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?

Solution 1

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]!; 
}

Solution 2

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']!;
  

Solution 3

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;

Comments

[Ivo]

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;

[Christopher Moore]

@IvoBeckers Nice! That looks even "cleaner".