Android 应用程序 - 尝试在空对象引用上调用接口(interface)方法 'int java.util.List.size()'

Question

我正在创建一个在 GridView 中显示电影海报的应用程序。我正在使用 themoviedb.org 的 API。问题是我在运行我的应用程序时遇到以下错误，我正在努力修复它:

07-18 20:27:39.244    2751-2751/? E/AndroidRuntime﹕ FATAL EXCEPTION: main
    Process: com.projmobileapp.pmdbadd.pmdb, PID: 2751
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.projmobileapp.pmdbadd.pmdb/com.projmobileapp.pmdbadd.pmdb.MainActivity}: android.view.InflateException: Binary XML file line #1: Error inflating class fragment
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2693)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2758)
            at android.app.ActivityThread.access$900(ActivityThread.java:177)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1448)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:145)
            at android.app.ActivityThread.main(ActivityThread.java:5942)
            at java.lang.reflect.Method.invoke(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:372)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1400)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1195)
     Caused by: android.view.InflateException: Binary XML file line #1: Error inflating class fragment
            at android.view.LayoutInflater.createViewFromTag(LayoutInflater.java:770)
            at android.view.LayoutInflater.inflate(LayoutInflater.java:483)
            at android.view.LayoutInflater.inflate(LayoutInflater.java:415)
            at android.view.LayoutInflater.inflate(LayoutInflater.java:366)
            at android.support.v7.app.AppCompatDelegateImplV7.setContentView(AppCompatDelegateImplV7.java:249)
            at android.support.v7.app.AppCompatActivity.setContentView(AppCompatActivity.java:106)
            at com.projmobileapp.pmdbadd.pmdb.MainActivity.onCreate(MainActivity.java:15)
            at android.app.Activity.performCreate(Activity.java:6289)
            at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1119)
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2646)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2758)
            at android.app.ActivityThread.access$900(ActivityThread.java:177)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1448)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:145)
            at android.app.ActivityThread.main(ActivityThread.java:5942)
            at java.lang.reflect.Method.invoke(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:372)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1400)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1195)
     Caused by: java.lang.NullPointerException: Attempt to invoke interface method 'int java.util.List.size()' on a null object reference
            at android.widget.ArrayAdapter.getCount(ArrayAdapter.java:330)
            at android.widget.GridView.setAdapter(GridView.java:201)
            at com.projmobileapp.pmdbadd.pmdb.MainActivityFragment.onCreateView(MainActivityFragment.java:53)
            at android.support.v4.app.Fragment.performCreateView(Fragment.java:1789)
            at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:924)
            at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1116)
            at android.support.v4.app.FragmentManagerImpl.addFragment(FragmentManager.java:1218)
            at android.support.v4.app.FragmentManagerImpl.onCreateView(FragmentManager.java:2170)
            at android.support.v4.app.FragmentActivity.onCreateView(FragmentActivity.java:300)
            at android.support.v7.app.AppCompatDelegateImplV7.callActivityOnCreateView(AppCompatDelegateImplV7.java:838)
            at android.support.v7.app.AppCompatDelegateImplV11.callActivityOnCreateView(AppCompatDelegateImplV11.java:34)
            at android.support.v7.app.AppCompatDelegateImplV7.onCreateView(AppCompatDelegateImplV7.java:826)
            at android.support.v4.view.LayoutInflaterCompatHC$FactoryWrapperHC.onCreateView(LayoutInflaterCompatHC.java:44)
            at android.view.LayoutInflater.createViewFromTag(LayoutInflater.java:732)
            at android.view.LayoutInflater.inflate(LayoutInflater.java:483)
            at android.view.LayoutInflater.inflate(LayoutInflater.java:415)
            at android.view.LayoutInflater.inflate(LayoutInflater.java:366)
            at android.support.v7.app.AppCompatDelegateImplV7.setContentView(AppCompatDelegateImplV7.java:249)
            at android.support.v7.app.AppCompatActivity.setContentView(AppCompatActivity.java:106)
            at com.projmobileapp.pmdbadd.pmdb.MainActivity.onCreate(MainActivity.java:15)
            at android.app.Activity.performCreate(Activity.java:6289)
            at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1119)
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2646)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2758)
            at android.app.ActivityThread.access$900(ActivityThread.java:177)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1448)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:145)
            at android.app.ActivityThread.main(ActivityThread.java:5942)
            at java.lang.reflect.Method.invoke(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:372)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1400)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1195)
07-18 20:27:39.244    2702-2702/? W/ResourcesManager﹕ Asset path '/system/framework/twframework.jar' does not exist or contains no resources.

