Extract Hierarchy

You have a class that is doing too much work, at least in part through many conditional statements.

Create a hierarchy of classes in which each subclass represents a special case.

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Motivation

In evolutionary design, it is common to think of a class as implementing one idea and come to realize later that it is really implementing two or three or ten. You create the class simply at first. A few days or weeks later you see that if only you add a flag and a couple of tests, you can use it in a new case. A month later you have another such opportunity. A year later you have a real mess: flags and conditional expressions all over the place.

When you encounter a Swiss-Army-knife class that has grown to open cans, cut down small trees, shine a laser point at reluctant presentation bullet items, and, oh yes, I suppose cut things, you need a strategy for teasing apart the various strands. The strategy here works only if your conditional logic remains static during the life of the object. If not, you may have to use Extract Class before you can begin separating the cases from each other.

Don't be discouraged if Extract Hierarchy is a refactoring that you can't finish in a day. It can take weeks or months to untangle a design that has become snarled. Do the steps that are easy and obvious, then take a break. Do some visibly productive work for a few days. When you've learned something, come back and do a few more easy and obvious steps.

Mechanics

We've put in two sets of mechanics. In the first case you aren't sure what the variations should be. In this case you want to take one step at a time, as follows:

  • Identify a variation.

    If the variations can change during the life of the object, use Extract Class to pull that aspect into a separate class.

  • Create a subclass for that special case and use Replace Constructor with Factory Method on the original. Alter the factory method to return an instance of the subclass where appropriate.
  • One at a time, copy methods that contain conditional logic to the subclass, then simplify the methods given what you can say for certain about instances of the subclass that you can't say about instances of the superclass.

    Use Extract Method in the superclass if necessary to isolate the conditional parts of methods from the unconditional parts.

  • Continue isolating special cases until you can declare the superclass abstract.
  • Delete the bodies of methods in the superclass that are overridden in all subclasses and make the superclass declarations abstract.

When the variations are very clear from the outset, you can use a different strategy, as follows:

Example

The example is a nonobvious case. You can follow the refactorings for Replace Type Code with Subclasses, Replace Type Code with State/Strategy, and Replace Conditional with Polymorphism to see how the obvious case works.

We start with a program that calculates an electricity bill. The initial objects look like Figure 12.11.

Figure 12.11. Customer and billing scheme
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The billing scheme contains a lot of conditional logic for billing in different circumstances. Different charges are used for summer and winter, and different billing plans are used for residential, small business, customers receiving Social Security (lifeline), and those with a disability. The resulting complex logic makes the Billing Scheme class rather complex.

Our first step is to pick a variant aspect that keeps cropping up in the conditional logic. This might be various conditions that depend on whether the customer is on a disability plan. This can be a flag in Customer, Billing Scheme, or somewhere else.

We create a subclass for the variation. To use the subclass we need to make sure it is created and used. So we look at the constructor for Billing Scheme. First we use Replace Constructor with Factory Method. Then we look at the factory method and see how the logic depends on disability. We then create a clause that returns a disability billing scheme when appropriate.

We look at the various methods on Billing Scheme and look for those that contain conditional logic that varies on the basis of disability. CreateBill is one of those methods, so we copy it to the subclass (Figure 12.12).

Figure 12.12. Adding a subclass for disability
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Now we examine the subclass copy of createBill and simplify it on the basis that we know it is now within the context of a disability scheme. So code that says

if (disabilityScheme()) doSomething

can be replaced with doSomething

If disabilities are exclusive of the business scheme we can eliminate any code that is conditional on the business scheme.

As we do this, we like to ensure that varying code is separated from code that stays the same. We use Extract Method and Decompose Conditional to do that. We continue doing this for various methods of Billing Scheme until we feel we've dealt with most of the disability conditionals. Then we pick another variation, say lifeline, and do the same for that.

As we do the second variation, however, we look at how the variations for lifeline compare with those for disability. We want to identify cases in which we can have methods that have the same intention but carry it out differently in the two separate cases. We might have variation in the calculation of taxes for the two cases. We want to ensure that we have two methods on the subclasses that have the same signature. This may mean altering disability so we can line up the subclasses. Usually we find that as we do more variations, the pattern of similar and varying methods tends to stabilize, making additional variations easier.