The Beverages Prices Refactoring kata: a kata to practice refactoring away from an awful application of inheritance.

Published by Manuel Rivero on 07/04/2019

Katas, Learning, Community, Object-Oriented Design

I created the Beverages Prices Refactoring kata for the Deliberate Practice Program I’m running at Lifull Connect offices in Barcelona (previously Trovit). Its goal is to practice refactoring away from a bad usage of inheritance.

The code computes the price of the different beverages that are sold in a coffee house. There are some supplements that can be added to those beverages. Each supplement increases the price a bit. Not all combinations of drinks and supplements are possible.

Just having a quick look at the tests of the initial code would give you an idea of the kind of problems it might have:

	package unit_tests;

	import beverages.*;
	import org.junit.Test;

	import static org.hamcrest.CoreMatchers.is;
	import static org.hamcrest.MatcherAssert.assertThat;
	import static org.hamcrest.Matchers.closeTo;

	public class BeveragesPricingTest {
	private static final double PRECISION = 0.001;

	@Test
	public void computes_coffee_price() {
	Beverage coffee = new Coffee();
	assertThat(coffee.price(), is(closeTo(1.20, PRECISION)));
	}

	@Test
	public void computes_tea_price() {
	Beverage tea = new Tea();
	assertThat(tea.price(), is(closeTo(1.50, PRECISION)));
	}

	@Test
	public void computes_hot_chocolate_price() {
	Beverage hotChocolate = new HotChocolate();
	assertThat(hotChocolate.price(), is(closeTo(1.45, PRECISION)));
	}

	@Test
	public void computes_tea_with_milk_price() {
	Tea teaWithMilk = new TeaWithMilk();
	assertThat(teaWithMilk.price(), is(closeTo(1.60, PRECISION)));
	}

	@Test
	public void computes_coffee_with_milk_price() {
	Coffee coffeeWithMilk = new CoffeeWithMilk();
	assertThat(coffeeWithMilk.price(), is(closeTo(1.30, PRECISION)));
	}

	@Test
	public void computes_coffee_with_milk_and_cream_price() {
	Coffee coffeeWithMilkAndCream = new CoffeeWithMilkAndCream();
	assertThat(coffeeWithMilkAndCream.price(), is(closeTo(1.45, PRECISION)));
	}

	@Test
	public void computes_hot_chocolate_with_cream_price() {
	HotChocolateWithCream hotChocolateWithCream = new HotChocolateWithCream();
	assertThat(hotChocolateWithCream.price(), is(closeTo(1.60, PRECISION)));
	}
	}

view raw BeveragesPricingTest.java hosted with ❤ by GitHub

If that’s not enough have a look at its inheritance hierarchy:

Inheritance hierarchy in the initial code

To make things worse, we are asked to add an optional cinnamon supplement that costs 0.05€ to all our existing catalog of beverages. We think we should refactor this code a bit before introducing the new feature.

We hope you have fun practicing refactoring with this kata.

Originally published in Manuel Rivero's blog.

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