Dart Multiple Constructors

You can only have one unnamed constructor, but you can have any number of additional named constructors

class Player {
  Player(String name, int color) {
    this._color = color;
    this._name = name;
  }

  Player.fromPlayer(Player another) {
    this._color = another.getColor();
    this._name = another.getName();
  }  
}

new Player.fromPlayer(playerOne);

This constructor can be simplified

  Player(String name, int color) {
    this._color = color;
    this._name = name;
  }

to

  Player(this._name, this._color);

Named constructors can also be private by starting the name with _

class Player {
  Player._(this._name, this._color);

  Player._foo();
}

Constructors with final fields initializer list are necessary:

class Player {
  final String name;
  final String color;

  Player(this.name, this.color);

  Player.fromPlayer(Player another) :
    color = another.color,
    name = another.name;
}

If your class uses final parameters the accepted answer will not work. This does:

class Player {
  final String name;
  final String color;

  Player(this.name, this.color);

  Player.fromPlayer(Player another) :
    color = another.color,
    name = another.name;
}

If you already used a constructor with params in the project and now you figured out that you need some no params default constructor you can add an empty constructor.

class User{
String name;

   User({this.name}); //This you already had before
   User.empty(); //Add this later 
}

Try the below code on DartPad

class MyClass {
  //These two are private attributes
  int _age;
  String _name;

  //This is a public attribute
  String defaultName = "My Default Name!";

  //Default constructor
  MyClass() {
    _age = 0;
    _name = "Anonymous";
  }

  MyClass.copyContructor(MyClass fromMyClass) {
    this._age = fromMyClass._age;
    this._name = fromMyClass._name;
  }

  MyClass.overloadedContructor(String name, int age) {
    this._age = age;
    this._name = name;
  }

  MyClass.overloadedContructorNamedArguemnts({String name, int age}) {
    this._age = age;
    this._name = name;
  }

  //Overriding the toString() method
  String toString() {
    String retVal = "Name:: " + _name + " | " + "Age:: " + _age.toString();
    return retVal;
  }
}

//The execution starts from here..
void main() {
  MyClass myClass1 = new MyClass();

  //Cannot access oprivate attributes
  //print(myClass1.name);
  //print(myClass1.age);

  //Can access the public attribute
  print("Default Name:: " + myClass1.defaultName);

  print(myClass1.toString());

  MyClass myClass2 = new MyClass.copyContructor(myClass1);

  print(myClass2.toString());

  MyClass myClass3 = new MyClass.overloadedContructor("Amit", 42);

  print(myClass3.toString());

  MyClass myClass4 =
      new MyClass.overloadedContructorNamedArguemnts(age: 42, name: "Amit");

  print(myClass4.toString());
}

You can use factory constructors

factory Player.fromPlayer(Player another) => Player(another.name, another.color);