Step 1 - Non-nullable by default

void main() {
  String name = 'Asha';
  name = 'Rafi';
}

• String cannot hold null. Assigning null is a compile error.

Step 2 - Nullable type

void main() {
  String? nick = null;
  nick = 'Ace';
  print(nick);
}

• T? means T or null.

Step 3 - Passing between worlds

String requireName(String? input) {
  if (input == null) {
    return 'Anonymous';
  }
  return input;
}

void main() {
  print(requireName(null));
  print(requireName('Asha'));
}

• After a null check, Dart promotes input to String in that branch.

Good habit

• Prefer T for values that must exist; use T? when absence is real data.

Practice tasks

• Change requireName to return String? and return null for empty string after trim.
• List three APIs in dart:core that return nullable types and why.