flutter - What is the difference between the two constructors?

Question

What is the difference between the constructors?

    class UserRepository {
    final FirebaseAuth _firebaseAuth;
    final GoogleSignIn _googleSignIn;

    UserRepository({FirebaseAuth firebaseAuth, GoogleSignIn googleSignin})
  : _firebaseAuth = firebaseAuth ?? FirebaseAuth.instance,
    _googleSignIn = googleSignin ?? GoogleSignIn();
    }

VERSUS

class UserRepository {
final FirebaseAuth _firebaseAuth;
final GoogleSignIn _googleSignIn;

UserRepository({this._firebaseAuth, this._googleSignin});

question from:https://stackoverflow.com/questions/65855266/what-is-the-difference-between-the-two-constructors

Answer 1

Briefly, on the above function, the default values are assigned when the constructor parameter are null. So It always has default value. (The ?? operator). While the below one is not.