Basic inheritance and polymorphism in Dart

main.dart

// Basic inheritance and polymorphism in Dart (as of v2.5)

// see https://dartpad.dartlang.org/030764a47bc7599b0e1853bda75c7639

void main() {
  
  print('');
  print('------------------ Tree object, Tree Reference ----------------------');
  print('');
  
  final Tree t = Tree();
  t.grow();
  
  print('');
  print('------------------ ChristmasTree object, Tree Reference ----------------------');
  print('');
  
  final Tree ct = ChristmasTree();
  ct.grow();
  
//   Expect error - Though this is a ChristmasTree object, BUT it is a Tree reference.
//   i.e., the holdOrnaments() method is not seen.

//   ct.holdOrnaments();
  
  
  print('');
  print('------------------ ChristmasTree object, ChristmasTree Reference ----------------------');
  print('');
  
  final ChristmasTree christmasTree = ChristmasTree();
  christmasTree.holdOrnaments();
  // Notice here too, that even though our "ct" variable is a Tree reference,
  // The Tree object has leafType as a field, and since it is visible, we use the overriden
  // version (i.e. we see "Needle" rather than "General")
  print("""
  
    A christmas tree has leaves of type "${christmasTree.leafType}"
    Really! They have "${ct.leafType}" type of leaves!
    But... Most trees have "${t.leafType}" types of leaves.
  """);
}


class Plant {
  String leafType = "General";
  
  Plant() {
    print('Created a plant!');
  }
  
  void grow() {
    print('Growing plantly.');
  }
}


class Tree extends Plant {
  Tree() {
    print('Created a tree!');
  }
  
  // Overrides the grow method of the parent.
  void grow() {
    // Call to grow method of the parent class via super keyword.
    super.grow();
    print('Growing tree-like');
  }
}


class ChristmasTree extends Tree {
  String leafType = "Needle";
  
  ChristmasTree() {
    print('Created a Christmas tree!');
  }
  
  void grow() {
    print('Growing ChristmasTree-like');
  }
  
  void holdOrnaments() {
    print('Only a Christmas tree can hold ornaments!');
  }
}