Firebase数据库引用错误:没有这样的实例字段：'mDatabaseReference‘

DatabaseReference mDatabaseReference;

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

    // 初始化 Firebase 数据库引用
    mDatabaseReference = FirebaseDatabase.getInstance().getReference();
}

DatabaseReference mDatabaseReference; // 注意大小写

public class MainActivity extends AppCompatActivity {
    DatabaseReference mDatabaseReference;

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

        // 初始化 Firebase 数据库引用
        mDatabaseReference = FirebaseDatabase.getInstance().getReference();
    }
}

import android.os.Bundle;
import androidx.appcompat.app.AppCompatActivity;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;

public class MainActivity extends AppCompatActivity {
    DatabaseReference mDatabaseReference;

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

        // 初始化 Firebase 数据库引用
        mDatabaseReference = FirebaseDatabase.getInstance().getReference();

        // 示例：写入数据
        mDatabaseReference.child("users").child("user1").setValue("John Doe");
    }
}