What is the difference between `?`, `!`, `late`, and `required` keywords in Dart?

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Four keywords for working with Dart's null safety. ? — appended to a type to opt in to nullability. String? can be null or a String. ! — force-unwrap. Asserts the value is non-null; throws if it isn't. Almost always a code smell — defeats the type system. Use sparingly. late — deferred initialization. Variable is non-nullable but assigned after declaration. late final user is idiomatic for 'init once in initState , never again'. LateInitializationError if read before assigned. required — modifier on named parameters; caller must pass them. Common on widget constructors: Box({required this.size, this.color = Colors.black}) . Senior signal: **'`!` is almost always a smell'** — force-unwrap means you've defeated the type system. Most uses should be replaced with pattern matching, `?? throw`, or early-return guards. The other senior detail: **`late final` is the idiomatic way to express 'I'll initialize this in initState and never again'** — it gives you immutability *and* deferred init. Use `late` (non-final) sparingly — it can mask bugs.