).commit();
        }
    });

    return rootView;
}

// RemoteDataTask AsyncTask
private class RemoteDataTask extends AsyncTask<Void, Void, Void> {
    @Override
    protected void onPreExecute() {
        super.onPreExecute();
        // Create a progressdialog
        mProgressDialog = new ProgressDialog(getActivity());
        // Set progressdialog title
        mProgressDialog.setTitle("");
        // Set progressdialog message
        mProgressDialog.setMessage("Loading...");
        mProgressDialog.setIndeterminate(false);
        // Show progressdialog
        mProgressDialog.show();

    }

    protected Void doInBackground(Void... arg0) {
        // Create the array
        familytreelist = new ArrayList<FamilyTree>();
        try {
            // Locate the class table named "Country" in Parse.com
            ParseQuery<ParseObject> query = new ParseQuery<ParseObject>("FamilyTree");
            // Locate the column named "ranknum" in Parse.com and order list
            // by ascending
            query.orderByAscending("createdAt");
            ob = query.find();
            for (ParseObject familytree : ob) {
                // Locate images in flag column
                ParseFile image = (ParseFile) familytree.get("image");

                FamilyTree tree = new FamilyTree();
                tree.setName((String) familytree.get("name"));
                tree.setRelation((String) familytree.get("relation"));
                tree.setImage(image.getUrl());
                familytreelist.add(tree);
            }
        } catch (ParseException e) {
            Log.e("Error", e.getMessage());
            e.printStackTrace();
        }
        return null;
    }

    protected void onPostExecute(Void result) {

        // Locate the listview in listview_main.xml
        listview = (ListView) rootView.findViewById(R.id.list);
        // Pass the results into ListViewAdapter.java
        adapter = new FamilyTreeListViewAdapter(getActivity(), familytreelist);
        // Binds the Adapter to the ListView
        listview.setAdapter(adapter);
        // Close the progressdialog
        // mProgressDialog.dismiss();
    }
}

}

XML：

<?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:id="@+id/familytreecontainer"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical"
tools:context="com.example.parcel.FamilyTreeFragment$PlaceholderFragment" >

<ListView
    android:id="@+id/list"
    android:layout_width="match_parent"
    android:layout_height="407dp"
    android:layout_weight="2.86" >
</ListView>

<Button
    android:id="@+id/btn_add"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:text="Add" />

Logcat：

11-06 17:01:06.500: E/AndroidRuntime(22072): FATAL EXCEPTION: main
11-06 17:01:06.500: E/AndroidRuntime(22072): Process: com.example.parcel, PID: 22072
11-06 17:01:06.500: E/AndroidRuntime(22072): java.lang.NullPointerException
11-06 17:01:06.500: E/AndroidRuntime(22072):    at  com.example.parcel.FamilyTreeFragment$RemoteDataTask.onPostExecute(FamilyTreeFragment.java:111)
11-06 17:01:06.500: E/AndroidRuntime(22072):    at com.example.parcel.FamilyTreeFragment$RemoteDataTask.onPostExecute(FamilyTreeFragment.java:1)
11-06 17:01:06.500: E/AndroidRuntime(22072):    at android.os.AsyncTask.finish(AsyncTask.java:632)
11-06 17:01:06.500: E/AndroidRuntime(22072):    at android.os.AsyncTask.access$600(AsyncTask.java:177)
11-06 17:01:06.500: E/AndroidRuntime(22072):    at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:645)
11-06 17:01:06.500: E/AndroidRuntime(22072):    at android.os.Handler.dispatchMessage(Handler.java:102)
11-06 17:01:06.500: E/AndroidRuntime(22072):    at android.os.Looper.loop(Looper.java:157)
11-06 17:01:06.500: E/AndroidRuntime(22072):    at android.app.ActivityThread.main(ActivityThread.java:5867)
11-06 17:01:06.500: E/AndroidRuntime(22072):    at java.lang.reflect.Method.invokeNative(Native Method)
11-06 17:01:06.500: E/AndroidRuntime(22072):    at java.lang.reflect.Method.invoke(Method.java:515)
11-06 17:01:06.500: E/AndroidRuntime(22072):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:858)
11-06 17:01:06.500: E/AndroidRuntime(22072):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:674)
11-06 17:01:06.500: E/AndroidRuntime(22072):    at dalvik.system.NativeStart.main(Native Method)

