当我尝试通过视图持有人从数据库中检索数据时出现此错误

Question

我正在尝试通过recyclerView从数据库检索数据。但是它运行不正常。应用程序需要通过检索Firebase数据库中的所有数据转到第二页（这是详细信息页面）。但我收到此错误。 原因：java.lang.NullPointerException：尝试调用虚拟方法“ com.google.firebase.database.ChildEventListener com.google.firebase.database.Query.addChildEventListener（com.google.firebase.database.ChildEventListener）”在空对象引用上 ..会出现什么问题。

在我的mainActivity类中，我正在编码如何向数据库添加数据。当我单击按钮时，它运行正常并且数据正在添加到表中。但是当我单击按钮时，它应该重定向到详细信息界面。但它没有发生。数据已正确添加到数据库，然后应用程序开始停止。然后我得到了上面的错误。

这是我的mainActivity类。

package com.example.medisafe;

import androidx.annotation.NonNull;
import androidx.appcompat.app.AppCompatActivity;
import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;

import android.content.Intent;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;

import com.firebase.ui.database.FirebaseRecyclerAdapter;
import com.firebase.ui.database.FirebaseRecyclerOptions;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;

public class MainActivity extends AppCompatActivity {

    EditText txtCname, txtSpecial,txtDocName, txtadd, txtphone, date, time;
    Button button;
    DatabaseReference dbRef;
    Appointment appointment;
    FirebaseDatabase firebaseDatabase;


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

        txtCname = (EditText)findViewById(R.id.eTTCenterName);
        txtSpecial = (EditText)findViewById(R.id.eTTSpeciality);
        txtDocName= (EditText)findViewById(R.id.eTTdocName);
        txtadd = (EditText)findViewById(R.id.eTTAddress);
        txtphone = (EditText)findViewById(R.id.eTTPhone);
        date = (EditText)findViewById(R.id.editTextDate);
        time = (EditText)findViewById(R.id.editTextTime);

        button = (Button) findViewById(R.id.button);

        appointment = new Appointment();


        dbRef = FirebaseDatabase.getInstance().getReference().child("Appointment");
        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {

                int phone = Integer.parseInt(txtphone.getText().toString().trim());


                appointment.setName(txtCname.getText().toString().trim());
                appointment.setSpecial(txtSpecial.getText().toString().trim());
                appointment.setDocName(txtDocName.getText().toString().trim());
                appointment.setAddress(txtadd.getText().toString().trim());
                appointment.setPhone(phone);
                appointment.setDate(date.getText().toString().trim());
                appointment.setTime(time.getText().toString().trim());

                String id = dbRef.push().getKey();
                dbRef.child(id).setValue(appointment);

                Toast.makeText(MainActivity.this, "saved successfully",Toast.LENGTH_LONG).show();
                opennewpage();

            }


        });
    }

    public void opennewpage (){
        Intent intent = new Intent(this, AppointmentDetail.class);
        startActivity(intent);
    }





}

这是我的viewHolder类。

package com.example.medisafe;

import android.content.Context;
import android.view.View;
import android.widget.TextView;

import androidx.annotation.NonNull;
import androidx.recyclerview.widget.RecyclerView;

import java.util.ConcurrentModificationException;

public class ViewHolder extends RecyclerView.ViewHolder {


    public ViewHolder(@NonNull View itemView) {
        super(itemView);

        itemView.setOnLongClickListener(new View.OnLongClickListener() {
            @Override
            public boolean onLongClick(View view) {

                mClickListener.onItemLongClick(view,getAdapterPosition());
                return false;
            }
        });
    }

    public void setData(Context context, String cname, String sp, String docName, String Add, int cphone, String adate, String aTime){

