Dart Initializing Class Fields "Non-Nullable instance field must be initialized" - dart

Question

I'm new to Dart, and I was learning about Classes in Dart.
I'm facing a problem understanding how constructors in Dart actually work.
I was used to initialize fields within constructors in the following way with other programming languages, but in Dart I get many warnings and errors with the following way of initializing fields.
class Car {
String brand;
String model;
int year;
Car(String brand, String model, int year) {
this.brand = brand;
this.model = model;
this.year = year;
}
}
I think that the problem is concerned with the null safety in Dart, but I don't want to use the ? operator or the keyword late at the time of the fields declaration.
So, could anyone tell me how I could initialize the fields properly using constructors in Dart?
Thanks

Answer 1

The problem is that the constructor body is essentially just a function. Which means you can do anything that a function can normally do. Including this:
class Car {
String brand;
String model;
int year;
void printCarDetails() => print("This car is from $year");
Car(String brand, String model, int year) {
printCarDetails(); // <-- what should this print?
this.brand = brand;
this.model = model;
this.year = year; // <-- if [year] is only initialized here?
}
}
The problem is that you can use fields before you've initialized them (remember that int isn't null by default -- int? is). Dart needs a section before the constructor body where you can guarantee you've initialized everything, and then you can run your constructor body as normal.
That section is called an initializer list:
class Car {
String brand;
String model;
int year;
Car(String brand, String model, int year) :
brand = brand,
model = model,
year = year
{
printCarDetails();
}
void printCarDetails() => print("This car is from $year");
}
So everything after the : is just initializing fields, and then everything in the { } is the constructor body, which can then access those fields.
But the initializer list is ugly to look at. Thankfully, Dart provides a shorthand called initializing formal parameters:
class Car {
String brand;
String model;
int year;
Car(this.brand, this.model, this.year) {
// brand, model, and year are all initialized now
printCarDetails();
}
void printCarDetails() => print("This car is from $year");
}
This is the idiomatic way of initializing fields, and you should try to stick to this pattern when you can.
As a side note, you may run into this problem when using the constructor of a super class:
class Vehicle {
String brand;
Vehicle(this.brand);
}
class Car extends Vehicle {
String model;
// Passes [model] to the initializer list, and [numWheels] to Vehicle
Car(this.model, super.numWheels);
}
This feature is called super parameters, and it is relatively new.

Answer 2

Instance fields must either have an initializer at the declaration, use an initializing formal, or be initialized in the constructor's initialization list.
Here are the examples:
// initializer at the declaration
class Car {
String brand = 'Brand';
String model = 'Model';
int year = 2023;
Car();
}
// initializing formal
class Car {
String brand;
String model;
int year;
Car(this.brand, this.model, this.year);
}
// constructor's initialization list
class Car {
String brand;
String model;
int year;
Car(String brand, String model, int year)
: this.brand = brand,
this.model = model,
this.year = year;
}
For more information and better understanding why dart works in that way please read the official documentation.