这是我的代码

package com.projmobileapp.pmdbadd.pmdb;

import android.os.AsyncTask;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuInflater;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.widget.GridView;

import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.URL;
import java.util.ArrayList;
import java.util.List;

public class MainActivityFragment extends Fragment {

    private ArrayList<Movie> flavorList;


    public MainActivityFragment() {
    }

    MovieAdapter mMovieAdapter;


    public void onSaveInstanceState(Bundle outState) {
        outState.putParcelableArrayList("flavors", flavorList);
        super.onSaveInstanceState(outState);
    }


    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {

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

        mMovieAdapter = new MovieAdapter(getActivity(), flavorList);
        GridView listView = (GridView) rootView.findViewById(R.id.gridview_movie);
        listView.setAdapter(mMovieAdapter);
        updateMovie();

    return rootView;

    }


    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setHasOptionsMenu(true);
    }

    @Override
    public void onCreateOptionsMenu(Menu menu, MenuInflater inflater) {
        // Inflate the menu; this adds items to the action bar if it is present.
        inflater.inflate(R.menu.mainactivityfragment, menu);
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        int id = item.getItemId();
        if (id == R.id.action_refresh) {
            updateMovie();
            return true;
        }
        return super.onOptionsItemSelected(item);
    }

    private void updateMovie() {
        FetchMovieTask movieTask = new FetchMovieTask();
        movieTask.execute();
    }

    class FetchMovieTask extends AsyncTask<Void, Void, List<Movie>> {
        private final String LOG_TAG = FetchMovieTask.class.getSimpleName();

        @Override
        protected List<Movie> doInBackground(Void... params) {
            HttpURLConnection urlConnection = null;
            BufferedReader reader = null;

            // Will contain the raw JSON response as a string.
            String movieJsonStr = null;

            try {
                URL url = new URL("http://api.themoviedb.org/3/discover/movie?sort_by=popularity.desc&api_key=InsertAPIKey");

                urlConnection = (HttpURLConnection) url.openConnection();
                urlConnection.setRequestMethod("GET");
                urlConnection.connect();

                // Read the input stream into a String
                InputStream inputStream = urlConnection.getInputStream();
                StringBuffer buffer = new StringBuffer();
                if (inputStream == null) {
                    // Nothing to do.
                    return null;
                }
                reader = new BufferedReader(new InputStreamReader(inputStream));

                String line;
                while ((line = reader.readLine()) != null) {

                    buffer.append(line + "\n");
                }

                if (buffer.length() == 0) {
                    // Stream was empty.  No point in parsing.
                    return null;
                }
                movieJsonStr = buffer.toString();

            } catch (IOException e) {
                Log.e(LOG_TAG, "Error ", e);
                return null;
            } finally {
                if (urlConnection != null) {
                    urlConnection.disconnect();
                }
                if (reader != null) {
                    try {
                        reader.close();
                    } catch (final IOException e) {
                        Log.e(LOG_TAG, "Error closing stream", e);
                    }
                }
            }
            try {
                return getMovieDataFromJson(movieJsonStr);
            } catch (JSONException j) {
                Log.e(LOG_TAG, "JSON Error", j);
            }
            return null;
        }

        private List<Movie> getMovieDataFromJson(String forecastJsonStr)
                throws JSONException {
            List<Movie> movies = new ArrayList<>();
            JSONObject movieJson = new JSONObject(forecastJsonStr);
            JSONArray movieArray = movieJson.getJSONArray("results");
            for (int i = 0; i < movieArray.length(); i++) {
                JSONObject movie = movieArray.getJSONObject(i);
                Movie movie1 = new Movie(movieJson.getString("original_title"), movieJson.getDouble("vote_average"), movieJson.getString("release_date"), movieJson.getString("overview"), movieJson.getString("poster_path"));
                movies.add(movie1);

            }
            return movies;
        }





        @Override
        protected void onPostExecute(List<Movie> movies) {
            if (movies != null) {
                mMovieAdapter.clear();
                for(Movie dayForecastStr : movies) {
                    mMovieAdapter.add(dayForecastStr);
                }
                // New data is back from the server.  Hooray!
            }
        }
    }


}

有人可以请教

Answer 1

我相信您应该将列表保存到 saveInstanceState 上的包中对于旋转，如果包不为空，您实际上并没有在 onCreate 或 onCreateView 中读回它。另外，我什至不确定 Movie实际上是一个 parcelable(使用 http://parcelabler.com )，但即使是，这也行不通，因为您需要使用 writeTypedList和 readTypedList而不是 writeParcelableArrayList (读取时返回 List<Parcelable>，这不是您想要的)。

但是现在的问题是你不应该给null到此行中的适配器:

mMovieAdapter = new MovieAdapter(getActivity(), flavorList);

初始化适配器并仅在您实际有数据时设置它，或者使用空列表对其进行初始化。

但实际上，使用 writeTypedList和 readTypedList ，否则您稍后会在状态保存方面遇到很多问题。