Understanding Null Safety in Flutter

3 min readJul 20, 2023

When working with Flutter, you might come across various symbols and operators, such as ?, !, ??, and late. These symbols play an essential role in the Dart programming language, which is the language used for building Flutter applications. In this article, we'll explain the differences between these symbols and their usage.

The ‘?’ Symbol (Null Safety Operator)

In Dart, the ? symbol is used as part of the null safety feature, which helps prevent null reference exceptions and make your code more robust. By default, Dart variables can have a null value, but with null safety, you need to explicitly declare whether a variable can be null or not.

String? nullableString = "Hello"; // The '?' after String indicates nullableString can be null.
String nonNullableString = "World"; // nonNullableString cannot be null.

When using nullable variables, you must handle null values appropriately, using the null-aware operators, such as the ?? operator (explained below).

The ‘!’ Symbol (Postfix Non-null Assertion Operator)

The ! symbol is used to assert that a nullable variable indeed has a non-null value at runtime. It tells the Dart compiler that you know the variable is not null, even if it's nullable. If the variable is null when you use the ! operator, it will result in a runtime exception.

String? nullableName = "John";
String nonNullableName = nullableName!; // Using '!' to assert that nullableName is not null.

It’s essential to be cautious when using the ! operator since it bypasses the null safety check and can lead to crashes if the variable is actually null.

The ‘??’ Symbol (Null-aware Coalescing Operator)

The ?? symbol, also known as the null-aware coalescing operator, is used to provide a default value for a nullable variable if it is null. It allows you to handle null values more gracefully.

String? nullableGreeting;
String greeting = nullableGreeting ?? "Hello, Guest"; // If nullableGreeting is null, use the default value "Hello, Guest".

In the above example, if nullableGreeting is null, the value of greeting will be set to "Hello, Guest". Otherwise, it will take the value of nullableGreeting.

The ‘late’ Keyword (Late Initialization)

The late keyword is used when you need to declare a non-nullable variable that is not initialized immediately. It informs the Dart compiler that the variable will be assigned a value before it is used, and it won't be null.

late String initializedLater;
void initializeLater() {
  initializedLater = "This value is initialized later";
}

You must ensure that you assign a value to a late variable before accessing it; otherwise, it will throw a runtime exception.

In conclusion, understanding the differences between ?, !, ??, and late is crucial when working with Flutter and Dart. The ? symbol is for null safety, ! is for asserting non-null values, ?? is for providing default values, and late is for declaring non-nullable variables with deferred initialization. Proper usage of these symbols will help you write safer and more reliable Flutter applications.

Understanding Null Safety in Flutter

The ‘?’ Symbol (Null Safety Operator)

The ‘!’ Symbol (Postfix Non-null Assertion Operator)

The ‘??’ Symbol (Null-aware Coalescing Operator)

The ‘late’ Keyword (Late Initialization)

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