polymorph.dart

// Abstract class that other classes can implement
abstract class Item {
  use();
}

// Inherits the 'Item' class's 'use()' method
class Box<T> implements Item {
  // ADT list that can contain other objects
  List<T> contents;

  Box(this.contents);

  use() => print("$this has ${contents.length} items in it.");
}

// Inherits the 'Item' class's 'use()' method
class Ball implements Item {
  int value = 0;

  use() => print("$this is worth \$$value.");
}

// Child of 'Ball' with different 'value' integer
class RedBall extends Ball {
  int value = 5;
}

// Child of 'Ball' with different 'value' integer
class BlueBall extends Ball {
  int value = 10;
}

// Child of 'Ball' with different 'value' integer
class GreenBall extends Ball {
  int value = 20;
}

main() {
  // Instance of 'Box' with list type 'Item'
  var b = Box<Item>([
    RedBall(),
    BlueBall(),
    GreenBall()
  ]);

  // Prints the contents length of box
  b.use();

  // Prints the price of each object within the box's list
  for (var item in b.contents) {
    item.use();
  }
}