        TextView textView = itemView.findViewById(R.id.txtViewRow);

        textView.setText("Center Name: "+cname+ "\n" + "Specialization"+ sp + "\n"+ "Doctor Name: "+docName+ "\n"
                            + "Location: "+Add+ "\n"+ "Phone"+ "\n"  + "Date: "+adate+ "\n" + "Time: "+aTime);
    }

    private ViewHolder.Clicklistener mClickListener;
    public interface Clicklistener{

        void onItemLongClick(View view, int position);
    }

    public void setOnClickListener(ViewHolder.Clicklistener clickListener)
    {
        mClickListener = clickListener;
    }
}

这是我的AppointmentDetail类。

package com.example.medisafe;

import androidx.annotation.NonNull;
import androidx.appcompat.app.AppCompatActivity;
import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;

import android.app.AlertDialog;
import android.content.DialogInterface;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Toast;

import com.firebase.ui.database.FirebaseRecyclerAdapter;
import com.firebase.ui.database.FirebaseRecyclerOptions;
import com.google.firebase.database.DataSnapshot;
import com.google.firebase.database.DatabaseError;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;
import com.google.firebase.database.Query;
import com.google.firebase.database.ValueEventListener;

public class AppointmentDetail extends AppCompatActivity {

    DatabaseReference dbRef;
    String name;
    RecyclerView recyclerView;
    FirebaseDatabase firebaseDatabase;
    Appointment appointment;


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

        recyclerView = findViewById(R.id.detailView);

        recyclerView.setHasFixedSize(true);
        recyclerView.setLayoutManager(new LinearLayoutManager(this));
    }

    protected void onStart(){
        super.onStart();

        FirebaseRecyclerOptions<Appointment> options = new FirebaseRecyclerOptions.Builder<Appointment>()
                .setQuery(dbRef,Appointment.class).build();

        FirebaseRecyclerAdapter<Appointment, ViewHolder> firebaseRecyclerAdapter = new FirebaseRecyclerAdapter<Appointment, ViewHolder>(options){

            @Override
            protected void onBindViewHolder(@NonNull ViewHolder holder, int position, @NonNull Appointment model) {

                holder.setData(getApplicationContext(), model.getName(), model.getSpecial(), model.getDocName(),
                        model.getAddress(), model.getPhone(), model.getDate(), model.getTime());

                holder.setOnClickListener(new ViewHolder.Clicklistener() {
                    @Override
                    public void onItemLongClick(View view, int position) {

                        name = getItem(position).getName();
                        showDeleteDialog(name);
                    }
                });
            }
            @NonNull
            @Override
            public ViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {

                View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.row, parent, false);
                return new ViewHolder(view);
            }
        };
        firebaseRecyclerAdapter.startListening();
        recyclerView.setAdapter(firebaseRecyclerAdapter);


    }

    private void showDeleteDialog(final String name){

        AlertDialog .Builder builder = new AlertDialog.Builder(AppointmentDetail.this);
        builder.setTitle("Delete");
        builder.setMessage("Sure You want to delete this?");
        builder.setPositiveButton("yes", new DialogInterface.OnClickListener() {
            @Override
            public void onClick(DialogInterface dialog, int i) {

                Query query = dbRef.orderByChild("name").equalTo(name);
                query.addListenerForSingleValueEvent(new ValueEventListener() {
                    @Override
                    public void onDataChange(@NonNull DataSnapshot snapshot) {

                        for (DataSnapshot ds : snapshot.getChildren()) {
                            ds.getRef().removeValue();
                        }
                        Toast.makeText(AppointmentDetail.this, "Data deleted", Toast.LENGTH_SHORT).show();
                    }

                    @Override
                    public void onCancelled(@NonNull DatabaseError error) {

                    }
                });
            }
        });

        builder.setNegativeButton("No", new DialogInterface.OnClickListener() {
            @Override
            public void onClick(DialogInterface dialog, int i) {
                dialog.dismiss();
            }
        });

        AlertDialog alertDialog = builder.create();
        alertDialog.show();
    }

}

他们说错误出在此行的详细信息类的第76行。

firebaseRecyclerAdapter.startListening();

此行位于 viewHolder 方法之后的Appointmentmentdetail类中。