Monday, 26 March 2012

Extract, Inject, Kill: Breaking hierarchies (part 3)

In part one I explain the main idea behind this approach and in part two I start this example. Please read parts one and two before reading this post

Although the main ideas of Extract, Inject, Kill is already expressed, it's good to finish the exercise just for completion's sake. Here is where we stopped:

Let's have a look at the VoucherPricingService, that now is the only concrete class at the bottom of our hierarchy. 

Note that it uses the VoucherService class to calculate the voucher value.

Before anything, let's write some tests to

Once thing to notice is that the User parameter is not used for anything. So let's remove it.

Now it is time to user the Extract, Inject, Kill on the VoucherPricingService. Let's Extract the content of the VoucherPricingService.applyAdditionalDiscounts(double, String) method and add it to a class called VoucherDiscountCalculation. Let's call the method calculateVoucherDiscount(). Of course, let's do that writing our tests first. They need to test exactly the same things that are tested on VoucherPricingService.applyAdditionalDiscounts(double, String). We also take the opportunity to pass the VoucherService object into the constructor of VoucherDiscountCalculation.

If you noticed, when doing the extraction, we took the opportunity to give proper names to our new classes and methods and also to pass their essential dependencies to the constructor instead of using method injection.
Let's now change the code in the VoucherPricingService to use the new VoucherDiscountCalculation and see if all the tests still pass.

Cool. All the tests still pass, meaning that we have the same behaviour, but now in the VoucherDiscountCalculation class, and we are ready to move to the Inject stage.

Let's now inject VoucherDiscountCalculation into PricingService, that is the top class in the hierarchy. As always, let's add a test that will test this new collaboration.

And here is the changed PriningService.

Now it is time to kill the VoucherPricingService class and kill the PricingService.applyAdditionalDiscounts(double total, String voucher) template method, since it is not being used anymore. We can also kill the VoucherPricingServiceTest class and fix the PricingServiceTest removing the applyAdditionalDiscounts() method from the testable class.

So now, of course, we don't have a concrete class in our hierarchy anymore, since the VoucherPricingService was the only one. We can now safely promote UserDiscountPricingService to concrete.

That is now how our object graph looks like:

Our hierarchy is another level short. The only thing we need to do now is to apply  Extract, Inject, Kill once again, extracting the logic inside UserDiscountPricingService into another class (e.g. UserDiscountCalculation), inject UserDiscountCalculation into PricingService, finally kill UserDiscountPricingService and the calculateDiscount(User user) template method.  UserDiscountPricingService

Since the approach was described before, there is no need to go step by step anymore. Let's have a look at the final result.

Here is the diagram representing where we started:

  After the last Extract, Inject, Kill refactoring, this is what we've got:

The cool thing about the final model pictured above is that now we don't have any abstract classes anymore. All classes and methods are concrete and every single class is independently testable. 

That's how the final PricingService class looks like:

For a full implementation of the final code, please look at

Note: For this three part blog post I used three different approaches to drawing UML diagrams. By hand, using ArgoUML and Astah community edition. I'm very happy with the latter. 


Post a Comment