我们如何从 dart 中的抽象类实例化？

Question

How we can instantiate from abstract class in dart like below:

int a = 5;
String a = ‘Hello’;
List<int> myList1= [50,60,70];

these classes are all abstract but we get instance from each one.

Answer 1

Abstract classes is allowed to have factory constructors and/or static methods which returns instances which are compatible with the abstract class.

So we can do things like this where the actual implementation is hidden from users of our library:

abstract class A {
  String getString();

  factory A() => _A();
  static A getA() => _A();
}

class _A implements A {
  @override
  String getString() => 'Hello World';
}

void main() {
  A a1 = A();
  A a2 = A.getA();
  print(a1.getString()); // Hello World
  print(a2.getString()); // Hello World
  print(a1.runtimeType); // _A
  print(a2.runtimeType); // _A
}

Answer 2

The expressions 5 , 'hello' and [50, 60, 70] are literals , not class instantiations (no constructor is invoked in the creation, well, as far as you know). The literals produce values which satisfy the interfaces int , String and List<int> , in some magical way which the language doesn't specify (it just says what the end result must be), and the objects do not have to be actual instances of those types.

In practice, they aren't. Each platform has their own implementation. On the native VM, 5 is an instance of _Smi (short for "small integer"), 'hello' is an instance of _OneByteString and [50, 60, 70] is an instance of _GrowableList<String> . All these implementations classes are hidden from users because they are platform-specific. On the web, each class uses an underlying JavaScript type, Number , String and Array , because those are very efficient, and then lies about it if asked, pretending that they are really Dart classes.

You cannot create an instance of an abstract class. You can create an instance of a class which implements the interface of an abstract class. That's what's happening here — plus some platform-native magic to hide which class that is, because you shouldn't care about the native implementations below. (Also because that might just change in the next Dart SDK release, because some other implementation is more efficient.)