Java 如何检查房间数据库是否为空
在打网络电话之前,我试图检查房间数据库是否为空。 但它显示了下面的错误 错误:不确定如何将游标转换为此方法的返回类型(java.lang.Integer) 我一直在做的是,当应用程序启动时,我正在检查房间数据库中的行数,如果它为空,那么我正在进行网络呼叫 下面是我的代码 UserDao.javaJava 如何检查房间数据库是否为空,java,android,android-room,dao,Java,Android,Android Room,Dao,在打网络电话之前,我试图检查房间数据库是否为空。 但它显示了下面的错误 错误:不确定如何将游标转换为此方法的返回类型(java.lang.Integer) 我一直在做的是,当应用程序启动时,我正在检查房间数据库中的行数,如果它为空,那么我正在进行网络呼叫 下面是我的代码 UserDao.java @Dao public interface UserDao { @Query("SELECT * FROM Users") LiveData<Integer> isDbEmpt
@Dao
public interface UserDao {
@Query("SELECT * FROM Users")
LiveData<Integer> isDbEmpty();
}
public class UserRepository {
private Context context;
private UserDb userDb;
private LiveData<Integer> checkDb;
private UserDao userDao;
public UserRepository(Context context) {
this.context = context;
userDb = UserDb.getInstance(context);
userDao = userDb.userDao();
checkDb = userDao.isDbEmpty();
}
public LiveData<Integer> isDbEmpty(){
return checkDb;
}
}
public class UserViewModel extends AndroidViewModel {
private UserRepository repo;
private LiveData<Integer> checkDb;
public UserViewModel(@NonNull Application application) {
super(application);
repo = new UserRepository(application);
checkDb = repo.isDbEmpty();
}
public LiveData<Integer> getCheckDb() {
return checkDb;
}
}
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Toolbar toolbar = findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
userRepository = new UserRepository(this);
userModel = ViewModelProviders.of(MainActivity.this).get(UserViewModel.class);
userModel.getCheckDb().observe(this, new Observer<Integer>() {
@Override
public void onChanged(Integer rowCount) {
if(rowCount == 0){
userRepository.getUserList();
}
}
});
}
@Dao
公共接口UserDao{
@查询(“从用户中选择*)
LiveData是空的();
}
UserRepository.java
@Dao
public interface UserDao {
@Query("SELECT * FROM Users")
LiveData<Integer> isDbEmpty();
}
public class UserRepository {
private Context context;
private UserDb userDb;
private LiveData<Integer> checkDb;
private UserDao userDao;
public UserRepository(Context context) {
this.context = context;
userDb = UserDb.getInstance(context);
userDao = userDb.userDao();
checkDb = userDao.isDbEmpty();
}
public LiveData<Integer> isDbEmpty(){
return checkDb;
}
}
public class UserViewModel extends AndroidViewModel {
private UserRepository repo;
private LiveData<Integer> checkDb;
public UserViewModel(@NonNull Application application) {
super(application);
repo = new UserRepository(application);
checkDb = repo.isDbEmpty();
}
public LiveData<Integer> getCheckDb() {
return checkDb;
}
}
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Toolbar toolbar = findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
userRepository = new UserRepository(this);
userModel = ViewModelProviders.of(MainActivity.this).get(UserViewModel.class);
userModel.getCheckDb().observe(this, new Observer<Integer>() {
@Override
public void onChanged(Integer rowCount) {
if(rowCount == 0){
userRepository.getUserList();
}
}
});
}
公共类用户存储库{
私人语境;
私有用户数据库;
私有LiveData-checkDb;
私有UserDao UserDao;
公共用户存储库(上下文){
this.context=上下文;
userDb=userDb.getInstance(上下文);
userDao=userDb.userDao();
checkDb=userDao.isDbEmpty();
}
公共LiveData是空的(){
返回checkDb;
}
}
UserViewModel.java
@Dao
public interface UserDao {
@Query("SELECT * FROM Users")
LiveData<Integer> isDbEmpty();
}
public class UserRepository {
private Context context;
private UserDb userDb;
private LiveData<Integer> checkDb;
private UserDao userDao;
public UserRepository(Context context) {
this.context = context;
userDb = UserDb.getInstance(context);
userDao = userDb.userDao();
checkDb = userDao.isDbEmpty();
}
public LiveData<Integer> isDbEmpty(){
return checkDb;
}
}
public class UserViewModel extends AndroidViewModel {
private UserRepository repo;
private LiveData<Integer> checkDb;
public UserViewModel(@NonNull Application application) {
super(application);
repo = new UserRepository(application);
checkDb = repo.isDbEmpty();
}
public LiveData<Integer> getCheckDb() {
return checkDb;
}
}
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Toolbar toolbar = findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
userRepository = new UserRepository(this);
userModel = ViewModelProviders.of(MainActivity.this).get(UserViewModel.class);
userModel.getCheckDb().observe(this, new Observer<Integer>() {
@Override
public void onChanged(Integer rowCount) {
if(rowCount == 0){
userRepository.getUserList();
}
}
});
}
公共类UserViewModel扩展了AndroidViewModel{
私有用户存储库回购;
私有LiveData-checkDb;
public UserViewModel(@NonNull应用程序){
超级(应用);
repo=新用户存储库(应用程序);
checkDb=repo.isDbEmpty();
}
公共LiveData getCheckDb(){
返回checkDb;
}
}
MainActivity.java
@Dao
public interface UserDao {
@Query("SELECT * FROM Users")
LiveData<Integer> isDbEmpty();
}
public class UserRepository {
private Context context;
private UserDb userDb;
private LiveData<Integer> checkDb;
private UserDao userDao;
public UserRepository(Context context) {
this.context = context;
userDb = UserDb.getInstance(context);
userDao = userDb.userDao();
checkDb = userDao.isDbEmpty();
}
public LiveData<Integer> isDbEmpty(){
return checkDb;
}
}
public class UserViewModel extends AndroidViewModel {
private UserRepository repo;
private LiveData<Integer> checkDb;
public UserViewModel(@NonNull Application application) {
super(application);
repo = new UserRepository(application);
checkDb = repo.isDbEmpty();
}
public LiveData<Integer> getCheckDb() {
return checkDb;
}
}
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Toolbar toolbar = findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
userRepository = new UserRepository(this);
userModel = ViewModelProviders.of(MainActivity.this).get(UserViewModel.class);
userModel.getCheckDb().observe(this, new Observer<Integer>() {
@Override
public void onChanged(Integer rowCount) {
if(rowCount == 0){
userRepository.getUserList();
}
}
});
}
@覆盖
创建时受保护的void(Bundle savedInstanceState){
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Toolbar Toolbar=findviewbyd(R.id.Toolbar);
设置支持操作栏(工具栏);
userRepository=新的userRepository(此);
userModel=ViewModelProviders.of(MainActivity.this.get)(UserViewModel.class);
userModel.getCheckDb().observe(这是一个新的观察者(){
@凌驾
更改后的公共void(整数行计数){
如果(行计数==0){
userRepository.getUserList();
}
}
});
}
有人请让我知道我做错了什么。任何帮助都将不胜感激
谢谢您的
UserDao
上的返回类型应该是LiveData
,选择后只需检查从该查询返回的列表的大小
当您放置整数
并且查询是从该表中选择时,房间
不知道用户
表如何匹配整数
@Query("SELECT * FROM Users")
LiveData<List<User>> selectAllUsers();
@Query(“从用户中选择*)
LiveData selectAllUsers();
调用该方法时,只需检查如果(usersList.size()!=0){}
@Query(“从用户中选择计数(*))
使用计数查询。