java.lang.ClassCastException：尝试将 Activity 转换为 Fragment 时无法将 Activity 强制转换为适配器

Question

我正在尝试将 Activity 转换为 Fragment，但我正在努力了解如何正确地使用此转换。

基本上，我正在尝试调整 this MainActivity 以便它适合底部导航栏，除了在上下文中迷路之外，我正在处理这个特定问题：

E/AndroidRuntime: FATAL EXCEPTION: main Process: com.example.com, PID: 5213 java.lang.ClassCastException: com.example.com.MainActivity cannot be cast to com.example.com.adapter.NotesAdapter$OnNoteItemClick at com.example.com.adapter.NotesAdapter.(NotesAdapter.java:27) at com.example.com.RoomMarkdownActivity.onCreateView(RoomMarkdownActivity.java:54) at android.support.v4.app.Fragment.performCreateView(Fragment.java:2439) at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1460) at android.support.v4.app.FragmentManagerImpl.moveFragmentToExpectedState(FragmentManager.java:1784) at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1852) at android.support.v4.app.BackStackRecord.executeOps(BackStackRecord.java:802) at android.support.v4.app.FragmentManagerImpl.executeOps(FragmentManager.java:2625) at android.support.v4.app.FragmentManagerImpl.executeOpsTogether(FragmentManager.java:2411) at android.support.v4.app.FragmentManagerImpl.removeRedundantOperationsAndExecute(FragmentManager.java:2366) at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:2273) at android.support.v4.app.FragmentManagerImpl.run(FragmentManager.java:733) at android.os.Handler.handleCallback(Handler.java:873) at android.os.Handler.dispatchMessage(Handler.java:99) at android.os.Looper.loop(Looper.java:193) at android.app.ActivityThread.main(ActivityThread.java:6669) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)

I/Process: Sending signal. PID: 5213 SIG: 9 Application terminated.

这是我正在处理的代码片段的副本：

public class RoomMarkdownActivity extends Fragment implements NotesAdapter.OnNoteItemClick {

    public static RoomMarkdownActivity newInstance() { return new RoomMarkdownActivity(); }

    public RecyclerView recyclerView;
    public TextView textViewMsg;
    public NoteDatabase noteDatabase;
    public List<Note> notes;
    public NotesAdapter notesAdapter;
    public int pos;

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

        textViewMsg = view.findViewById(R.id.textview_empty);
        FloatingActionButton fab = view.findViewById(R.id.fab);
        fab.setOnClickListener(listener);
        recyclerView = view.findViewById(R.id.recycler_view);
        recyclerView.setLayoutManager(new LinearLayoutManager(getActivity()));

        notes = new ArrayList<>();
        notesAdapter = new NotesAdapter(notes, getActivity());
        recyclerView.setAdapter(notesAdapter);
        noteDatabase = NoteDatabase.getInstance(getActivity());
        new RetrieveTask(this).execute();

        return view;
    }

    private static class RetrieveTask extends AsyncTask<Void, Void, List<Note>> {

        private WeakReference<RoomMarkdownActivity> activityReference;

        RetrieveTask(RoomMarkdownActivity context) {
            activityReference = new WeakReference<>(context);
        }

        @Override
        protected List<Note> doInBackground(Void... voids) {
            if (activityReference.get() != null)
                return activityReference.get().noteDatabase.getNoteDao().getNotes();
            else
                return null;
        }

        @Override
        protected void onPostExecute(List<Note> notes) {
            if (notes != null && notes.size() > 0) {
                activityReference.get().notes.clear();
                activityReference.get().notes.addAll(notes);
                activityReference.get().textViewMsg.setVisibility(View.GONE);
                activityReference.get().notesAdapter.notifyDataSetChanged();
            }
        }
    }

    private View.OnClickListener listener = new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            startActivityForResult(new Intent(getActivity(), RoomMarkdownAddNoteActivity.class), 100);
        }
    };

    @Override
    public void onActivityResult(int requestCode, int resultCode, Intent data) {
        if (requestCode == 100 && resultCode > 0) {
            if (resultCode == 1) {
                notes.add((Note) data.getSerializableExtra("note"));
            } else if (resultCode == 2) {
                notes.set(pos, (Note) data.getSerializableExtra("note"));
            }
            listVisibility();
        }
    }

