How do I call a super constructor in Dart?
InheritanceConstructorDartSuperclassExtendsInheritance Problem Overview
How do I call a super constructor in Dart? Is it possible to call named super constructors?
Inheritance Solutions
Solution 1 - Inheritance
Yes, it is, the syntax is close to C#, here is an example with both default constructor and named constructor:
class Foo {
Foo(int a, int b) {
//Code of constructor
}
Foo.named(int c, int d) {
//Code of named constructor
}
}
class Bar extends Foo {
Bar(int a, int b) : super(a, b);
}
class Baz extends Foo {
Baz(int c, int d) : super.named(c, d);
}
If you want to initialize instance variables in the subclass, the super()
call must be last in an initializer list.
class CBar extends Foo {
int c;
CBar(int a, int b, int cParam) :
c = cParam,
super(a, b);
}
You can read about the motivation behind this super()
call guideline on /r/dartlang.
Solution 2 - Inheritance
This is a file I am sharing with you, run it as is. You'll learn how to call super constructor, and how to call super parameterized constructor.
// Objectives
// 1. Inheritance with Default Constructor and Parameterised Constructor
// 2. Inheritance with Named Constructor
void main() {
var dog1 = Dog("Labrador", "Black");
var dog2 = Dog("Pug", "Brown");
var dog3 = Dog.myNamedConstructor("German Shepherd", "Black-Brown");
}
class Animal {
String color;
Animal(String color) {
this.color = color;
print("Animal class constructor");
}
Animal.myAnimalNamedConstrctor(String color) {
print("Animal class named constructor");
}
}
class Dog extends Animal {
String breed;
Dog(String breed, String color) : super(color) {
this.breed = breed;
print("Dog class constructor");
}
Dog.myNamedConstructor(String breed, String color) : super.myAnimalNamedConstrctor(color) {
this.breed = breed;
print("Dog class Named Constructor");
}
}
Solution 3 - Inheritance
case of a constructor with Optional Parameters
class Foo {
String a;
int b;
Foo({this.a, this.b});
}
class Bar extends Foo {
Bar({a,b}) : super(a:a, b:b);
}
Solution 4 - Inheritance
Can I call a private constructor of the superclass?
Yes, but only if the superclass and the subclass you are creating are in the same library. (Since private identifiers are visible across the whole library they are defined in). Private identifiers are those that start with underscore.
class Foo {
Foo._private(int a, int b) {
//Code of private named constructor
}
}
class Bar extends Foo {
Bar(int a, int b) : super._private(a,b);
}
Solution 5 - Inheritance
As dart supports implementing a class as interface (Implicit interfaces), you can't call the parent constructor if you implemented it you should use extends. If you use implements change it to extends and use Eduardo Copat's Solution.
Solution 6 - Inheritance
class AppException implements Exception {
final _message;
final _prefix;
//constructor with optional & not named paramters
//if not the parameter is not sent, it'll be null
AppException([this._message, this._prefix]);
String toString() {
return "$_prefix$_message";
}
}
class FetchDataException extends AppException {
//redirecting constructor with a colon to call parent two optional parameters
FetchDataException([String msg]) : super(msg, "Error During Communication: ");
}
class BadRequestException extends AppException {
BadRequestException([msg]) : super(msg, "Invalid Request: ");
}
class UnauthorisedException extends AppException {
UnauthorisedException([msg]) : super(msg, "Unauthorised: ");
}
class InvalidInputException extends AppException {
InvalidInputException([String msg]) : super(msg, "Invalid Input: ");
}