Sound null safety
- Dart course
- Expert
- Lesson 2/47
- 5 min
Learn sound null safety and the type system.
Sound null safety
Sound null safety, introduced in Dart 2.12, is a type system feature that catches null reference errors at compile time. A type is non-nullable by default: int x cannot hold null.
To make it nullable, write int?. This lets the type system answer 'can this variable be null?'. null can only be assigned to nullable types; otherwise a compile-time error is raised.
Working with nullable values
To use a nullable value in a non-nullable context, a check is required. Three common ways: guarantee non-null with ! (maybeAge!), provide a default with ?? (maybeAge ?? 0) and check with if (maybeAge != null).
! is risky: it throws at runtime if the value is null. ?? is safe; it returns the default instead. Prefer ?? to keep code safe.
Practice Resolve a nullable age
Define a function int resolveAge(int? maybeAge) => maybeAge ?? 0; that takes a nullable int? and returns an int. In main add print(resolveAge(null)); and print(resolveAge(25));.
Starter code
void main() {
// define a resolveAge function
// print calls with null and 25
}Sample solution
int resolveAge(int? maybeAge) => maybeAge ?? 0;
void main() {
print(resolveAge(null));
print(resolveAge(25));
}Expected output
0 25
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Nullable type declaration
int? maybeAge = ____;
print(maybeAge);int? maybeAge = null;
print(maybeAge);Explanation: a nullable (int?) variable can be assigned null.
2 What does sound null safety provide?
- Removes the garbage collector
- Produces faster output
- Catches null reference errors at compile time Correct answer
- Makes all variables nullable
Explanation: sound null safety catches null errors at compile time.
3 Match the null safety operators
| Term | Meaning |
|---|---|
int |
Non-nullable |
int? |
Nullable |
! |
Non-null assertion |
?? |
Default value |