Type constraints

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.

Exercise 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));.

Show example 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));
}

Test yourself

Match the constraint parts

extends

Constraint keyword
T

Type parameter
num

Upper bound
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)
Show answer

T can only be num (int or double) — extends num limits T to int and double.

Add a constraint to max

T max<T ____>(T a, T b) {
  return a > b ? a : b;
}

Show answer

T max<T extends num>(T a, T b) {
  return a > b ? a : b;
}

Next lesson: Generic collections

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