    @Override
    public void onNoteClick(final int pos) {
        new AlertDialog.Builder(recyclerView.getContext())
                .setTitle("Select:")
                .setItems(new String[]{"Read", "Update", "Delete"}, new DialogInterface.OnClickListener() {
                    @Override
                    public void onClick(DialogInterface dialogInterface, int i) {
                        switch (i) {
                            case 0:
                                RoomMarkdownActivity.this.pos = pos;
                                startActivityForResult(
                                        new Intent(getActivity(),
                                                RoomMarkdownViewNoteActivity.class).putExtra(
                                                "note", notes.get(pos)), 100);
                                break;
                            case 1:
                                RoomMarkdownActivity.this.pos = pos;
                                startActivityForResult(
                                        new Intent(getActivity(),
                                                RoomMarkdownAddNoteActivity.class).putExtra(
                                                "note", notes.get(pos)), 100);
                                break;
                            case 2:
                                noteDatabase.getNoteDao().deleteNote(notes.get(pos));
                                notes.remove(pos);
                                listVisibility();
                                break;
                        }
                    }
                }).show();
    }

    private void listVisibility() {
        int emptyMsgVisibility = View.GONE;
        if (notes.size() == 0) {
            if (textViewMsg.getVisibility() == View.GONE)
                emptyMsgVisibility = View.VISIBLE;
        }
        textViewMsg.setVisibility(emptyMsgVisibility);
        notesAdapter.notifyDataSetChanged();
    }

    @Override
    public void onDestroy() {
        noteDatabase.cleanUp();
        super.onDestroy();
    }

}

这是适配器：

public class NotesAdapter extends RecyclerView.Adapter<NotesAdapter.BeanHolder> {

    private List<Note> list;
    public Context context;
    private LayoutInflater layoutInflater;
    private OnNoteItemClick onNoteItemClick;

    public NotesAdapter(List<Note> list, Context context) {
        layoutInflater = LayoutInflater.from(context);
        this.list = list;
        this.context = context;
        this.onNoteItemClick = (OnNoteItemClick) context;
    }

    @Override
    public BeanHolder onCreateViewHolder(ViewGroup parent, int viewType) {
        View view = layoutInflater.inflate(R.layout.room_markdown_note_list_item, parent, false);
        return new BeanHolder(view);
    }

    @Override
    public void onBindViewHolder(BeanHolder holder, int position) {
        Log.e("bind", "onBindViewHolder: " + list.get(position));
        holder.textViewTitle.setText(list.get(position).getTitle());
    }

    @Override
    public int getItemCount() {
        return list.size();
    }

    public class BeanHolder extends RecyclerView.ViewHolder implements View.OnClickListener {

        TextView textViewTitle;

        private BeanHolder(View itemView) {
            super(itemView);
            itemView.setOnClickListener(this);
            textViewTitle = itemView.findViewById(R.id.textview_title);
        }

        @Override
        public void onClick(View view) {
            onNoteItemClick.onNoteClick(getAdapterPosition());
        }
    }

    public interface OnNoteItemClick {
        void onNoteClick(int pos);
    }

}

notesAdapter = new NotesAdapter(notes, getActivity()); 和 this.onNoteItemClick = (OnNoteItemClick) context; 之间似乎有问题，但我找不到替代 getActivity() 的好方法，我想我早些时候用另一种方法解决了这个问题，但是，如果我没记错的话，它仍然会抛出另一个类似的错误（这个文件充满了 this，我不怀疑有些 getActivity() 放错了地方）。

代码按原样编译，但是，一旦点击屏幕上调用特定 Fragment 的按钮，就会抛出错误并且应用程序立即停止运行。

在此先感谢您的关注和帮助，如果您需要了解有关此问题的更多信息，请告诉我，以便我能够在此处提供更多信息。

Answer 1

您不能将 Activity 转换为 OnNoteItemClick，因为您在片段中实现了它，您可以将 OnNoteItemClick 分配给适配器，这样您就可以将代码更改为：适配器：

public NotesAdapter(List<Note> list, OnNoteItemClick onNoteItemClick) {
    layoutInflater = LayoutInflater.from(context);
    this.list = list;
    this.context = context;
    this.onNoteItemClick = onNoteItemClick;
}

以这种方式在片段中初始化您的适配器：

notesAdapter = new NotesAdapter(notes, this); //Your class implements OnNoteItemClick interface so you can use 'this'

Answer 2

替换

notesAdapter = new NotesAdapter(notes, getActivity());

和

notesAdapter = new NotesAdapter(notes, RoomMarkdownActivity.this);

Answer 3

感谢 and 在答案和评论中所说的内容，我能够理解正在发生的事情并克服这个问题。

问题出在准备从 Activity 获取 Context 的 Adapter 中，所以我不得不将参数转换为 Fragment，它最终像这个：

public NotesAdapter(List<Note> list, RoomMarkdownActivity fragment) {
    this.layoutInflater = LayoutInflater.from(fragment.getActivity());
    this.list = list;
    this.fragment = fragment;
    this.onNoteItemClick = fragment;
}

