All 7 Dart Null Safety questions
More in Dart Null Safety
What does the question mark symbol mean when it appears after a type in a Dart variable declaration, such as String?, and how is a nullable type actually different from a regular non nullable type? What do the null aware operators, specifically the null coalescing operator and the null aware access operator, actually do, and how do they let you write more concise code when working with nullable values? What is the null assertion operator, written as an exclamation mark, and why should it generally be used carefully and only when you are genuinely certain a specific nullable value cannot actually be null at that particular point? How does Dart's late keyword let you declare a non nullable variable that will genuinely be initialized before it is ever actually first used, even though it cannot be given an initial value immediately at the exact point of its declaration? How does Dart's type promotion, sometimes also called flow analysis, let the compiler automatically treat a nullable variable as non nullable within a specific block of code immediately after you have already explicitly checked that it is not null? How does migrating a large, existing pre null safety Dart or Flutter codebase to properly support null safety typically work, and what tools does Dart provide to help automate a significant portion of that migration process?
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What is null safety in Dart, and how does it help prevent a common category of runtime error known as a null reference exception?

Beginner

Answer

Null safety is a Dart language feature that requires you to explicitly declare whether a variable is allowed to hold a null value or not, and by default every variable in Dart is considered non nullable, meaning the compiler guarantees it will always genuinely hold a real value and can never accidentally be null, which eliminates an entire common category of runtime crash, historically often called a null pointer exception in many other programming languages, by catching these kinds of mistakes at compile time instead of allowing them to only surface later as an unexpected crash while the app is actually running.
String name = 'Alex';  // Cannot ever be null
String? nickname;      // Can be null, must be explicitly marked with a question mark
Real-world example A developer accidentally forgets to provide a value for a non nullable String field, and Dart's compiler immediately flags this as an error at compile time, preventing what would have otherwise very likely caused a genuine crash later in production.

Common follow-ups: What was Dart's null handling behavior like before null safety was introduced?;How does null safety affect working with existing third party packages that were written before it was introduced?

Dart Language Basics & Syntax;Dart Collections & Generics
All 7 Dart Null Safety questions