Inheritance on Dart Null 可测试性安全

Question

I have a question related to Dart Null Safety concept.

Imagine I have a class called Bird

class Bird{
Object character;

Bird();
}

and Pigeon

class Pigeon extends Bird{
String name;

Pigeon();
}

Now, because of Null Safety on Dart, the character object must be instantiated. I want to instantiate that in the constructor because I want the bird class can be testable using Mockito.

But, when I wrote this

class Bird{
late Object character;

Bird(this.character);
}

The Pigeon class shows an error because the Bird class doesn't have any zero-argument constructor.

My workaround is by using a not required argument like this.

class Bird{
late Object character;

Bird({Object? character}) : this.character= character ?? GetIt.I<Object>();;
}

But, I don't like the GetIt part for my default value. Moreover, another programmer may think it is an optional argument because I didn't mark it as required which can lead to an error.

So, how is the best practice to solve this case?

Answer 1

You can either use Named Argument Constructor to make character optional or remove late and ? like this

class Bird{
  Object character;

  Bird(Object character) : this.character = character ?? GetIt.I<Object>();
}

Your Named Argument Constructor will look like this

class Bird{
  Object character;

  Bird({this.character = DEFAULT_CONST_VALUE});
}

Answer 2

What about this solution?

class Bird {
  Object character;

  Bird({this.character});
}

class Pigeon extends Bird {
  String name;

  Pigeon({this.name, Object character}) : super(character: character);
}

Or even this?

class Bird<T> {
  T character;

  Bird({this.character});
}

class Pigeon extends Bird<String> {
  String name;

  Pigeon({this.name, String character}) : super(character: character);
}

Or this?

class Character {
  //
}

class Bird<T extends Character> {
  T character;

  Bird({this.character});
}

class Pigeon extends Bird<PigeonCharacter> {
  String name;

  Pigeon({this.name, PigeonCharacter character}) : super(character: character);
}

class PigeonCharacter extends Character {
  //
}

Answer 3

You need to use the named constructor as below

class Bird{
 Object character;
  
 Bird({this.character});
  
 Bird withCharater(Object character) {
   return Bird(character: character);
 }
}

class Pigeon extends Bird{
 String name;

 Pigeon(); 
}