基于Room数据库的提醒APP出现NullPointerException问题求助

我跟着YouTube教程开发基于Room的提醒APP，运行时抛出空指针异常：

java.lang.NullPointerException: Attempt to invoke virtual method 'com.dev.alarmreminder.Database.EventDao com.dev.alarmreminder.Database.DatabaseClass.EventDao()' on a null object reference

错误堆栈：

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'com.dev.alarmreminder.Database.EventDao com.dev.alarmreminder.Database.DatabaseClass.EventDao()' on a null object reference
        at com.dev.alarmreminder.MainActivity.setAdapter(MainActivity.java:49)
        at com.dev.alarmreminder.MainActivity.onResume(MainActivity.java:43)

问题出在MainActivity的setAdapter方法第一行，调用databaseClass.EventDao()时databaseClass对象为空。以下是相关代码：

DatabaseClass.java #

@Database(entities = {EntityClass.class}, version = 1)
public abstract class DatabaseClass extends RoomDatabase
{
    public abstract  EventDao EventDao();
    private static DatabaseClass INSTANCE;


    static DatabaseClass getDatabase(final Context context)
    {
        if(INSTANCE == null)
        {
            synchronized (DatabaseClass.class)
            {
                if(INSTANCE == null)
                {
                    INSTANCE = Room.databaseBuilder(context.getApplicationContext(), DatabaseClass.class, "product_database").build();
                }
            }
        }

        return INSTANCE;
    }
}

EventDao.java #

import androidx.room.Dao;
import androidx.room.Insert;
import androidx.room.Query;

import java.util.List;

@Dao
public interface EventDao
{
    @Insert
    void insertAll(EntityClass entityClass);

    @Query("SELECT * FROM myTable")
    List<EntityClass> getAllData();
}

EntityClass.java #

import androidx.room.Entity;
import androidx.room.PrimaryKey;

@Entity(tableName = "myTable")
    public class EntityClass
    {
        @PrimaryKey(autoGenerate = true)
        int id;

        String eventname;
        String eventdate;
        String eventtime;


        public int getId() {
            return id;
        }

        public void setId(int id) {
            this.id = id;
        }

        public String getEventname() {
            return eventname;
        }

        public void setEventname(String eventname) {
            this.eventname = eventname;
        }

        public String getEventdate() {
            return eventdate;
        }

        public void setEventdate(String eventdate) {
            this.eventdate = eventdate;
        }

        public String getEventtime() {
            return eventtime;
        }

        public void setEventtime(String eventtime) {
            this.eventtime = eventtime;
        }
    }

MainActivity.java #

package com.mahidev.alarmreminder;

import androidx.appcompat.app.AppCompatActivity;
import androidx.recyclerview.widget.RecyclerView;

import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;

import com.mahidev.alarmreminder.Adapter.EventAdapter;
import com.mahidev.alarmreminder.Database.DatabaseClass;
import com.mahidev.alarmreminder.Database.EntityClass;

import java.util.List;

public class MainActivity extends AppCompatActivity implements View.OnClickListener
{

    Button createEvent;
    EventAdapter eventAdapter;
    RecyclerView recyclerView;
    DatabaseClass databaseClass;


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

        createEvent = findViewById(R.id.btn_createEvent);
        recyclerView = findViewById(R.id.recyclerview);
        createEvent.setOnClickListener(this);
    }

    @Override
    protected void onResume() {
        super.onResume();
        setAdapter();
    }

    private void setAdapter()
    {
        List<EntityClass> classList = databaseClass.EventDao().getAllData();
        eventAdapter = new EventAdapter(getApplicationContext(), classList);
        recyclerView.setAdapter(eventAdapter);
    }


    @Override
    public void onClick(View view)
    {
        if(view == createEvent)
        {
        goToCreateEventActivity();
        }
    }

    private void goToCreateEventActivity()
    {
        Intent intent = new Intent(getApplicationContext(), CreateEvent.class);
        startActivity(intent);
    }
}

解决方法 #

1. 初始化databaseClass对象
   问题核心是MainActivity中声明的databaseClass变量从未赋值，直接调用方法导致空指针。修改onCreate方法，通过DatabaseClass.getDatabase()获取实例：

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

    createEvent = findViewById(R.id.btn_createEvent);
    recyclerView = findViewById(R.id.recyclerview);
    createEvent.setOnClickListener(this);
    
    // 初始化数据库实例
    databaseClass = DatabaseClass.getDatabase(this);
}

2. 异步执行数据库操作
   Room禁止在主线程执行数据库查询，直接调用getAllData()会抛出异常，需用协程或AsyncTask处理异步操作。示例用协程修改setAdapter：

private void setAdapter()
{
    // 在IO线程执行查询
    CoroutineScope(Dispatchers.IO).launch {
        List<EntityClass> classList = databaseClass.EventDao().getAllData();
        // 回到主线程更新UI
        runOnUiThread(() -> {
            eventAdapter = new EventAdapter(getApplicationContext(), classList);
            recyclerView.setAdapter(eventAdapter);
        });
    }
}

需确保项目已添加协程依赖，或使用其他异步方案替代。

内容的提问来源于stack exchange，提问作者Man Steel