Nullable vs Non-Nullable

Part of the Object Oriented Programming section of Coddy's Dart journey. Lesson 24 of 110.

Now that you understand why null safety matters, let's look at how to declare variables that can hold null. In Dart, you make a type nullable by adding a ? after the type name.

String name = 'Alice';   // Non-nullable - must always have a value
String? nickname;        // Nullable - can be null or a String

print(name);      // Alice
print(nickname);  // null

The key difference is that non-nullable types guarantee a value exists, while nullable types explicitly allow the absence of a value. This distinction applies to all types:

int count = 10;       // Must have a value
int? optionalCount;   // Can be null

List<String> items = [];      // Must be a list (can be empty)
List<String>? maybeItems;     // Could be null (no list at all)

Notice that an empty list [] is different from null. An empty list exists but contains nothing; null means no list exists at all.

The compiler enforces these rules strictly. You cannot assign null to a non-nullable variable, and you cannot use a nullable variable where a non-nullable one is expected without first checking for null:

String? maybeName = 'Bob';
// String definite = maybeName;  // Error! Can't assign nullable to non-nullable

In the next lesson, you'll learn the operators Dart provides to safely work with nullable values.

Challenge

Easy

Let's build a movie watchlist system that demonstrates the difference between required information and optional information using nullable and non-nullable types.

You'll create two files to organize your code:

movie.dart: Define a Movie class that represents a film in your watchlist. Every movie must have a title and release year (these are always required), but some additional details might not be available for every movie. Your class should have:
- A non-nullable String title - every movie must have a title
- A non-nullable int year - every movie must have a release year
- A nullable String? director - the director might be unknown
- A nullable double? rating - not all movies have been rated yet
- A constructor that takes title and year as required parameters
- A displayInfo() method that prints the movie's information, showing Unknown for missing director and Not rated for missing rating
main.dart: Import your movie class and create a small watchlist demonstrating both complete and incomplete movie data:
- Create a movie with title 'Inception' and year 2010, then set its director to 'Christopher Nolan' and rating to 8.8
- Create a movie with title 'The Matrix' and year 1999, then set only its rating to 8.7 (leave director as null)
- Create a movie with title 'Upcoming Film' and year 2025 (leave both director and rating as null)
- Call displayInfo() on each movie in the order listed above

The displayInfo() method should print in this exact format:

[title] ([year])
Director: [director or "Unknown"]
Rating: [rating or "Not rated"]

Notice how the non-nullable fields (title and year) are always guaranteed to have values, while the nullable fields (director? and rating?) can remain unset. This reflects real-world scenarios where some data is essential and some is optional.

Expected output:

Inception (2010)
Director: Christopher Nolan
Rating: 8.8
The Matrix (1999)
Director: Unknown
Rating: 8.7
Upcoming Film (2025)
Director: Unknown
Rating: Not rated

Try it yourself

import 'movie.dart';

void main() {
  // TODO: Create a movie with title 'Inception' and year 2010
  // Then set its director to 'Christopher Nolan' and rating to 8.8
  
  // TODO: Create a movie with title 'The Matrix' and year 1999
  // Then set only its rating to 8.7 (leave director as null)
  
  // TODO: Create a movie with title 'Upcoming Film' and year 2025
  // Leave both director and rating as null
  
  // TODO: Call displayInfo() on each movie in order
}