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Non-Nullable Dart: Understanding Null Safety

Learn how to use null safety in Dart. Get to know Dart’s type system and how to utilize language features in production code.

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Version

  • Dart 2.12, Android Studio 4.1

Null safety helps you solve one of the most common errors in software development: NullPointerException. This exception and other types of bugs cost millions of dollars for businesses and corporations. As a result, many companies recruit developers with good error-handling skills.

Null safety is not a new concept. Other languages such as Swift, Kotlin and Rust, have implemented null safety based on their type systems and language features. Dart has also recently introduced sound null safety.

In this tutorial, you’ll build ProfileApp, a small app that saves data about users and their relationships. In the process, you’ll learn about:

  • Sound null safety.
  • How to create a Flutter app with null-safe code.
  • The type system in Dart.
  • How to work with nullable types.
  • What type promotion is and how Dart automatically promotes types.
  • The migration types you can use to make your existing code null safe.
Note: This tutorial assumes you have experience with Dart and Flutter widgets. If you don’t, check out this Flutter UI widgets video course and our Getting Started With Flutter tutorial.

Getting Started

Download the starter project by clicking the Download Materials button at the top or bottom of the tutorial. After the download completes, unzip the file to a suitable location.

Note: This tutorial uses Android Studio, but feel free to use any editor that supports the Flutter SDK and Dart. If you use a different editor, however, keep in mind that some screenshots are specific to Android Studio, so they might vary from what you see on your screen.

ProfileApp consists of two screens and one dialog. In the first screen, the user enters their own data. In the second, they can add relatives or friends based on their preferences. Finally, users can review the entered data in the dialog widget. Follow the instructions in the next section to try the app out for yourself.

Getting to Know the App

Open the starter project in Android Studio by clicking Open an Existing Project in the Welcome to Android Studio window:

Open an Existing Project in Android Studio

Browse to the location where you unzipped the file. Choose the starter folder and click Open:

Choose starter project

The project structure should look like this:

Project structure

The project already contains most of the UI code. You’ll concentrate on using model classes to save user-entered data and integrating that data into the existing UI components.

Inside Android Studio, open the terminal and enter flutter pub get to get all the packages:

Run flutter pub get

Build and run the app. You’ll see the home page:

Home Page with empty information

Tapping the + button opens the Add member page. You can add friends or family members here:

Add member page with text fields to enter details about friends and family members

Go back to the previous screen. Tap the Save & Preview button and the User details dialog appears:

Empty Profile Dialog

Right now, you can add people, but they don’t appear on the home page. Furthermore, the dialog is missing some information. You’ll fix these issues using null-safe code. Next, you’ll learn about what sound null safety is.

Understanding Sound Null Safety

Null is just the absence of a value for an object, which Dart needs to be able to represent. null helps developers deal with the unknown in their code.

When any exception is not handled, the app will show a red error screen. If you try to call a method or getter on a null object, Dart will throw a NoSuchMethodError. This is essentially the same as a NullPointerException from other languages such as Java. For the purposes of this article, the term NullPointerException (NPE) will be used to discuss null errors since it is the more commonly used term.

The main issue with the NullPointerException is that it can happen at almost any part of the app if not well handled. Checking for nulls every now and then throughout your app isn’t the best either. For these reasons and more, handling nulls at the language level is probably the best way to avoid the dreaded NullPointerEception.

To build on this idea, the Dart team introduced sound null safety. This means that variables are considered non-nullable by default. If you don’t give an object null support when you create it, it will never get a null. As a result, you avoid null reference errors in your code.

Before going into details about null safety and how it works, you have to understand the type system in Dart. The Dart team relied on the type system when they implemented null safety.

Exploring the Dart Type System

Dart divides types into two groups:

  • Nullable types
  • Non-Nullable types

Nullable types can contain either a value or null. There are certain types of operators that help you work with nullable types. The figure below is an example representing a nullable String:

Nullable Strings can contain either a String or null

Non-Nullable types can only contain values. Nulls are not permitted.

Non-nullable Dart Strings can only contain String values

The type hierarchy of Dart is as follows: Object? is at the top and Never is at the bottom. You’ll rarely use Never, though. Any code that returns or evaluates to Never either throws an exception or aborts the program.

Nullable Type system in Dart

Now that you know what null safety is, you need to enable it before you can work with it.

Enabling Null Safety

Dart introduced a stable version of null safety in version 2.12.0. On the terminal, you can run flutter –version to verify your version is at least up to version 2.12.0. Alternatively, you can simply follow the steps below to download the latest release from the stable channel.

