RXJAVA ROOM android

Question

[尝试学习Room和RXJAVA。我对此有大约80％的了解，但是我被困在找出其余的问题上。

这是我在插入数据上遇到的错误。

java.lang.NullPointerException：尝试调用接口方法'无效com.example.learnroom.EntityDao.insert（com.example.learnroom.Entitys）'在空对象引用上

如果我不运行try catch，则会收到以下似乎与之相关的错误。

java.lang.RuntimeException：无法启动活动ComponentInfo {com.example.learnroom / com.example.learnroom.MainActivity}：java.lang.NullPointerException：尝试调用接口方法'io.reactivex。也许com.example.learnroom.EntityDao.getEntity（java.lang.String）'上的null对象参考

我该如何解决？

我已尝试使用recyclerviews从网络上的大多数教程简化为仅两个文本字段。他们说这是3个部分，但看起来却不太像，因为从未设置过数据库，所以我在运行代码的方法中运行了它。也许有人可以帮我解释一下这是如何工作的。我的代码

Dao

public interface EntityDao {

 @Query("SELECT * FROM Entitys WHERE ID  = :ID LIMIT 1")
Maybe<List<Entitys>> getEntity(String ID);

@Insert(onConflict = OnConflictStrategy.REPLACE)
void insert(Entitys entitys);

@Query("DELETE FROM Entitys")
void deleteAllEntity();
}

实体

 public class Entitys {

@PrimaryKey
@NonNull
public String ID;

public  String ts;
public  String tss;

public Entitys(@NonNull String ID, String ts, String tss) {
    this.ID = ID;
    this.ts = ts;
    this.tss = tss;
}

public String getTss() {
    return tss;
}

public void setTss(String tss) {
    this.tss = tss;
}

public void setID(String ID) {
    this.ID = ID;
}

public void setTs(String ts) {
    this.ts = ts;
}

public String getID() {
    return ID;
}

public String getTs() {
    return ts;
}
 } 

数据库

 @Database(entities = {Entitys.class}, version = 1)
 public abstract class PathwaysDB extends RoomDatabase {

private static volatile PathwaysDB INSTANCE;

public static EntityDao entityDao() {
    return null;
}

public static PathwaysDB getInstance(Context context) {
    if (INSTANCE == null) {
        synchronized (PathwaysDB.class) {
            if (INSTANCE == null) {
                INSTANCE = Room.databaseBuilder(context.getApplicationContext(),
                        PathwaysDB.class, "Pathwaysdb")
                        .build();
            }
        }
    }
    return INSTANCE;
}


 }

MainActivity

public class MainActivity extends AppCompatActivity implements View.OnClickListener {

private static final String TAG = MainActivity.class.getSimpleName();
Button tb;
EditText te, tes;
String ts, tss, ID;

CompositeDisposable compositeDisposable = new CompositeDisposable();

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

    ID ="test";
    te = findViewById(R.id.te);

    tb = findViewById(R.id.tb);
    tb.setOnClickListener(this);

    tes = findViewById(R.id.tes);

    Builddb();

    try{
    getData();}catch (Exception e){}
}

   private void Builddb() {

    Completable.fromAction(() -> PathwaysDB.getInstance(this))
            .subscribeOn(Schedulers.io())
            .observeOn(AndroidSchedulers.mainThread())
            .subscribe(new CompletableObserver() {
                @Override
                public void onSubscribe(Disposable d) {
                    compositeDisposable.add(d);
                }

                @Override
                public void onComplete() {
                    // action was completed successfully
                }

                @Override
                public void onError(Throwable e) {
                    // something went wrong
                }
            });
}

private void getData() {

    Maybe<List<Entitys>> single = entityDao().getEntity(ID);
    single.subscribeOn(Schedulers.io())
            .observeOn(AndroidSchedulers.mainThread())
            .subscribe(new MaybeObserver<List<Entitys>>() {
                @Override
                public void onSubscribe(Disposable d) {
                    // add it to a CompositeDisposable
                }

                @Override
                public void onSuccess(List<Entitys> entity) {
                    te.setText(entity.indexOf(ts));
                    tes.setText(entity.indexOf(tss));
                }
                @Override
                public void onError(Throwable e) {
                    // show an error message
                }

                @Override
                public void onComplete() {
                }
            });

    compositeDisposable.add((Disposable) single);
}

@Override
protected void onDestroy() {
    super.onDestroy();

    compositeDisposable.dispose();
}

private void updateUserName() {

   ts = te.getText().toString();
   tss = tes.getText().toString();
   Entitys entitys = new Entitys(ID, ts, tss);

    Completable.fromAction(() -> entityDao().insert(entitys))
            .subscribeOn(Schedulers.io())
            .observeOn(AndroidSchedulers.mainThread())
            .subscribe(new CompletableObserver() {
                @Override
                public void onSubscribe(Disposable d) {
                    compositeDisposable.add(d);
                }

                @Override
                public void onComplete() {
                    // action was completed successfully
                }

                @Override
                public void onError(Throwable e) {
                    // something went wrong
                }
            });


}

@Override
public void onClick(View view) {
    updateUserName();

    Intent forward = new Intent(this, secondpage.class);
    startActivity(forward);
}
 }

Answer 1

崩溃的原因是您的PathwaysDB类中的这一行

public static EntityDao entityDao() {
return null;
}

它返回null。它应该像

public abstract EntityDao entityDao()

Answer 2

您忘记将@Dao注释添加到EntityDao接口类。您还需要更改以下方法：

public static EntityDao entityDao() {
    return null;
}

收件人

public abstract EntityDao entityDao();