Java 从Android Room检索双精度值导致NullPointerException

我有一个安卓房间DB和下一个POJO

  Entity(tableName = "transactions")
    public class BoardItem {
        @PrimaryKey(autoGenerate = true)
        int itemID;
        String account;
        String date;
        String category;
        double value:
//getters and setters
    }

对于指定类别的所有值的加载和，我尝试使用下一个DAO

@Query ("SELECT category, SUM (value) as total FROM transactions WHERE category =:transactionCategory GROUP BY category ")
double getAllTransactionInCategory(String transactionCategory);

以及存储库android室中的下一个代码：

public class RoomTransactionsRepository {
    private final RoomTransactionsDAO mTransactionsDao;
    private final LiveData<List<TransactionBoardItem>> mAllTransactions;

    RoomTransactionsRepository(Application application){
            RoomTransactionsDatabase db = RoomTransactionsDatabase.getDatabase(application);
            mTransactionsDao = db.transactions_dao();
            mAllTransactions = mTransactionsDao.getAllTransactions();
    }

    LiveData<List<TransactionBoardItem>> getAllTransactions(){
        return mAllTransactions;
    }

    double getAllTransactionInCategory (String category){
        return mTransactionsDao.getAllTransactionInCategory (category);
    }

    //Livedata code...

要显示，我使用下一个代码：

public class StatisticActivity extends AppCompatActivity {
    RoomTransactionsRepository repository;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        // init code 

         binding.totalValue.setText (String.valueOf (repository.getAllTransactionInCategory ("Food")));
    }
}

RoomTransactionsViewModel model = new ViewModelProvider (this).get(RoomTransactionsViewModel.class);

        model.getAllTransactionInCategory("Food", item -> {
            binding.totalValue.setText (String.valueOf (item.getTransactionValue ()));
            binding.category.setText (String.valueOf (item.getTransactionCategory ()));
        });

在DAO或存储库代码中，当我试图显示要从db中提取的双精度值时，我做错了什么导致了NullPointerException？ 提前谢谢

更新：VievModel类。在单独的活动中，动态地将数据库中的数据插入到RecyclerView中是很有用的

public class RoomTransactionsViewModel extends AndroidViewModel {
    private final RoomTransactionsRepository mRepo;
    private final LiveData<List<TransactionItem>> mAllTransactions;

    public RoomTransactionsViewModel(Application application){
        super(application);
        mRepo = new RoomTransactionsRepository(application);
        mAllTransactions  = mRepo.getAllTransactions();
    }

    LiveData<List<TransactionBoardItem>> getAllTransactions(){
        return mAllTransactions;
    }

    public void insert(TransactionItem item){mRepo.insertTransaction(item);}
    public void deleteAll(){mRepo.deleteAllTransactions();}
    
}

步骤2：我创建了应答中的接口，但将double替换为POJO class

void onReturned（TransactionStatististicItem项）

步骤3：根据答案升级存储库代码，但有更改：

//To retrieve data asynchronous from DB you can use : ExecutorService (java) / Coroutines (kotlin) or AsyncTask (deprecated in API level 30)
 public static final ExecutorService singleThreadExecutor =
            Executors.newSingleThreadExecutor();

    void getAllTransactionInCategory (String category, TransactionStatisticListener listener){
            singleThreadExecutor.execute(() -> listener.onReturned(mTransactionsDao.getAllTransactionInCategory (category)));
        }

第4步：在ViewModel中添加以下几行

 void getAllTransactionInCategory (String category, TransactionStatisticListener listener){
        mRepo.getAllTransactionInCategory (category, listener);
    }

步骤5：最后在活动中，我添加了下一个代码：

public class StatisticActivity extends AppCompatActivity {
    RoomTransactionsRepository repository;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        // init code 

         binding.totalValue.setText (String.valueOf (repository.getAllTransactionInCategory ("Food")));
    }
}

RoomTransactionsViewModel model = new ViewModelProvider (this).get(RoomTransactionsViewModel.class);

        model.getAllTransactionInCategory("Food", item -> {
            binding.totalValue.setText (String.valueOf (item.getTransactionValue ()));
            binding.category.setText (String.valueOf (item.getTransactionCategory ()));
        });