In the terminal, run the flutter channel command to check which channel you are currently using:

Example of the result of a Flutter channel command, showing the Beta channel with an asterisk next to it

Run the flutter channel stable command to switch to the stable channel:

Change Flutter channel command showing the result: Switched to branch 'Stable'

Update to the latest version by running the flutter upgrade command:

Flutter Upgrade after channel switch

Open pubspec.yaml and confirm your SDK version under the environment section looks like below:

environment:
  sdk: ">=2.12.0 <3.0.0"

This changes the Dart version to 2.12.0.

Finally, run flutter upgrade again to avoid future problems with Android Studio:

Flutter Upgrade after changing version

Congratulations! You’ve just migrated your project to null safety. You can now use null safety in your project.

Migrating to Null-Safe Dart

Dart introduced null safety in 2.12.0, but there are old Dart codes in production. Since the Dart team can’t migrate your code automatically. You have two options:

  1. Migrate using a tool that makes easily predictable changes for you. Based on your needs, you can choose to apply those changes or not.
  2. Migrate manually, where you can easily predict most of the changes that need null safety and migrate accordingly. This involves manually refactoring your code.
Note: The Dart team provides migration guides to help you learn more about migrating your code to null safety.

Creating Model Classes

For this project, you’ll create three models: Friend, FamilyMember and User. These model classes will help you hold and manage user-entered data.

User‘s main responsibility is to hold Friend, FamilyMember and primitive information about the user.

All three extend an abstract class, Person, which defines fields common to these models. This class is already defined for you in the starter project.

Reviewing the Person Class

Project models relationship

Inside lib/model/abstract, open person.dart. Person‘s constructor takes a set of required properties, namely: name, surname, birth date and gender:

abstract class Person {
  String name;
  String surname;
  String birthDate;
  String gender;

  //1
  Person({
    required this.name,
    required this.surname,
    required this.birthDate,
    required this.gender});

  //2
  abstract String whoAmI;

}

Here’s how it works:

  1. You need to initialize every class property in Dart. If you can’t initialize the property via a class constructor, you must declare it as a nullable type. By using the required keyword, you make the property required so you don’t have to declare it as nullable.
  2. Dart treats abstract classes differently. It gives you a compile-time error if you don’t initialize fields or make them nullable. To allow the fields to be implemented and to prevent compile-time errors, you mark the fields as abstract, which lets the child class implement them.

Creating the User Class

Create user.dart under lib/model and extend it from Person:

class User extends Person {
  User() : super();

  @override
  String whoAmI;
}

Because Person requires a set of parameters, you pass these parameters from User to Person using super(). The final class should look like this:

class User extends Person {

  User({required String name,
       required String surname,
       required String birthDate,
       required String gender})
     : super(name: name, surname: surname, birthDate: birthDate, gender: gender);

  @override
  String whoAmI = ' a user';

}

Creating Friend and FamilyMember Classes With Nullable Types

Add a new file friend.dart under lib/model. Then create a class called Friend extending it from Person:

import 'abstract/person.dart';


 class Friend extends Person {
 //1
  Friend(
     {required String name,
       required String surname,
       required String birthDate,
       required String gender})
     :super(name: name, surname: surname, birthDate: birthDate, gender: gender);
  //2
  @override
  String whoAmI = 'a friend';

}

In the code above, you:

  1. Declare Friend with all required arguments, then call super with the arguments.
  2. Implement an abstract field based on Friend.

Declaring Nullable Types

In real life, a friend has a relationship with the user. They could be a high school friend, a colleague, or a next-door neighbor. To define the relationship, add a nullable called relation to Friend:

String? relation;

Dart uses the nullable operator ?, to declare nullable types. Thus, you just need to append ? to the variable type and it becomes nullable.

Note: In null-safe Dart, you can’t define class properties without initialization or you have to make them nullable. If you remove the ? symbol from relation, Dart complains that it isn’t initialized and forces you to make it nullable or initialize it with a value.

Now, add relation to Friend as an optional argument:

String? relation;

Friend(
     {required String name,
       required String surname,
       required String birthDate,
       required String gender, this.relation})
     :super(name: name, surname: surname, birthDate: birthDate, gender: gender);

You didn’t mark relation as required in the constructor because you want to be able to create an instance of a Friend without declaring the relationship. Dart will assign null to it at runtime.

