Mastering Dart Null Safety: Say Goodbye to Runtime Crashes

// ❌ Compile-time Error in Dart 3:
String name = null; 

// ✅ Correct Non-nullable Variable:
String name = "Flutter Developer";

// ✅ Explicit Nullable Type using ? operator:
String? username = null;

// ✅ The late keyword — promises initialization before first read:
late String userId; // must be set before reading or throws LateInitializationError
userId = fetchUserId();
print(userId); // ✅ safe

String? fetchName() => null; // returns null

// ✅ Safe pattern — check before using
String? name = fetchName();
if (name != null) {
  print(name.length); // Dart knows name is non-null here
}

// ⚠️ Assertion — only use when you are CERTAIN it's not null
String? apiName = fetchFromCache(); // might return non-null
print(apiName!.length); // throws if apiName is actually null

// ❌ Anti-pattern — never do this blindly
print(fetchName()!.length); // always throws here (returns null above)

// Without required — callers can omit name and age (both default to null)
void greet({String? name, int? age}) {
  print('Hello ${name ?? "stranger"}, age ${age ?? "unknown"}');
}

// With required — callers MUST provide name and age or it won't compile
void greetUser({required String name, required int age}) {
  print('Hello $name, you are $age years old');
}

greetUser(name: "Priya", age: 28); // ✅ compiles
greetUser(age: 28);                // ❌ compile-time error: Missing required param 'name'

// Flutter real-world example:
class ProfileCard extends StatelessWidget {
  const ProfileCard({
    super.key,
    required this.username,
    required this.avatarUrl,
    this.bio,              // ← optional (nullable)
  });

  final String username;
  final String avatarUrl;
  final String? bio;

  @override
  Widget build(BuildContext context) => Text(username);
}