Null safety and Maps

Question

How do you handle the following case:

void main() {
  print(getInt('d'));
}

Map<String, int> myMap = {'a':1, 'b':2, 'c':3};

int getInt(String key) {
  if (!myMap.containsKey(key)) {
    myMap[key] = 0;
  }
  return myMap[key]!; 
}

myMap[key] is never null, but if I remove the ! of return myMap[key]!; I get the null safety error:

A value of type 'int?' can't be returned from the function 'getInt' because it has a return type of 'int'.

Is it proper to cast away the nullability with !, or is there a cleaner way?

score 3 · Answer 1 · answered Mar 12 '21 at 16:32

3

Not sure what cleaner means, and the ! does exist for a reason.

You could simplify that if statement to a single line:

int getInt(String key) {
  myMap[key] ??= 0;
  return myMap[key]!; 
}

answered Mar 12 '21 at 16:32

Christopher Moore

15,626
10
42
52

fyi, assignments in dart also return the value, and then it's even smart enough here to know it's not null. so you could simplify it even more by saying `return myMap[key] ??= 0;` – Ivo Mar 24 '22 at 13:07
@IvoBeckers Nice! That looks even "cleaner". – Christopher Moore Mar 24 '22 at 13:17

score 3 · Answer 2 · answered May 23 '21 at 08:12

From the docs:

The index [] operator on the Map class returns null if the key isn’t present. This implies that the return type of that operator must be nullable.

Map<String, int> map = {'a': 1};
int i = map['a']; // Error

Solutions:

Use ?? and provide a default value.
```
int i = map['a'] ?? 0;
```
Use Bang ! operator.
```
int i = map['a']!;
```

score 1 · Answer 3 · answered Mar 12 '21 at 20:03

1

There is no way you can get rid of the bang when you're using a map.

This might be a cleaner way to code the same thing.

int getInt(String key) => myMap[key] ?? 0;

answered Mar 12 '21 at 20:03

Suman Maharjan

1,070
7
14

Null safety and Maps

3 Answers3