Creating FamilyMember With a Nullable Property

Create family_member.dart under lib/model and extend it from Person. Then declare a nullable profession field.

import 'abstract/person.dart';

class FamilyMember extends Person {
  String? profession;

  FamilyMember(
     {required String name,
       required String surname,
       required String birthDate,
       required String gender, this.profession})
     : super(name: name, surname: surname, birthDate: birthDate, gender: gender);

  @override
  String whoAmI = 'a family member';

}

Here, you’re extending Person and implementing its abstract variable, whoAmi. You also added profession. This property is nullable because a person may or may not have a profession.

Using Late Variables and Lazy Initialization

To display friends and family members on the home screen, you need to create them in _AddMemberPageState. This class is in lib/add_member_page.dart.

Add late Person _person inside _addMember() in _AddMemberPageState:

void _addMember() {
  late Person _person;
}

This object will hold a Friend or FamilyMember.

Use late on variables when you’re sure you’ll initialize them before using them. Use late with class properties.

Sometimes, you can’t initialize properties in the constructor, but you’ll define them in other methods of your class. In that case, you mark those properties with late.

Another advantage of late is lazy initialization. Dart will not initialize late properties until they’re used for the first time. This can be useful during app initialization, when an expression is costly or might not be needed.

Retrieving Data From Widgets

To create a _person object, you need to retrieve data from the widgets. To do that, update _addMember() to the following:

void _addMember() {
  //1
  late final Person _person;
  final name = _nameController.text;
  final surname = _surnameController.text;
  final birthDate = _birthDateController.text;
  final gender = _dropDownGender;
  final profession = _professionController.text;
  final friendRelation = _friendController.text;
  //2
  if (_dropDownMember.contains(ProjectConst.FAMILY_MEMBER)) {
    _person = FamilyMember(
      name: name,
      surname: surname,
      birthDate: birthDate,
      gender: gender,
      profession: profession.isEmpty ? null : profession,
    );
  } else {
    _person = Friend(
        name: name,
        surname: surname,
        birthDate: birthDate,
        gender: gender,
        relation: friendRelation.isEmpty ? null : friendRelation);
  }
  //3
  DataManager.addPerson(_person);
  Navigator.pop(context);
}

Here’s what’s happening:

  1. You retrieve user-entered information from the text fields.
  2. Based on the type of relationship the user selected, you create a Friend or FamilyMember. Pay attention to the last property of each object. If relation or profession is empty, it passes null because you defined these properties as nullable.
  3. DataManager is already defined in lib/utils/data_manager.dart. It adds a _person into a static list so you can access it from the home screen. Navigator.pop() closes the current screen.

Build and run. You can now add members to the list:

Empty Add member screen

But on the home screen, you can’t see members in the Friends or Family members sections:

Home Page with empty information

You’ll see how to fix that shortly. Before that, take a look at Dart’s Never type.

Understanding the Never Type

Never is at the bottom of the Dart type system. It has no value. You don’t really use Never in your code; when an expression returns Never, the program will throw an exception or abort when the execution reaches it.

However, for the sake of this tutorial, you’ll use Never to test a scenario when the app promotes errors to the user interface because it encounters an unhandled exception.

Add checkRelation() to Friend. This method checks whether relation is defined:

String? checkRelation() {
  //1
  if (relation != null) {
    return relation;
  //2
  } else {
    relationIsNotDefined();
  }
}

Never relationIsNotDefined() {
  throw ArgumentError('Friend relation is not defined');
}

This is how the method works:

  1. This condition checks whether relation is null. If it isn’t null, the condition returns relation.
  2. If relation is null, the condition calls relationsIsNotDefined(), which throws an ArgumentError exception. Notice how the code is not wrapped in a try-catch statement. Never signals flow analysis that the app will throw an exception when it reaches relationsIsNotDefined().

Understanding Flow Analysis

Flow analysis is a mechanism that determines the control flow of a program. Dart uses it most of the time at runtime for type promotion and code reachability analysis.

Flow analysis helps you write null-safe code. By analyzing the code at compile time, it prompts you to handle nullable types better in order to avoid NullPointerExceptions. It comes embedded in the Dart language.

In summary, the main responsibilities of flow analysis are:

  • Reachability analysis, which is the process of evaluating a function or expression.
  • Code warnings.
  • Null checks at compile time and runtime.
  • Type promotion.
  • Ensuring you assign values to all local and global variables.
Note: Read more about how it works in the official flow analysis documentation.

Testing the Never Type

Call checkRelation() inside _addMember() in _AddMemberPageState. Before calling checkRelation(), you need to cast _person to Friend using as. Make it the last call in the else block:

  (_person as Friend).checkRelation();

Build and run. Go to the Add member page and fill in the information:

Adding a new user with no relationship

Don’t provide a value for Friend Relation. Press Add member. Your program should throw an exception.