最后，我能够检索下一个示例数据：“食品”类别中所有事务的总和

为了遵循Google MVVM准则，您不应该从活动访问存储库，因为它被认为是数据库的一个层，不应该由视图处理，而应该由
ViewModel

另一个问题是，在不使用侦听器的情况下从数据库返回一个双精度值；由于从数据库获取值需要一些时间，因此
getAllTransactionCategory（）
不会返回预期值。这可以通过使用侦听器接口来解决，或者返回
LiveData
并在活动中观察它

应用它：

因此，建议您通过
ViewModel
访问此选项：

Dao：返回一个
LiveData

更新：

“这可以通过使用监听器界面来解决”你能详细说明一下吗

要使用侦听器，您需要一个接口，该接口具有一个回调方法，该方法接受您希望从文件室返回的类型，在本例中，它是一个
double

public interface TransactionListener {
     void onReturned(double count);
}

然后在Room返回如下值时使用此接口的实现：

道：

视图模型：

public class RoomTransactionsViewModel extends AndroidViewModel {
   
   // ...
    
    
    LiveData<Double> getAllTransactionInCategory (String category){
        return mRepo.getAllTransactionInCategory (category);
    }
}

public class RoomTransactionsViewModel extends AndroidViewModel {
   
   // ...
    
    
    void getAllTransactionInCategory (String category, TransactionListener listener){
        mRepo.getAllTransactionInCategory (category, listener);
    }
}

存储库：

public class RoomTransactionsRepository {

    //.....
    
    LiveData<Double> getAllTransactionInCategory (String category){
        return mTransactionsDao.getAllTransactionInCategory (category);
    }

public class RoomTransactionsRepository {

    //.....
    
    void getAllTransactionInCategory (String category, TransactionListener listener){
 
        listener.onReturned(mTransactionsDao.getAllTransactionInCategory (category));
    }

活动：

public class StatisticActivity extends AppCompatActivity {
    RoomTransactionsRepository repository;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        // init code 
        
        
        RoomTransactionsViewModel model = new ViewModelProvider(this).get(RoomTransactionsViewModel.class);
        
            
        model.getAllTransactionInCategory("Food", new TransactionListener() {
            @Override
            public void onReturned(double count) {
                binding.totalValue.setText (String.valueOf (count));
            }
        });
        
    }
}

能否显示您在何处初始化视图模型
RoomTransactionRepository
instances@Zain更新，这是吗？独立活动中涉及的ViewModel允许在RecyclerView中动态插入POJO项目。谢谢！“这可以通过使用监听器界面来解决”你能详细说明一下吗？我还更新了帖子，提供了关于我想要什么的其他信息。@GremlinShX请在回答中查看更新部分。谢谢你，我使用你的回答作为回答我自己问题的基础，但是如果有更改，你可以在问题中查看更新3。还有一件事，您有一个错误：ViewModel类中的void GetAllTransactionCategory不应该使用“return”关键字。
@Query ("SELECT category, SUM (value) as total FROM transactions WHERE category =:transactionCategory GROUP BY category ")
double getAllTransactionInCategory(String transactionCategory);

public class RoomTransactionsViewModel extends AndroidViewModel {
   
   // ...
    
    
    void getAllTransactionInCategory (String category, TransactionListener listener){
        mRepo.getAllTransactionInCategory (category, listener);
    }
}

public class RoomTransactionsRepository {

    //.....
    
    void getAllTransactionInCategory (String category, TransactionListener listener){
 
        listener.onReturned(mTransactionsDao.getAllTransactionInCategory (category));
    }

public class StatisticActivity extends AppCompatActivity {
    RoomTransactionsRepository repository;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        // init code 
        
        
        RoomTransactionsViewModel model = new ViewModelProvider(this).get(RoomTransactionsViewModel.class);
        
            
        model.getAllTransactionInCategory("Food", new TransactionListener() {
            @Override
            public void onReturned(double count) {
                binding.totalValue.setText (String.valueOf (count));
            }
        });
        
    }
}