Learn Variables and Null Safety in Flutter | Dart Language Core for Flutter

Definition

Null-aware operators in Dart, such as ??, ?., and ??=, help you write safer code by handling cases where a variable might be null. In Flutter, these operators are commonly used in widget build methods to provide default values or avoid calling methods on null objects, reducing the risk of runtime exceptions and ensuring smoother UI updates.

Understanding how to declare and use variables is essential when building Flutter apps with Dart. Dart provides three main ways to declare variables: var, final, and const. Use var when a variable's value might change, and Dart will infer its type. Use final for single-assignment variables whose value is set once and never changes at runtime. Use const for compile-time constants—values that are known and fixed at compile time. Dart's type system also supports null safety, which means you must explicitly allow a variable to be null by adding a question mark (?) to its type. By default, variables are non-nullable, which helps prevent many common runtime errors.

main.dart


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import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  // Non-nullable variable
  final String title = "Hello Flutter";

  // Nullable variable
  final String? subtitle = null;

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(title: Text(title)),
        body: Center(
          child: Text(subtitle ?? "No subtitle provided"),
        ),
      ),
    );
  }
}

Null safety in Dart helps you avoid runtime errors by ensuring that non-nullable variables cannot accidentally contain null values. This feature is especially important in Flutter, where widget trees rely on predictable, non-null data to render correctly. By making nullability explicit, Dart helps you catch potential issues at compile time rather than during app execution.

main.dart


              123456789101112131415161718192021
            
import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  final String? greeting = null;

  @override
  Widget build(BuildContext context) {
    // Use the null-aware operator ?? to provide a fallback value
    return MaterialApp(
      home: Scaffold(
        body: Center(
          child: Text(greeting ?? "Welcome!"),
        ),
      ),
    );
  }
}

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Section 1. Chapter 1

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Definition

main.dart


              12345678910111213141516171819202122232425
            
import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  // Non-nullable variable
  final String title = "Hello Flutter";

  // Nullable variable
  final String? subtitle = null;

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(title: Text(title)),
        body: Center(
          child: Text(subtitle ?? "No subtitle provided"),
        ),
      ),
    );
  }
}

main.dart


              123456789101112131415161718192021
            
import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  final String? greeting = null;

  @override
  Widget build(BuildContext context) {
    // Use the null-aware operator ?? to provide a fallback value
    return MaterialApp(
      home: Scaffold(
        body: Center(
          child: Text(greeting ?? "Welcome!"),
        ),
      ),
    );
  }
}

Everything was clear?

Thanks for your feedback!

Section 1. Chapter 1