Question

我正在尝试使用片段创建活动。该片段包含一个包含“电影对象”的网格视图。必须使用gridview中的图像来表示每个对象。图像是一个网址，我将其作为字符串保存在对象内部，并加载毕加索。

出于某种原因，当我实例化我的适配器时，它会触发nullPointerException，而我无法终生，找出原因。

这是我的活动：

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        if (savedInstanceState== null) {
            getSupportFragmentManager().beginTransaction()
                    .add(R.id.container, new MovieListFragment())
                    .commit();
        }
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }
}

这是我的片段：

public class MovieListFragment extends Fragment {

    private final String LOG_TAG = MovieListFragment.class.getSimpleName();

    MovieAdapter mMovieAdapter = new MovieAdapter(
            getContext(),
            R.id.image_thumbnail,
            new ArrayList<Movie>()
    );

    public MovieListFragment() {
    }

    @Override
    public void onCreate(Bundle SavedInstanceState) {
        super.onCreate(SavedInstanceState);
        // We want to add options to the Menu bar.
        setHasOptionsMenu(true);
    }

    @Override
    public void onCreateOptionsMenu(Menu menu, MenuInflater inflater) {
        inflater.inflate(R.menu.movielist_fragment, menu);
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // We inflate and store the rootView
        View rootView = inflater.inflate(R.layout.fragment_main, container, false);
        // We sotre the gridview and set a custom adapter to it. It is defined below
        GridView gridView = (GridView) rootView.findViewById(R.id.gridview);
        gridView.setAdapter(mMovieAdapter);

        // The gridView will react to a user clicking on it.
        gridView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
                Toast.makeText(getActivity(),position,Toast.LENGTH_SHORT);
            }
        });
        return rootView;
    }

    private void retrieveMovies() {
        FetchMoviesTask moviesTask = new FetchMoviesTask();
        moviesTask.execute();
    }

    @Override
    public void onStart() {
        super.onStart();
        retrieveMovies();
    }

    // Custom adapter to display Movies on a gridView
    public class MovieAdapter extends ArrayAdapter<Movie> {
        //Context member variable
        private Context mContext;
        //Constructor
        public MovieAdapter(Context context, int imgViewResourceId,
                            ArrayList<Movie> items) {
            super(context, imgViewResourceId, items);
            mContext = context;
        }

        public View getView(int position, View convertView, ViewGroup parent) {
            ImageView imageView;
            // If the view passed is null, we initialize it
            if (convertView == null) {
                imageView = new ImageView(mContext);
                imageView.setLayoutParams(new GridView.LayoutParams(80,80));
                imageView.setScaleType(ImageView.ScaleType.CENTER_CROP);
                imageView.setPadding(2,2,2,2);
            } else {
                imageView = (ImageView) convertView;
            }

            String url = getItem(position).getUrlPath();

            Picasso.with(mContext)
                    .load(url)
                    .resize(80,80)
                    .centerCrop()
                    .into(imageView);


            return imageView;
        }

    }

这本身就是例外：

Process: com.example.madelenko.movierating, PID: 5326
 java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.madelenko.movierating/com.example.madelenko.movierating.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Object android.content.Context.getSystemService(java.lang.String)' on a null object reference
     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2416)
     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
     at android.app.ActivityThread.-wrap11(ActivityThread.java)
     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
     at android.os.Handler.dispatchMessage(Handler.java:102)
     at android.os.Looper.loop(Looper.java:148)
     at android.app.ActivityThread.main(ActivityThread.java:5417)
     at java.lang.reflect.Method.invoke(Native Method)
     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
  Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Object android.content.Context.getSystemService(java.lang.String)' on a null object reference
     at android.view.LayoutInflater.from(LayoutInflater.java:229)
     at android.widget.ArrayAdapter.<init>(ArrayAdapter.java:178)
     at android.widget.ArrayAdapter.<init>(ArrayAdapter.java:163)
     at com.example.madelenko.movierating.MovieListFragment$MovieAdapter.<init>(MovieListFragment.java:101)
     at com.example.madelenko.movierating.MovieListFragment.<init>(MovieListFragment.java:44)
     at com.example.madelenko.movierating.MainActivity.onCreate(MainActivity.java:21)
     at android.app.Activity.performCreate(Activity.java:6251)
     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1107)
     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2369)
     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476) 
     at android.app.ActivityThread.-wrap11(ActivityThread.java) 
     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344) 
     at android.os.Handler.dispatchMessage(Handler.java:102) 
     at android.os.Looper.loop(Looper.java:148) 
     at android.app.ActivityThread.main(ActivityThread.java:5417) 
     at java.lang.reflect.Method.invoke(Native Method) 
     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) 
     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)

Answer 1

问题是，当您初始化MovieListFragment时，Context尚未与MovieAdapter相关联，因此getContext()正在返回null。

要解决此问题，首先只声明适配器：

MovieAdapter mMovieAdapter;

并在上下文附加到Fragment之后设置其值。这是在onAttach生命周期中调用Fragment之后的任何时候，例如您使用的onCreateView方法：

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    mMovieAdapter = new MovieAdapter(
        getContext(),
        R.id.image_thumbnail,
        new ArrayList<Movie>()
    );
    ....
}

创建自定义适配器时触发NullPointerException

1 个答案: