Type constraints
- Dart course
- Intermediate
- Lesson 32/116
- 5 min
Learn type constraints with extends.
Type constraints (extends)
Sometimes T must not be any type but only a specific subset. For this the type parameter gets an upper bound with extends: <T extends num>.
Example: T max<T extends num>(T a, T b) { return a > b ? a : b; }. Here T can only be a subtype of num; that is int or double. The > comparison is defined on num, so it compiles. Without the constraint T could be anything and > would not be available. A constraint gives generic code a strong contract.
Why constrain?
Without a constraint T covers all types; but that means you can only call Object methods on T. Operators like +, -, > raise a compile error.
When you write <T extends num>, Dart knows T carries num behavior; +, -, > and compareTo become available. This is how mathematical helper methods are written generically and type-safely. For example average, max and min methods work for every number type with a single T extends num definition.
Practice Write a max<T extends num> method
Define a constrained generic method named T max<T extends num>(T a, T b); its body should be return a > b ? a : b;. In main, call print(max<int>(3, 7));.
Starter code
// Define the max<T extends num> method
void main() {
// Call max<int>(3, 7) and print
}Sample solution
// Define the max<T extends num> method
T max<T extends num>(T a, T b) {
return a > b ? a : b;
}
void main() {
print(max<int>(3, 7));
}Expected output
7
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Match the constraint parts
| Term | Meaning |
|---|---|
extends |
Constraint keyword |
T |
Type parameter |
num |
Upper bound |
2 What does the <T extends num> constraint provide?
- T can only be String
- T can only be bool
- T can be any type
- T can only be num (int or double) Correct answer
Explanation: extends num limits T to int and double.
3 Add a constraint to max
T max<T ____>(T a, T b) {
return a > b ? a : b;
}T max<T extends num>(T a, T b) {
return a > b ? a : b;
}Explanation: The <T extends num> constraint limits max to numbers.