Null safety problem - map contains non nullable values

Question

Why is there an error:

A value of type 'TestBloc?' can't be returned from the method 'createFromId' because it has a return type of 'TestBloc.

The map contains values of type TestBloc (not of type TestBloc?) so it is impossible to assign a null value.

class TestBloc {
  String id;
  TestBloc({
    required this.id,
  });
}

class TestBlocFactory {
  final Map<String, TestBloc> _createdElements = HashMap<String, TestBloc>();

  TestBloc createFromId(String id) {
    if (_createdElements.containsKey(id)) {
      return _createdElements[id]; // !! ERROR !!
    } else {
      TestBloc b = TestBloc(id: id);
      _createdElements[id] = b;
      return b;
    }
  }
}

eamirho3ein · Answer 1 · 2022-12-28T16:07:23.527

0

You may define you map as <String, TestBloc> but still _createdElements[id] may return a null value, because the id key may not available so it may return null but because you check it in your if condition you can use ! (as @MendelG said) or you can cast it like this:

if (_createdElements.containsKey(id)) {
  return _createdElements[id] as TestBloc; // <--- change this
} else {
  TestBloc b = TestBloc(id: id);
  _createdElements[id] = b;
  return b;
}

edited Dec 28 '22 at 16:07

answered Dec 28 '22 at 15:57

eamirho3ein

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How will it return `null` if he checks for the key first? – MendelG Dec 28 '22 at 15:57
dart thinks that may be null its not gonna be null because there are checking for the key @MendelG – eamirho3ein Dec 28 '22 at 16:00
not mistake, its the way dart works, using `!`could do the work too. @MendelG – eamirho3ein Dec 28 '22 at 16:02
@PiotrTemp was it what you want? – eamirho3ein Dec 29 '22 at 06:29

MendelG · Answer 2 · 2022-12-28T16:02:52.683

0

Since you are checking that the map contains the correct id with _createdElements.containsKey, you know for sure that it won't return null. So, it's safe to use the ! operator which says "I won't be null"

class TestBloc {
  String id;
  TestBloc({
    required this.id,
  });
}

class TestBlocFactory {
  final Map<String, TestBloc> _createdElements = HashMap<String, TestBloc>();

  TestBloc createFromId(String id) {
    if (_createdElements.containsKey(id)) {
      return _createdElements[id]!;
    } else {
      TestBloc b = TestBloc(id: id);
      _createdElements[id] = b;
      return b;
    }
  }
}

I know that "!" will do the job, but I didn't understand why is it necessary when types are strictly given. So I looked for definition of [] operator of Map class which is: ```V? operator [](Object? key);``` Operator [] can return null (even when value type is non nullable) if key is null. – Piotr Temp Dec 28 '22 at 18:45
@PiotrTemp I didn't fully understand, are you saying or asking? are you saying that it isn't safe to use `!` in this example? – MendelG Dec 28 '22 at 18:49
My question was why in Map there are non nullable values, but when we access them thay are nullable. Now I understand. Thanks for disscussion. – Piotr Temp Dec 28 '22 at 18:51
@PiotrTemp Got you. Would you consider marking which ever answer has helped you the most [as accepted](https://meta.stackexchange.com/questions/5234/how-does-accepting-an-answer-work) (Click on the checkmark next to the answer) – MendelG Dec 28 '22 at 19:13

Null safety problem - map contains non nullable values

2 Answers2

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