Попытка вызвать виртуальный метод void com.firebase.ui.database.FirebaseRecyclerAdapter.startListening () для пустой ссылки на объект

Question

Я создаю свое первое приложение для Android Studio, где пользователь может публиковать записи в дневнике. Публикация работает (загружается в базу данных в реальном времени в Firebase), и я сейчас пытаюсь отобразить эти сообщения на главной странице активности, в макете карты с использованием представления переработчика. К сожалению, я получаю эту ошибку при запуске приложения. Я прикреплю свой

MainActivity.java

public class MainActivity extends AppCompatActivity {

private RecyclerView mDiaryList;
private DatabaseReference mDatabase;
private FirebaseAuth mAuth;
private FirebaseAuth.AuthStateListener mAuthListener;
private FirebaseRecyclerAdapter<Journal, JournalViewHolder> adapter;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    mDatabase = FirebaseDatabase.getInstance().getReference().child("Journal");

    mDiaryList = (RecyclerView) findViewById(R.id.diary_list);
    mDiaryList.setLayoutManager(new LinearLayoutManager(this));
    mDiaryList.setHasFixedSize(true);


    mDatabase = FirebaseDatabase.getInstance().getReference().child("Journal");

    DatabaseReference personRef = FirebaseDatabase.getInstance().getReference().child("Journal");
    Query query = personRef.orderByKey();

    mDatabase.keepSynced(true);
    FirebaseRecyclerOptions<Journal> options =
            new FirebaseRecyclerOptions.Builder<Journal>()
                    .setQuery(query, Journal.class)
                    .build();


    FirebaseRecyclerAdapter adapter = new FirebaseRecyclerAdapter<Journal, JournalViewHolder>(options) {
        @NonNull
        @Override
        public JournalViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {

            View view = LayoutInflater.from(parent.getContext())
                    .inflate(R.layout.diary_list, parent, true);

            return new JournalViewHolder(view);
        }

        //
        @Override
        protected void onBindViewHolder(@NonNull JournalViewHolder holder, int position, @NonNull Journal model) {

            final String post_key = getRef(position).getKey().toString();
            holder.setTitle(model.getTitle());
            holder.setDesc(model.getDesc());

        }
    };
    mDiaryList.setAdapter(adapter);

}


@Override
protected void onStart() {
    super.onStart();
    adapter.startListening();
}

@Override
protected void onStop() {
    super.onStop();
    adapter.stopListening();
}

//we need a viewholder to set up the Recycler  view.
public static class JournalViewHolder extends RecyclerView.ViewHolder {
    View mView;

    public JournalViewHolder(View itemView) {
        super(itemView);
        mView = itemView;
    }

    public void setTitle(String title) {
        TextView post_title = mView.findViewById(R.id.post_title);
        post_title.setText(title);
    }

    public void setDesc(String desc) {
        TextView post_desc = mView.findViewById(R.id.post_desc);
        post_desc.setText(desc);
    }
}


@Override
public boolean onCreateOptionsMenu(Menu menu) {
    getMenuInflater().inflate(R.menu.menu, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    switch (item.getItemId()) {
        case R.id.options:
            // Red item was selected
            startActivity(new Intent(MainActivity.this, ProfileActivity.class));
            return true;

        case R.id.add_post:
            //add_post was selected
            startActivity(new Intent(MainActivity.this, PostActivity.class));
        default:
            return super.onOptionsItemSelected(item);
    }
}

Journal.java

public class Journal {
private String title, desc, uid;
public Journal(String title, String desc, String uid) {
    this.title = title;
    this.desc = desc;
    this.uid = uid;
}
public Journal() {
}

public String getTitle() {
    return title;
}

public void setTitle(String title) {
    this.title = title;
}

public String getDesc() {
    return desc;
}

public void setDesc(String desc) {
    this.desc = desc;
}

public String getUid() {
    return uid;
}

public void setUid(String uid) {
    this.uid = uid;
}

Answer 1

public class MainActivity extends AppCompatActivity {

private RecyclerView mDiaryList;
private DatabaseReference mDatabase;
private FirebaseAuth mAuth;
private FirebaseAuth.AuthStateListener mAuthListener;
private FirebaseRecyclerAdapter<Journal, JournalViewHolder> adapter;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    mDatabase = FirebaseDatabase.getInstance().getReference().child("Journal");

    mDiaryList = (RecyclerView) findViewById(R.id.diary_list);
    mDiaryList.setLayoutManager(new LinearLayoutManager(this));
    mDiaryList.setHasFixedSize(true);

    //This is unnecessary.
    mDatabase = FirebaseDatabase.getInstance().getReference().child("Journal");

    //In my opinion this is unnecessary too. You already have a DatabaseReference object you declared as local variable.
    DatabaseReference personRef = FirebaseDatabase.getInstance().getReference().child("Journal");

    //Change this line to Query query = mDatabase.orderByKey();
    Query query = personRef.orderByKey();

    mDatabase.keepSynced(true);
    FirebaseRecyclerOptions<Journal> options =
            new FirebaseRecyclerOptions.Builder<Journal>()
                    .setQuery(query, Journal.class)
                    .build();

    //Instantiate your adapter here
    adapter = new FirebaseRecyclerAdapter<Journal, JournalViewHolder>(options) {
        @NonNull
        @Override
        public JournalViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {

            View view = LayoutInflater.from(parent.getContext())
                    .inflate(R.layout.diary_list, parent, true);

            return new JournalViewHolder(view);
        }

        //
        @Override
        protected void onBindViewHolder(@NonNull JournalViewHolder holder, int position, @NonNull Journal model) {

            final String post_key = getRef(position).getKey().toString();
            holder.setTitle(model.getTitle());
            holder.setDesc(model.getDesc());

        }
    };
    mDiaryList.setAdapter(adapter);

}


@Override
protected void onStart() {
    super.onStart();
    adapter.startListening();
}

@Override
protected void onStop() {
    super.onStop();
    adapter.stopListening();
}

//we need a viewholder to set up the Recycler  view.
public static class JournalViewHolder extends RecyclerView.ViewHolder {
    View mView;

    public JournalViewHolder(View itemView) {
        super(itemView);
        mView = itemView;
    }

    public void setTitle(String title) {
        TextView post_title = mView.findViewById(R.id.post_title);
        post_title.setText(title);
    }

    public void setDesc(String desc) {
        TextView post_desc = mView.findViewById(R.id.post_desc);
        post_desc.setText(desc);
    }
}


@Override
public boolean onCreateOptionsMenu(Menu menu) {
    getMenuInflater().inflate(R.menu.menu, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    switch (item.getItemId()) {
        case R.id.options:
            // Red item was selected
            startActivity(new Intent(MainActivity.this, ProfileActivity.class));
            return true;

        case R.id.add_post:
            //add_post was selected
            startActivity(new Intent(MainActivity.this, PostActivity.class));
        default:
            return super.onOptionsItemSelected(item);
    }
}

Answer 2

Вы локально определяете переменную с именем adapter в onCreate, которая заменяет переменную-член adapter на уровне класса. Вы должны удалить тип из его назначения, чтобы убедиться, что вы присваиваете переменную-член вместо локальной переменной:

// Remove "FirebaseRecyclerAdapter" from this line.
adapter = new FirebaseRecyclerAdapter<>(options) ...