Learn Practical Refactoring Example | Composition and Polymorphism

Refactoring procedural code into object-oriented programming (OOP) is a foundational skill for any Dart developer aiming to build maintainable and scalable applications. By moving from flat, function-based code to structured classes and interfaces, you can better organize logic, make code easier to extend, and reduce the risk of bugs as your project grows.

main.dart


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// Procedural approach to managing shapes

import 'dart:math';

// Calculate area for a circle
double circleArea(double radius) {
  return pi * radius * radius;
}

// Calculate area for a rectangle
double rectangleArea(double width, double height) {
  return width * height;
}

void printShapeArea(String shape, Map<String, double> params) {
  double area;
  if (shape == 'circle') {
    area = circleArea(params['radius']!);
    print('Circle area: $area');
  } else if (shape == 'rectangle') {
    area = rectangleArea(params['width']!, params['height']!);
    print('Rectangle area: $area');
  } else {
    print('Unknown shape');
  }
}

void main() {
  printShapeArea('circle', {'radius': 5});
  printShapeArea('rectangle', {'width': 4, 'height': 6});
}

In the procedural version, you manage shapes using separate functions and a control flow that checks shape types with strings. This approach leads to scattered logic, making it difficult to add new shapes or change existing behavior without editing multiple places in the code. The refactored OOP version introduces a Shape abstract class, which defines a common interface for all shapes. Both Circle and Rectangle inherit from Shape, implementing their own area methods. The printShapeArea function now accepts any Shape, supporting polymorphism and making it easy to add new shapes by simply extending the Shape class. This design centralizes logic, improves readability, and reduces the chance of errors when extending functionality.

main.dart


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// main.dart
// Refactored OOP with interfaces for extensibility

import 'dart:math';

// Interface for drawable objects
abstract class Drawable {
  void draw();
}

// Shape interface for polymorphism
abstract class Shape {
  double area();
}

// Circle implements both Shape and Drawable
class Circle implements Shape, Drawable {
  final double radius;
  Circle(this.radius);

  @override
  double area() => pi * radius * radius;

  @override
  void draw() {
    print('Drawing a circle with radius $radius');
  }
}

// Rectangle implements both Shape and Drawable
class Rectangle implements Shape, Drawable {
  final double width;
  final double height;
  Rectangle(this.width, this.height);

  @override
  double area() => width * height;

  @override
  void draw() {
    print('Drawing a rectangle of $width x $height');
  }
}

void printShapeArea(Shape shape) {
  print('${shape.runtimeType} area: ${shape.area()}');
}

void drawShape(Drawable drawable) {
  drawable.draw();
}

void main() {
  Shape circle = Circle(5);
  Shape rectangle = Rectangle(4, 6);

  printShapeArea(circle);
  printShapeArea(rectangle);

  drawShape(circle as Drawable);
  drawShape(rectangle as Drawable);
}

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Section 3. Chapter 3

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main.dart


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// Procedural approach to managing shapes

import 'dart:math';

// Calculate area for a circle
double circleArea(double radius) {
  return pi * radius * radius;
}

// Calculate area for a rectangle
double rectangleArea(double width, double height) {
  return width * height;
}

void printShapeArea(String shape, Map<String, double> params) {
  double area;
  if (shape == 'circle') {
    area = circleArea(params['radius']!);
    print('Circle area: $area');
  } else if (shape == 'rectangle') {
    area = rectangleArea(params['width']!, params['height']!);
    print('Rectangle area: $area');
  } else {
    print('Unknown shape');
  }
}

void main() {
  printShapeArea('circle', {'radius': 5});
  printShapeArea('rectangle', {'width': 4, 'height': 6});
}

main.dart


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// main.dart
// Refactored OOP with interfaces for extensibility

import 'dart:math';

// Interface for drawable objects
abstract class Drawable {
  void draw();
}

// Shape interface for polymorphism
abstract class Shape {
  double area();
}

// Circle implements both Shape and Drawable
class Circle implements Shape, Drawable {
  final double radius;
  Circle(this.radius);

  @override
  double area() => pi * radius * radius;

  @override
  void draw() {
    print('Drawing a circle with radius $radius');
  }
}

// Rectangle implements both Shape and Drawable
class Rectangle implements Shape, Drawable {
  final double width;
  final double height;
  Rectangle(this.width, this.height);

  @override
  double area() => width * height;

  @override
  void draw() {
    print('Drawing a rectangle of $width x $height');
  }
}

void printShapeArea(Shape shape) {
  print('${shape.runtimeType} area: ${shape.area()}');
}

void drawShape(Drawable drawable) {
  drawable.draw();
}

void main() {
  Shape circle = Circle(5);
  Shape rectangle = Rectangle(4, 6);

  printShapeArea(circle);
  printShapeArea(rectangle);

  drawShape(circle as Drawable);
  drawShape(rectangle as Drawable);
}

Everything was clear?

Thanks for your feedback!

Section 3. Chapter 3