_refactoring_inheritance.md

Improving on the refactoring in the clean code talk on inheritance, polymorphism, and testing by going from an inheritance approach to a composition approach.

Files should be read in order switch -> inheritance -> composition.

composition_version.dart

void main() {
  var root = new OpNode(add, new ValueNode(1),
      new OpNode(multiply, new ValueNode(2), new ValueNode(3)));
  print(root.toString());
  print('=');
  print(root.evaluate());
}

final multiply = new Multiplication();
final add = new Addition();

abstract class Operation {
  num evaluate(num left, num right);
}

class Multiplication implements Operation {
  num evaluate(num left, num right) => left * right;
  String toString() => '*';
}

class Addition implements Operation {
  num evaluate(num left, num right) => left + right;
  String toString() => '+';
}

abstract class Node {
  num evaluate();
}

class ValueNode implements Node {
  final num value;
  ValueNode(this.value);
  num evaluate() => value;
  String toString() => value.toString();
}

class OpNode implements Node {
  final Operation operation;
  final Node left;
  final Node right;
  OpNode(this.operation, this.left, this.right);
  num evaluate() => operation.evaluate(left.evaluate(), right.evaluate());
  String toString() => '$left $operation $right';
}

inheritance_version.dart

void main() {
  var root = new AdditionNode(new ValueNode(1),
      new MultiplicationNode(new ValueNode(2), new ValueNode(3)));
  print(root.toString());
  print('=');
  print(root.evaluate());
}

abstract class Node {
  num evaluate();
}

class ValueNode extends Node {
  final num value;
  ValueNode(this.value);
  num evaluate() => value;
  String toString() => value.toString();
}

abstract class OpNode extends Node {
  final Node left;
  final Node right;
  OpNode(this.left, this.right);
}

class AdditionNode extends OpNode {
  AdditionNode(Node left, Node right) : super(left, right);
  num evaluate() => left.evaluate() + right.evaluate();
  String toString() => '$left + $right';
}

class MultiplicationNode extends OpNode {
  MultiplicationNode(Node left, Node right) : super(left, right);
  num evaluate() => left.evaluate() * right.evaluate();
  String toString() => '$left * $right';
}

switch_version.dart

void main() {
  var root = new Node(
      '+',
      0,
      new Node('#', 1, null, null),
      new Node(
          '*', 0, new Node('#', 2, null, null), new Node('#', 3, null, null)));
  print(root.toString());
  print('=');
  print(root.evaluate());
}

class Node {
  final char operator;
  final num value;
  final Node left;
  final Node right;

  Node(this.operator, this.value, this.left, this.right);

  num evaluate() {
    switch (operator) {
      case '#':
        return value;
      case '+':
        return left.evaluate() + right.evaluate();
      case '*':
        return left.evaluate() * right.evaluate();
    }
  }

  String toString() {
    switch (operator) {
      case '#':
        return '$value';
      case '+':
        return '$left + $right';
      case '*':
        return '$left * $right';
    }
  }
}