class Animal{
//   1. initialize at the declaration
  num age = 1;
//   2. assign variable in the constructor
  num leg;
  Animal(this.leg);
}

class Plant{
//   3. use the initialozer list
  num weight;
  num height;
  Plant(weight,height):this.weight = weight,this.height = height;
}

class Human{
//   4. no-final variable can be initialize in the constructor.
  final String name;
  num age;
  Human(name,age):this.name = name{
    this.age = age;
  }

//     The constructor below will casue a compile error because a final variable do not have a setter.
//     this.name = name;
//   Human(name,age){
//     this.age = age;
//     this.name = name;
//   }

}

void main (){
  Animal animal = new Animal(4);
  Plant plant = new Plant(1,2);
  Human human = new Human("Tom", 14);
}