而 Fragment 喜欢：

public class RoomMarkdownActivity extends Fragment implements NotesAdapter.OnNoteItemClick {

    public static RoomMarkdownActivity newInstance() {
        return new RoomMarkdownActivity();
    }

    RecyclerView recyclerView;
    TextView textViewMsg;
    NoteDatabase noteDatabase;
    List<Note> notes;
    NotesAdapter notesAdapter;
    int pos;

    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.room_markdown_activity, container, false);
        textViewMsg = view.findViewById(R.id.textview_empty);
        FloatingActionButton fab = view.findViewById(R.id.fab);
        fab.setOnClickListener(listener);
        recyclerView = view.findViewById(R.id.recycler_view);
        initializeViews();
        displayList();
        return view;
    }

    private void displayList() {
        noteDatabase = NoteDatabase.getInstance(getActivity());
        new RetrieveTask(this).execute();
    }

    private static class RetrieveTask extends AsyncTask<Void, Void, List<Note>> {

        private WeakReference<RoomMarkdownActivity> activityReference;

        RetrieveTask(RoomMarkdownActivity context) {
            activityReference = new WeakReference<>(context);
        }

        @Override
        protected List<Note> doInBackground(Void... voids) {
            if (activityReference.get() != null)
                return activityReference.get().noteDatabase.getNoteDao().getNotes();
            else
                return null;
        }

        @Override
        protected void onPostExecute(List<Note> notes) {
            if (notes != null && notes.size() > 0) {
                activityReference.get().notes.clear();
                activityReference.get().notes.addAll(notes);
                activityReference.get().textViewMsg.setVisibility(View.GONE);
                activityReference.get().notesAdapter.notifyDataSetChanged();
            }
        }
    }

    public void initializeViews() {
        recyclerView.setLayoutManager(new LinearLayoutManager(getActivity()));
        notes = new ArrayList<>();
        notesAdapter = new NotesAdapter(notes, this);
        recyclerView.setAdapter(notesAdapter);
    }

    private View.OnClickListener listener = new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            startActivityForResult(new Intent(getActivity(), RoomMarkdownAddNoteActivity.class), 100);
        }
    };

    @Override
    public void onActivityResult(int requestCode, int resultCode, Intent data) {
        if (requestCode == 100 && resultCode > 0) {
            if (resultCode == 1) {
                notes.add((Note) data.getSerializableExtra("note"));
            } else if (resultCode == 2) {
                notes.set(pos, (Note) data.getSerializableExtra("note"));
            }
            listVisibility();
        }
    }

    @Override
    public void onNoteClick(final int pos) {
        new AlertDialog.Builder(getActivity())
                .setTitle("Select:")
                .setItems(new String[]{"Read", "Update", "Delete"}, new DialogInterface.OnClickListener() {
                    @Override
                    public void onClick(DialogInterface dialogInterface, int i) {
                        switch (i) {
                            case 0:
                                RoomMarkdownActivity.this.pos = pos;
                                startActivityForResult(
                                        new Intent(getActivity(),
                                                RoomMarkdownViewNoteActivity.class).putExtra(
                                                "note", notes.get(pos)), 100);
                                break;
                            case 1:
                                RoomMarkdownActivity.this.pos = pos;
                                startActivityForResult(
                                        new Intent(getActivity(),
                                                RoomMarkdownAddNoteActivity.class).putExtra(
                                                "note", notes.get(pos)), 100);
                                break;
                            case 2:
                                noteDatabase.getNoteDao().deleteNote(notes.get(pos));
                                notes.remove(pos);
                                listVisibility();
                                break;
                        }
                    }
                }).show();
    }

    public void listVisibility() {
        int emptyMsgVisibility = View.GONE;
        if (notes.size() == 0) {
            if (textViewMsg.getVisibility() == View.GONE)
                emptyMsgVisibility = View.VISIBLE;
        }
        textViewMsg.setVisibility(emptyMsgVisibility);
        notesAdapter.notifyDataSetChanged();
    }

    @Override
    public void onDestroy() {
        noteDatabase.cleanUp();
        super.onDestroy();
    }

}

非常感谢您的帮助，我希望这 question/answer 对其他人也有帮助！

java.lang.ClassCastException：尝试将 Activity 转换为 Fragment 时无法将 Activity 强制转换为适配器

java.lang.ClassCastException: Activity cannot be cast to Adapter while trying to convert Activity to Fragment

android

casting

android-adapter

android-fragments

android-activity