So I am trying to get into app development and following this guys yt-tutorial (yt-vid from The Net Ninja). However I stumbled over the following error: Non-nullable instance field ‘{0}’ must be initialized. (error on dart.dev explained). The whole code is shown in this image

img coding: comparison mine with yt:

The issue I am facing is that I don't understand the origin of that error. I understand that you can't use a variable that was not initialized yet and that this error is trying to tell you that before you run the code and run into an exception in runtime.

I also know that I can fix this using "late" in front of the variables, but as far as I'm concerned that's just "ignoring" the error.

So my main question is: why do I have this error while the yt doesn't even though we have (except for some names and assignments) the same code?

I appreciate every answer and hope you have a great day

the links:

error on dart.dev explained

CodePudding user response：

You should change your User class to:

class User {
  String username;
  int age;
  
  User(this.username, this.age);
}

By doing so, the variables will be initialized with values (since we are then able to use this values in the constructor body) as part of initialization of the object.

Dart does object creation in two steps:

1. Initialize object.
2. Run constructor body.

So when the constructor body is running, the object must be initialized with values (well, the non-nullable values at least). To do so, we have a code block which is executed before { } like:

class User {
  String username;
  int age;

  User(String username, int age)
      : this.username = username,
        this.age = age;
}

But since this is kinda redundant, we have the shortcut form as I first suggested.

When initializing an object, we are not allowed to use variables from the object since the object is ongoing its creation.