Understanding Null

Part of the Fundamentals section of Coddy's Dart journey — lesson 77 of 94.

Null in Dart represents the absence of a value. By default, variables in Dart cannot be null unless explicitly allowed.

Create a variable that can be null by adding a question mark after the type:

String? name = null;
print(name);

After executing the above code, the output will be:

null

Regular variables cannot be null and must have a value:

// This would cause an error:
// String name = null;

// This is valid:
String name = 'Dart';
print(name);

After executing the above code, the output will be:

Dart

Challenge

Easy

In this challenge, you'll practice working with nullable variables in Dart. A nullable variable can hold either a value or null.

Complete the code to check if the username variable is null. If it is, assign the string "Guest" to the displayName variable. If username is not null, assign the value of username to displayName.

Expected output:

Welcome, Guest!

Cheat sheet

In Dart, null represents the absence of a value. Variables cannot be null by default unless explicitly allowed.

Create a nullable variable by adding a question mark after the type:

String? name = null;

Regular variables must have a value and cannot be null:

String name = 'Dart'; // Valid
// String name = null; // Error

Try it yourself

void main() {
  // This nullable String variable is already defined for you
  String? username = null;
  
  // TODO: Check if username is null
  // If it is null, set displayName to "Guest"
  // If it is not null, set displayName to the value of username
  String displayName = "";
  
  // This will display the welcome message
  print("Welcome, $displayName!");
}