java.lang.ClassCastException：在嘗試將Activity轉換為Fragment時，無法將Activity強制轉換為Adapter

Question

我正在嘗試將Activity轉換為Fragment ，但我仍想了解如何正確地使用此代碼。

基本上，我正在嘗試修改此MainActivity ，以使其適合底部導航欄，除了在上下文中迷路以外，我還處理以下特定問題：

E / AndroidRuntime：致命例外：主進程：com.example.com，PID：5213 java.lang.ClassCastException：com.example.com.MainActivity無法轉換為com.example.com.adapter.NotesAdapter $ OnNoteItemClick com。 example.com.adapter.NotesAdapter。（NotesAdapter.java:27）位於com.example.com.RoomMarkdownActivity.onCreateView（RoomMarkdownActivity.java:54）位於android.support.v4.app.Fragment.performCreateView（Fragment.java:2439 ）在android.support.v4.app.FragmentManagerImpl.moveFragmentToExpectedState（FragmentManager.java:1784）在android.support.v4.app.FragmentManagerImpl.moveToState（FragmentManager.java:1460） android.support.v4.app.BackStackRecord.executeOps（BackStackRecord.java:802）的moveToState（FragmentManager.java:1852）android.support.v4.app.FragmentManagerImpl.executeOps（FragmentManager.java:2625）的moveToState（FragmentManager.java:2625） .v4.app.FragmentManagerImpl.executeOpsTogether（FragmentManager.java:2411）在android.support.v4.app.FragmentManagerImpl.rem 在android.support.v4.app.FragmentManagerImpl.execPendingActions（FragmentManager.java:2273）在android.support.v4.app.FragmentManagerImpl $ 1.run（FragmentManager.java:733）處的oveRedundantOperationsAndExecute（FragmentManager.java:2366）在android。 os.Handler.handleCallback（Handler.java:873）android.os.Handler.dispatchMessage（Handler.java:99）android.os.Looper.loop（Looper.java:193）android.app.ActivityThread.main （ActivityThread.java:6669）在com.android.internal.os.RuntimeInit $ MethodAndArgsCaller.run（RuntimeInit.java:493）在java.lang.reflect.Method.invoke（本機方法）在com.android.internal.os .ZygoteInit.main（ZygoteInit.java:858）

I / Process：正在發送信號。 PID：5213 SIG：9應用程序終止。

這是我正在處理的一段代碼的副本：

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();
    }

}

這是適配器：

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);
    }

}

似乎在notesAdapter = new NotesAdapter(notes, getActivity()); 和this.onNoteItemClick = (OnNoteItemClick) context; ，但我無法找到一個很好的替代getActivity()我想我在另一種方法解決了這一較早，但是，它仍然拋出另一個類似的錯誤，如果我沒有記錯（該文件是充滿了this和我不懷疑某些getActivity()放錯了位置）。

代碼按原樣進行編譯，但是，一旦單擊了將調用特定Fragment到屏幕的按鈕，就會引發錯誤，並且應用程序立即停止運行。

在此先感謝您的關注和幫助，如果您需要更多有關此問題的信息，請告訴我，以便在此處提供更多信息。

Answer 1

您不能將Activity轉換為OnNoteItemClick，因為您是在片段中實現的，因此可以將OnNoteItemClick分配給適配器，以便將代碼更改為：adapter：

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

通過以下方式初始化片段中的適配器：

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

Answer 2

更換

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

同

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

Answer 3

感謝Mike M.和Reza.Abedini在回答和評論中所說的，我能夠理解正在發生的事情並克服了這個問題。

問題出在Adapter ，該Adapter已准備好從Activity獲取Context ，因此我不得不將參數轉換為Fragment ，並且最終像這樣工作：

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

和Fragment一樣：

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();
    }

}

非常感謝您的幫助，希望這個問題/答案也能對其他人有所幫助！