Null safety

String title = "HTML Fundamentals";  // cannot be null
String? subtitle;                       // nullable, explicitly marked

print(subtitle?.length);       // safe access, prints null if subtitle is null
print(subtitle ?? "No subtitle"); // fallback if null

Like Kotlin and Swift, Dart requires you to opt in to nullability explicitly with ? -- the compiler won't let a non-nullable variable ever hold null.

Beyond ?. (safe access) and ?? (fallback value), Dart has a few more null-safety operators worth knowing:

String? subtitle;

subtitle ??= "Untitled"; // assign only if subtitle is currently null
print(subtitle!.length);  // "!" (the bang operator) asserts "trust me, this isn't null"

The ! operator is a promise to the compiler, not a safety check -- if the value actually is null at runtime, your program throws an exception immediately. Reach for it only when you're certain (for example, right after an explicit if (subtitle != null) check that Dart's analyzer can't otherwise see through), and prefer ?./?? wherever possible since they can never blow up at runtime.

Nullability also applies to class fields, and it interacts directly with constructors. A non-nullable field must be initialized -- either with a default value, in the constructor parameter list, or marked required:

class Course {
  final String title;      // non-nullable: must always be set
  final String? subtitle;  // nullable: optional, defaults to null if omitted
  final double price;

  Course({
    required this.title,
    this.subtitle,          // no `required` -- nullable fields can be safely left out
    required this.price,
  });
}

final c = Course(title: "Dart Mastery", price: 149.99); // subtitle defaults to null
print(c.subtitle ?? "No subtitle");

This is also why null safety is such a big deal for Flutter apps specifically: a huge share of real-world mobile app crashes historically came from a value the developer assumed would be present turning out to be null at runtime (a network response that failed, a widget that hasn't loaded yet). Dart's sound null safety catches the vast majority of these at compile time instead of in a crash report from a user's device.