How do I initialize a final class property in a constructor? - constructor

Question

In Java you are allowed to do this:
class A {
private final int x;
public A() {
x = 5;
}
}
In Dart, I tried:
class A {
final int x;
A() {
this.x = 5;
}
}
I get two compilation errors:
The final variable 'x' must be initialized.
and
'x' can't be used as a setter because its final.
Is there a way to set final properties in the constructor in Dart?

Answer 1

You cannot instantiate final fields in the constructor body. There is a special syntax for that:
class Point {
final num x;
final num y;
final num distanceFromOrigin;
// Old syntax
// Point(x, y) :
// x = x,
// y = y,
// distanceFromOrigin = sqrt(pow(x, 2) + pow(y, 2));
// New syntax
Point(this.x, this.y) :
distanceFromOrigin = sqrt(pow(x, 2) + pow(y, 2));
}

Answer 2

You can make it even shorter with this. syntax in the constructor (described in https://www.dartlang.org/guides/language/language-tour#constructors):
class Point {
final num x;
final num y;
final num distanceFromOrigin;
Point(this.x, this.y)
: distanceFromOrigin = sqrt(pow(x, 2) + pow(y, 2));
}
If you have some more complicated initialization you should use factory constructor, and the code become:
class Point {
final num x;
final num y;
final num distanceFromOrigin;
Point._(this.x, this.y, this.distanceFromOrigin);
factory Point(num x, num y) {
num distance = distanceFromOrigin = sqrt(pow(x, 2) + pow(y, 2));
return new Point._(x, y, distance);
}
}

Answer 3

Here is a simplified summary of the ways to initialize a final class variable.
class MyClass {
final int x; // <-- initialize this
}
Initializer value
class MyClass {
final int x = 'hello'.length;
}
You'd only use final if the initialization could only be done at runtime. Otherwise, static const is better:
class MyClass {
static const int x = 0;
}
Initializer formal
class MyClass {
MyClass(this.x);
final int x;
}
This is the most common approach.
Initializer list
class MyClass {
MyClass(int x)
: _x = x;
final int _x;
}
This is useful when you want to keep a field private.
Default parameter value
You can surround the parameter with square brackets ([]) for an unnamed parameter or curly braces ({}) for a named parameter and then give it a default value.
class MyClass {
MyClass({this.x = 0});
final int x;
}
This is useful if you want to make the parameter optional.
You could accomplish the same thing with an initializer list as well:
class MyClass {
MyClass({int? x})
: _x = x ?? 0;
final int _x;
}
Late initialization
class MyClass {
MyClass(String? a) {
x = a?.length ?? 0;
}
late final int x;
}
This is useful if you need to do more complex initialization than is allowed in the initializer list. For example, I've done this when initializing a gesture recognizer in Flutter.
Lazy initialization
Another advantage of using late is that it doesn't initialize a value until you access the value.
class MyClass {
late final int x = _doHeavyTask();
int _doHeavyTask() {
var sum = 0;
for (var i = 0; i < 100000000; i++) {
sum += 1;
}
return sum;
}
}
This is useful if you have a heavy calculation that you only want call if you absolutely need it.
This doesn't initialize x:
final myClass = MyClass();
But this does initialize x:
final myClass = MyClass();
final value = myClass.x;

Answer 4

I've had a similar problem: I was trying to initialise a final field from the constructor, while simultaneously calling a super constructor. You could think of the following example
class Point2d {
final int x;
final int y;
Point2d.fromCoordinates(Coordinates coordinates)
: this.x = coordinates.x,
this.y = coordinates.y;
}
class Point3d extends Point2d {
final int z;
Point3d.fromCoordinates(Coordinates coordinates)
:this.z = coordinates.z,
super.fromCoordinates(coordinates);
}
/// Demo class, to simulate constructing an object
/// from another object.
class Coordinates {
final int x;
final int y;
final int z;
}
Well apparently this works. You can initialise your final fields by using the above syntax (check Point3d's constructor) and it works just fine!
Run a small program like this to check:
void main() {
var coordinates = Coordinates(1, 2, 3);
var point3d = Point3d.fromCoordinates(coordinates);
print("x: ${point3d.x}, y: ${point3d.y}, z: ${point3d.z}");
}
It should prin x: 1, y: 2, z: 3

Answer 5

I've come to a dilemma here where I wanted to initialize a final List with no items, and a Stream to be defined inside the constructor (like in this case distanceFromOrigin).
I couldn't do that with any of the answers below, but I mixed them both and it worked.
Example:
class MyBloc {
final BehaviorSubject<List<String>> itemsStream;
final List<String> items = [];
MyBloc() : this.itemsStream = BehaviorSubject<List<String>>.seeded([]) {
items.addAll(List.generate(20, (index) => "Hola! I'm number $index"));
itemsStream.add(items);
}
}

Answer 6

class A{
final int x;
A(this.x){
}
}