我怎样才能在firebase中修复这个null？答案

【问题标题】：how can i fix this null in firebase?我怎样才能在firebase中修复这个null？
【发布时间】：2017-08-28 16:19:59
【问题描述】：

我正在尝试从 FirebaseDatabase 导入数据，它正在工作但不适用于此类，当我尝试获取它的数据时，它会给我一个空值，这是它称为 RoomsAvailable 的类，谁能告诉我为什么我得到空值，我该如何解决它。

public class RoomsAvailable {

private int onePerson;
private int twoPerson;
private int threePerson;
private int fourPerson;



public RoomsAvailable() {

}
public RoomsAvailable(int onePerson, int twoPerson, int threePerson, int fourPerson) {
    this.onePerson = onePerson;
    this.twoPerson = twoPerson;
    this.threePerson = threePerson;
    this.fourPerson = fourPerson;
}

public int getOnePerson() {
    return onePerson;
}

public void setOnePerson(int onePerson) {
    this.onePerson = onePerson;
}

public int getTwoPerson() {
    return twoPerson;
}

public void setTwoPerson(int twoPerson) {
    this.twoPerson = twoPerson;
}

public int getThreePerson() {
    return threePerson;
}

public void setThreePerson(int threePerson) {
    this.threePerson = threePerson;
}

public int getFourPerson() {
    return fourPerson;
}

public void setFourPerson(int fourPerson) {
    this.fourPerson = fourPerson;
}


@Override
public String toString() {
    return "RoomsAvailable{" +
            "onePerson=" + onePerson +
            ", twoPerson=" + twoPerson +
            ", threePerson=" + threePerson +
            ", fourPerson=" + fourPerson +
            '}';
}

}

这是我的代码

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    final List<Hotel> hotelList = new ArrayList<>();
    mDatabaseReference = FirebaseDatabase.getInstance().getReference();
    mDatabaseReference.child("0/"+"hotels").addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            for (int i = 0; i < dataSnapshot.getChildrenCount(); i++) {
                Hotel hotel = dataSnapshot.child(i + "").getValue(Hotel.class);
                hotelList.add(hotel);

                Log.d(TAG, hotel.getId() + "\n");
                Log.d(TAG, hotel.getCityId() + "\n");
                Log.d(TAG, hotel.getName() + "\n");


                Log.d(TAG, hotel.getRoomsAvailable() + "\n");

            }
            RecyclerView recyclerView = (RecyclerView) findViewById(R.id.cities_recyclerview);
            recyclerView.setLayoutManager(new LinearLayoutManager(getApplicationContext()));
            RecyclerView.Adapter adapter = new HotelAdapter(getApplicationContext(), hotelList);
            recyclerView.setAdapter(adapter);
        }
        @Override
        public void onCancelled(DatabaseError databaseError) {
            Log.w(TAG, "onCreate:Failed to read hotelList.", databaseError.toException());

        }



    });


}

这是我的 Firebase 数据库

问：有什么问题？

问：我该如何解决？

【问题讨论】：

请向我们展示您的Hotel 课程...

标签： java android json firebase firebase-realtime-database

【解决方案1】：

您总是得到null，因为您在onDataChange() 方法之外声明了hotelList。这是由于此方法的异步行为而发生的，甚至在您将这些酒店添加到列表之前就会调用该方法。所以为了解决这个问题，请将列表的声明移到onDataChange() 内，或者如果你想在外面使用它，请参阅我的答案表格post。

您的代码应如下所示：

mDatabaseReference.child("0/"+"hotels").addValueEventListener(new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        final List<Hotel> hotelList = new ArrayList<>();
        for (int i = 0; i < dataSnapshot.getChildrenCount(); i++) {
            Hotel hotel = dataSnapshot.child(i + "").getValue(Hotel.class);
            hotelList.add(hotel);

            Log.d(TAG, hotel.getId() + "\n");
            Log.d(TAG, hotel.getCityId() + "\n");
            Log.d(TAG, hotel.getName() + "\n");
            Log.d(TAG, hotel.getRoomsAvailable() + "\n");
        }
        Log.d("TAG", hotelList);
        RecyclerView recyclerView = (RecyclerView) findViewById(R.id.cities_recyclerview);
        recyclerView.setLayoutManager(new LinearLayoutManager(getApplicationContext()));
        RecyclerView.Adapter adapter = new HotelAdapter(getApplicationContext(), hotelList);
        recyclerView.setAdapter(adapter);
    }
    @Override
    public void onCancelled(DatabaseError databaseError) {
        Log.w(TAG, "onCreate:Failed to read hotelList.", databaseError.toException());

    }
});

【讨论】：

【解决方案2】：

你为什么不这样尝试

@Override
 protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
final List<Hotel> hotelList = new ArrayList<>();
mDatabaseReference = FirebaseDatabase.getInstance().getReference();
mDatabaseReference.child("0/"+"hotels").addValueEventListener(new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {

            hotelList = (List<Hotel>) dataSnapshot.getValue(Hotel.class);

        RecyclerView recyclerView = (RecyclerView) findViewById(R.id.cities_recyclerview);
        recyclerView.setLayoutManager(new LinearLayoutManager(getApplicationContext()));
        RecyclerView.Adapter adapter = new HotelAdapter(getApplicationContext(), hotelList);
        recyclerView.setAdapter(adapter);
    }
    @Override
    public void onCancelled(DatabaseError databaseError) {
        Log.w(TAG, "onCreate:Failed to read hotelList.", databaseError.toException());

    }



});

}

【讨论】：