java - 尝试在空对象引用上调用虚拟方法 '...(java.util.ArrayList)'

Question

我对 Android 开发非常陌生，因为我现在才上了一个类，所以我对当前的情况感到非常困惑，因为我正在使用目前拥有的引用文献编写代码。

我也一直在尝试使用其他来源的引用资料，但遗憾的是，我无法真正理解它们的真正工作原理。

我当前的任务是，我必须制作一个应用程序，作为使用 fragment 的电影和电视节目的目录，以下是其中一个 fragment 的代码:

public class MovieFragment extends Fragment {

    View view;
    private String[] titleMovie;
    private String[] descMovie;
    private TypedArray posterMovie;
    private String[] genreMovie;
    private String[] castMovie;
    private String[] duration;
    private String[] directorMovie;
    private MovieAdapter adapter;
    private ArrayList<Movie> movies;
    private RecyclerView recyclerView;

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

        view = inflater.inflate(R.layout.mv_fragment,container,false);
        recyclerView = view.findViewById(R.id.mv_list);
        adapter = new MovieAdapter(getContext(), movies);
        recyclerView.setLayoutManager(new LinearLayoutManager(getActivity()) {});
        recyclerView.setAdapter(adapter);
        return view;
    }

    @Override
    public void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        prepare();
        addItem();
    }

    private void prepare() {
        titleMovie = getResources().getStringArray(R.array.name_mv);
        descMovie = getResources().getStringArray(R.array.desc_mv);
        posterMovie = getResources().obtainTypedArray(R.array.poster_mv);
        genreMovie = getResources().getStringArray(R.array.genre_mv);
        castMovie = getResources().getStringArray(R.array.cast_mv);
        directorMovie = getResources().getStringArray(R.array.director_mv);
        duration = getResources().getStringArray(R.array.duration);
    }

    private void addItem() {
        movies = new ArrayList<>();

        for (int i = 0; i < titleMovie.length; i++){
            Movie movie = new Movie();
            movie.setTitleMovie(titleMovie[i]);
            movie.setDescMovie(descMovie[i]);
            movie.setPosterMovie(posterMovie.getResourceId(i,-1));
            movie.setGenreMovie(genreMovie[i]);
            movie.setDuration(duration[i]);
            movie.setDirectorMovie(directorMovie[i]);
            movie.setCastMovie(castMovie[i]);
            movies.add(movie);
        }
        adapter.setMovie(movies);
    }

}

当我尝试从 Android Studio 提供的模拟器运行应用程序时，出现以下错误:

    E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.wisnu_1605450.utsmobpro, PID: 18250
    java.lang.NullPointerException: Attempt to invoke virtual method 'void com.wisnu_1605450.utsmobpro.MovieAdapter.setMovie(java.util.ArrayList)' on a null object reference
    at com.wisnu_1605450.utsmobpro.MovieFragment.addItem(MovieFragment.java:74)
    at com.wisnu_1605450.utsmobpro.MovieFragment.onCreate(MovieFragment.java:47)
    at androidx.fragment.app.Fragment.performCreate(Fragment.java:2586)
    at androidx.fragment.app.FragmentManagerImpl.moveToState(FragmentManagerImpl.java:838)
    at androidx.fragment.app.FragmentTransition.addToFirstInLastOut(FragmentTransition.java:1197)
    at androidx.fragment.app.FragmentTransition.calculateFragments(FragmentTransition.java:1080)
    at androidx.fragment.app.FragmentTransition.startTransitions(FragmentTransition.java:119)
    at androidx.fragment.app.FragmentManagerImpl.executeOpsTogether(FragmentManagerImpl.java:1866)

非常感谢任何关于我搞砸的原因的解释，如果上下文需要的话，还会提供更多代码。

Answer 1

来自 fragment 生命周期documentation , onCreate 事件将在 onCreateView 事件之前调用。这意味着当您调用 adapter.setMovie(movies) 时，不会创建适配器。它会导致 NullPointerException。

您应该在 onViewCreated 或 onStart 事件中调用 addItem。