Connascence of Algorithm revisited

Last time I fixed two instances of Connascence of Algorithm. Then I realised that I had introduced some more, so I fixed that too. After I had put that post to bed I also noticed that I had missed some alternative solutions. This article puts that right.

Towards the end of the previous article I noticed that the following test would fail:

public void independentCheckouts() {
  Money priceOfA = randomPrice();
  MultibuyDiscount discount = new MultibuyDiscount("A", Money.fromPence(20), 2);
  Checkout checkout1 = new Checkout(discount);
  Checkout checkout2 = new Checkout(discount);
  checkout1.scan("A", priceOfA);
  checkout2.scan("A", priceOfA);
  assertEquals(priceOfA, checkout1.currentBalance());
  assertEquals(priceOfA, checkout2.currentBalance());

I fixed it by introducing a MultibuyDiscountFactory, so that the Checkout could always create its own instance of the discounter. And then I went for a walk, during which I realised that I was uncomfortable about having two classes — the rule and the factory — for one concept, and both with the same constructor parameters.

So, after much tramping across fields, I back out the change that introduced the factory. I now know that I can fix that Connascence of Algorithm without even touching the test: I will update the MultibuyDiscount so that it can be cloned by the Checkout:

public class Checkout {
  public Checkout(MultibuyDiscount discount) { = discount.reset();
public class MultibuyDiscount {
  public MultibuyDiscount reset() {
    return new MultibuyDiscount(watchedSku, discount, trigger);

(On method naming: I decided that “clone” wasn’t a domain term, whereas it seems reasonable to ask a discount rule to reset itself before use. I may well find an even better name in the future.)

Next time I plan to look ahead at what remains to be done in this code, and also to look back and reflect on the journey so far…


One thought on “Connascence of Algorithm revisited

  1. Pingback: Connascence of Algorithm | silk and spinach

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