我该如何解决这个问题？任何帮助表示赞赏。谢谢。

Answer 1

试试这个方法，希望能帮助你解决问题。

View rootView = inflater.inflate(R.layout.fragment_familytree, container, false);

替换为

rootView = inflater.inflate(R.layout.fragment_familytree, container, false);

错误原因：

您在类级别有 decalre rootView 但未在 onCreateView() 上初始化，您将 View rootView 作为 onCreateView() 中的局部变量，因此 onPostExecute() rootView 将指向仍未初始化的类级别变量。

格式良好的代码：

public class FamilyTreeFragment extends Fragment {

    // Declare Variables
    private ListView listview;
    private Button btn_add;
    private FamilyTreeListViewAdapter adapter;
    private List<FamilyTree> familytreelist;

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        View rootView = inflater.inflate(R.layout.fragment_familytree, container, false);

        listview = (ListView) rootView.findViewById(R.id.list);
        btn_add = (Button) rootView.findViewById(R.id.btn_add);

        mProgressDialog.dismiss();
        btn_add.setOnClickListener(new OnClickListener() {
            @Override
            public void onClick(View view) {
                // Toast.makeText(SuggestionsFragment.this.getActivity(),
                // "You have selected Parks.", Toast.LENGTH_SHORT).show();
                getFragmentManager().beginTransaction().replace(R.id.container, new FamilyTreeFragment2()).addToBackStack(null).commit();
            }
        });

        new RemoteDataTask().execute();
        return rootView;
    }

    // RemoteDataTask AsyncTask
    private class RemoteDataTask extends AsyncTask<Void, Void, List<FamilyTree>> {
        private ProgressDialog mProgressDialog;
        @Override
        protected void onPreExecute() {
            super.onPreExecute();
            // Create a progressdialog
            mProgressDialog = new ProgressDialog(getActivity());
            // Set progressdialog title
            mProgressDialog.setTitle("");
            // Set progressdialog message
            mProgressDialog.setMessage("Loading...");
            mProgressDialog.setIndeterminate(false);
            // Show progressdialog
            mProgressDialog.show();

        }

        protected List<FamilyTree> doInBackground(Void... arg0) {
            // Create the array
            List<FamilyTree> list = new ArrayList<FamilyTree>();
            try {
                // Locate the class table named "Country" in Parse.com
                ParseQuery<ParseObject> query = new ParseQuery<ParseObject>("FamilyTree");
                // Locate the column named "ranknum" in Parse.com and order list
                // by ascending
                query.orderByAscending("createdAt");
                List<ParseObject> ob = query.find();
                for (ParseObject familytree : ob) {
                    // Locate images in flag column
                    ParseFile image = (ParseFile) familytree.get("image");

                    FamilyTree tree = new FamilyTree();
                    tree.setName((String) familytree.get("name"));
                    tree.setRelation((String) familytree.get("relation"));
                    tree.setImage(image.getUrl());
                    list.add(tree);
                }
            } catch (ParseException e) {
                e.printStackTrace();
                Log.e("Error", e.getMessage());
            }
            return list;
        }

        protected void onPostExecute(List<FamilyTree> result) {
            super.onPostExecute(result);

            if(mProgressDialog!=null && mProgressDialog.isShowing()){
                mProgressDialog.dismiss();
            }
            if(familytreelist==null){
                familytreelist = new ArrayList<FamilyTree>();
            }

            familytreelist.addAll(result);

            // Pass the results into ListViewAdapter.java
            adapter = new FamilyTreeListViewAdapter(getActivity(), familytreelist);
            // Binds the Adapter to the ListView
            listview.setAdapter(adapter);
        }
    }

}