When you open your Dart analysis terminal, you’ll see the exception:

Illegal argument exception when Friend Relation is empty

To add a friend to the list, comment out the call to checkRelation(). Now, hot restart the app and submit the information. The app will work as it did before.

Using Type Promotion

To display user names on the home screen, you need to write two methods that filter the list of people into separate groups: friends and family members. Each method appends the names of the people to the names variable.

Open data_manager.dart inside lib/utils. Define these methods as static in DataManager:

import 'package:profile_app/model/family_member.dart';
import 'package:profile_app/model/friend.dart';


class DataManager {

  DataManager._();
  static List people = List.empty(growable: true);

  static void addPerson(Person person) {
    people.add(person);
  }


  //1
  static String getFamilyMemberNames() {
    var names = '';
    for (var i = 0; i < people.length; i++) {
      final person = people[i];
      //2
      if (person is FamilyMember) {
        names += '${person.name} ${person.surname},';
      }
    }
    return names;
  }
  //3
  static String getFriendNames() {
    var names = '';
    for (var i = 0; i < people.length; i++) {
      final person = people[i];
      //4
      if (person is Friend) {
        names += '${person.name} ${person.surname},';
      }
    }
    return names;
  }
}

The code above:

  1. Goes through the list of people to get only FamilyMembers, then puts their names into names.
  2. Checks if person is of type FamilyMember. Notice how you don’t have to cast person to FamilyMember.  Automatic type promotion takes care of that. Dart automatically promotes person to FamilyMember, allowing you to access its properties and methods.
  3. Similar to the method you used to get FamilyMembers, except it gets Friends instead.
  4. Checks if person is of type Friend, then, if true, automatically promotes person to the type Friend. This allows you to access the methods and properties available to Friend.

Displaying Member Names on the Home Screen

To display the names of the Family members and Friends, you need to define _updateNames() in _HomePageState, which you’ll find in home_page.dart.

void _updateNames() {
  setState(() {
    _friendNames = DataManager.getFriendNames();
    _familyMemberNames = DataManager.getFamilyMemberNames();
  });
}

This simply assigns friend and family member names to _friendNames and _familyMemberNames using DataManager. setState() rebuilds the widget tree.

Inside _HomePageState, you’ll find IconButton. Call _updateNames() in .then():

IconButton(
      icon: const Icon(Icons.add_circle),
      onPressed: () {
          Navigator.of(context)
              .push(MaterialPageRoute(
                   builder: (context) => const AddMemberPage()))
              .then((value) => {_updateNames()});
}),

This updates the names in _HomePageState after you add a member to AddMemberPage.

Build and run. Go to the Add member page to add a friend:

Add a new member to see them on the home page

Add family members to the user profile:

Add new family member

On the home screen, you can now view the people you added in the friend and family sections:

Added members

At this point, you’ve set up the two screens, but you still can’t see the member details in the dialog. You’ll address that next.

Displaying User Details in the Dialog

To show user data in the dialog, you first need to finish setting up User. Specifically, you need to add friendsAndFamily to User:

late List friendsAndFamily;

Although friendsAndFamily could be either nullable or late, you declared it late to avoid null checks and casting before you use it.

Retrieving User Input From the TextFields

Declare _user in _HomePageState:

User? _user;

When a user taps Save & Preview, it triggers _displayUserInfo(). Add this block of code to _displayUserInfo() to collect the information the user entered:

void _displayUserInfo() {
  // 1
  final name = _nameController.text;
  final surname = _surnameController.text;
  final birthDate = _birthDateController.text;
  final gender = _dropdownValue;
  //2
  _user = User(name: name, surname: surname, birthDate: birthDate, gender: gender);
  //3
  _user!.friendsAndFamily = DataManager.persons;

  //4
  _showPreview();
}

Here’s what the code does:

  1. Retrieves user-provided data from the TextFields.
  2. Creates a new _user.
  3. Assigns a list of people to _user. However, before you assign anything, you need to ensure that _user isn’t null. To do this, you use the Postfix null assertion bang operator, !, to cast the nullable _user to its non-nullable type. This is called casting away nullability. But if _user is not null from this code, why do you need to cast it to a non-nullable type? Because you declared that _user is nullable. Consequently, Dart believes it might have been assigned a null somewhere in the code. To be on the safe side, Dart requires you to cast it before you can access its properties.
  4. _showPreview() is a predefined method that shows user information in a dialog.

Displaying the Dialog With the User Information

You need to display the user’s provided information in the corresponding UserTextWidget widgets in _showPreview().

