为什么我的异常需要在 try/catch 中？ [关闭]

Question

我正在尝试从数据库中获取信息。并检查用户名是否已经存在 我正在使用 google cloud firestore 数据库。 我将该方法声明为抛出我创建的异常。 当我尝试调用抛出异常时，它告诉我需要将其放入 try/catch 块中。 我想知道为什么，因为我做了类似的事情（但是和我的老师一起使用 sqlite 并且效果很好）

我的代码：

@Override
public void isUserExists(final String username, String email) throws UserExistsException {
    Log.e(TAG, "isUserExists: in start of method" );
    // gets the document reference
    CollectionReference usersRef = db.collection(COLLECTION_NAME);
    //Creates and returns a new Query with the additional filter that documents must contain the specified field and the value should be equal to the specified value.
    Query query = usersRef.whereEqualTo(KEY_USERNAME, username);
    //Executes the query and returns the results as a QuerySnapshot.
    // QuerySnapshot = A QuerySnapshot contains the results of a query. It can contain zero or more DocumentSnapshot objects. (is an iterable)
    query.get().addOnCompleteListener(new OnCompleteListener<QuerySnapshot>() {
        @Override
        public void onComplete(@NonNull Task<QuerySnapshot> task) {
            Log.e(TAG, "isUserExists: in the onComplete" );
            //A DocumentSnapshot contains data read from a document in your Cloud Firestore database.
            if (task.isSuccessful()) {
                Log.e(TAG, "isUserExists: in the if statement" );
                for (DocumentSnapshot item : task.getResult()) {
                    Log.e(TAG, "isUserExists: in for loop" );
                    //getString() = Returns the value of the field as a String.
                    String user = item.getString(KEY_USERNAME);
                    if (user.equals(username)) {
                      //this throw line will not compile
                        throw new UserExistsException();
                    }
                }
            }
        }
    });
}

我正在实现这个接口：

public interface IUser {

void isUserExists(String username, String email) throws UserExistsException;
void isPassesMatch(String password, String rePass) throws PasswordMismatchException;
void checkLength(String username, String password) throws PasswordLengthException, UserNameLengthException;
void checkUserCred(String username, String password) throws UserCredentialException;
void registerUser (String username, String password, String email) throws UserException;
}

我在课堂上和老师做过的类似的事情：

 @Override
public void userExists() throws UserExistsException {
    String sqlStatement = "SELECT * FROM " + TABLE_NAME + " WHERE userName = '" + this.userName + "'";
    Cursor res = db.rawQuery(sqlStatement, null);
    if(res.moveToFirst()){
        throw new UserExistsException();
    }
}

编辑（添加我抛出的异常）：

public class UserExistsException extends UserException {

public UserExistsException(String message) {
    super(message);
}

public UserExistsException() {
    super("User Already Exists");
}
}

另一个编辑（添加处理方法及其异常的位置 ):

private void register() {
    String uName = regActEtUname.getText().toString();
    String uPass = regActEtPass.getText().toString();
    String rePass = regActEtRePass.getText().toString();
    String uEmail = regActEtEmail.getText().toString();

    try {
        // checks if the user exissts
        utils.isUserExists(uName,uEmail);
        //checks if the length is fulfilled
        utils.checkLength(uName,uPass);
        //checks if the passwords match
        utils.isPassesMatch(uPass, rePass);
        //registers the user
        utils.registerUser(uName,uPass,uEmail);

    } catch (UserExistsException | PasswordLengthException | UserNameLengthException | PasswordMismatchException e) {
        Log.e("fbdb", "register: " + e.getMessage() );
        TastyToast.makeText(context,e.getMessage(), TastyToast.LENGTH_LONG, TastyToast.ERROR);
    } catch (UserException e) {
        Log.e("regErr", "register: " + e.getMessage() );
    } catch (Exception e) {
        Log.e("regErr", "register: " + e.getMessage() );
    }
}

Answer 1

我建议在 isUserExists 方法中使用布尔变量，我们可以在 onComplete 方法中更新该变量并依靠它来抛出或不抛出 UserExistsException。

@Override
public void isUserExists(final String username, String email) throws UserExistsException {
    Log.e(TAG, "isUserExists: in start of method" );
    boolean isUserExist = false;
    // gets the document reference
    CollectionReference usersRef = db.collection(COLLECTION_NAME);
    //Creates and returns a new Query with the additional filter that documents must contain the specified field and the value should be equal to the specified value.
    Query query = usersRef.whereEqualTo(KEY_USERNAME, username);
    //Executes the query and returns the results as a QuerySnapshot.
    // QuerySnapshot = A QuerySnapshot contains the results of a query. It can contain zero or more DocumentSnapshot objects. (is an iterable)
    query.get().addOnCompleteListener(new OnCompleteListener<QuerySnapshot>() {
        @Override
        public void onComplete(@NonNull Task<QuerySnapshot> task) {
            Log.e(TAG, "isUserExists: in the onComplete" );
            //A DocumentSnapshot contains data read from a document in your Cloud Firestore database.
            if (task.isSuccessful()) {
                Log.e(TAG, "isUserExists: in the if statement" );
                for (DocumentSnapshot item : task.getResult()) {
                    Log.e(TAG, "isUserExists: in for loop" );
                    //getString() = Returns the value of the field as a String.
                    String user = item.getString(KEY_USERNAME);
                    if (user.equals(username)) {
                      isUserExist = true;
                    }
                }
            }
        }
    });
    if (isUserExist) throw new UserExistsException();
}

Answer 2

试试这个代码：

@Override
public void isUserExists(final String username, String email) throws 
 UserExistsException {

     CollectionReference allUsersRef = db.collection(COLLECTION_NAME);
     Query userNameQuery = allUsersRef.whereEqualTo(KEY_USERNAME, username);
     userNameQuery.get().addOnCompleteListener(new OnCompleteListener<QuerySnapshot>() {
       @Override
       public void onComplete(@NonNull Task<QuerySnapshot> task) {
          if (task.isSuccessful()) {
               for (DocumentSnapshot document : task.getResult()) {
                  if (document.exists()) {
                     String userName = document.getString(username);
                     Log.d(TAG, "username already exists");
                  } else {
                     Log.d(TAG, "username does not exists");
                  }
               }
          } else {
             Log.d("TAG", "Error getting documents: ", task.getException());
          }
      }
    });
}

如果对你有帮助，请告诉我。