First, fix the user’s name:

UserTextWidget('${ProjectConst.NAME}:${_user?.name}'),

Here, you access the name of the user using Dart’s null-aware operator, ?.. You use that operator because you declared _user as a nullable field, implying the field can be null. Without the null-aware operator, the program could crash if _user is null. In this case, it will return null.

Notice that you didn’t use the bang (!) operator because, even when _user is null, the bang operator will try to forcefully cast it to a non-nullable _user type, which will result in an error.

Next, fix the other widgets in _showPreview() with their respective user details:

...
UserTextWidget('${ProjectConst.SURNAME}:${_user?.surname}'),
UserTextWidget('${ProjectConst.GENDER}:${_user?.gender.toString()}'),
UserTextWidget('${ProjectConst.BIRTH_DATE_LABEL}:${_user?.birthDate}'),

Build and run, then enter information about the user:

Home Page with friends and family members

Tap Save & Preview to see the information in the dialog:

Dialog without friends and family information

You’ve set up everything except the user relationship. You’ll do that next.

Displaying the User Relationship

To display friends and family members, you need to modify the ListView in _showPreview(). Make sure the ListView looks like this:

ListView.builder(
  padding: const EdgeInsets.all(ProjectConst.value8),
  // 1
  itemCount: _user?.friendsAndFamily.length ?? 0,
  itemBuilder: (BuildContext context, int index) {
    //2
    final person = _user!.friendsAndFamily[index];
    return Center(child: Text('${person.name} is ${person.whoAmI}'));
  })

This is how ListView implements the friends and family members:

  1. Checks whether _user object is null. Based on the result, it sets ListView‘s size either to the list size or to 0. You don’t need to check friendsAndFamily because it is not nullable. friendsAndFamily is always initialized when you create _user inside _displayUserInfo().
  2. Gets a person based on the index from friendsAndFamily and displays it in Text.

At this point, you’ll see a preview dialog containing all the information about the user.

Build and run, then enter the user’s information:

Home Page

Tap the Save & Preview button to see the information in the dialog widget:

Dialog with all user information

Clearing the UI

If the user taps the Upload & Clear button, the dialog disappears but the input fields do not. To clear them, implement a new _clearUI():

void _clearUI(){
  setState(() {
    //1
    _nameController.text = '';
    _surnameController.text = '';
    _birthDateController.text = '';
    _familyMemberNames = '';
    _friendNames = '';
    _dropdownValue = ProjectConst.FEMALE;
    _genderImage = ProjectConst.FEMALE_IMAGE;
    //2
    _user = null;
    //3
    DataManager.people.clear();
  });
}

Essentially, this code:

  1. Clears the inputs by assigning empty values to them.
  2. Resets _user to null. Pay attention to the null. Remember, all code by default is safe. If you’d created a non-nullable _user like this: User _user, then you wouldn’t be able to assign a null to it.
  3. Clears the list of people from DataManager.

Now, call _clearUI() inside _onClearClicked(). This method triggers when the user clicks the Upload & Clear button.

void _onClearClicked() {
  _clearUI();
  Navigator.pop(context);
}

This clears the user input and closes the dialog.

Build and run, then enter a user’s information:

Filled Dialog

Press Upload & Clear and you’ll see that the fields clear now:

Home Page with information cleared

Congratulations, you finished ProfileApp! Best of all, it’s null-safe. Even if some NullPointer errors come up, your app can handle them.

Sound Null Safety In a Nutshell

Dart has three main operators to work with null:

  • Null-aware operators: The null aware accessor ?., which accesses properties of its operand, is an example. If the operand is null, then it will not throw an exception. Instead, it shows null text.
  • Bang operator: Use (!) to cast away nullability. It tries to cast a nullable type to a non-nullable type, throwing an exception if the operand is null.
  • If-null operator: ?? is shorthand for an if else condition. If the left side of the ?? operator is null, then it will use its right side.

Where to Go From Here?

Download the completed project files by clicking the Download Materials button at the top or bottom of the tutorial.

In this non-nullable Dart tutorial, you learned:

  • How to write null-safe code for a new or an existing Flutter project.
  • The Dart type system.
  • Nullable types and how to declare them.
  • Type promotions.
  • Abstract types and how Dart treats them.

You can expand your knowledge about null safety in Dart further by reading Dart’s Understanding Null Safety documentation.

If you’d like to learn more about Flutter, check out our Flutter video courses and articles.

We hope you enjoyed this tutorial. If you have any questions or comments, please join the forum discussion below!

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