Tease Apart Inheritance

You have an inheritance hierarchy that is doing two jobs at once.

Create two hierarchies and use delegation to invoke one from the other.

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Motivation

Inheritance is great. It helps you write dramatically "compressed" code in subclasses. A single method can take on importance out of proportion with its size because of where it sits in the hierarchy.

Not surprisingly for such a powerful mechanism, it is easy to misuse inheritance. And the misuse can easily creep up on you. One day you are adding one little subclass to do a little job. The next day you are adding other subclasses to do the same job in other parts of the hierarchy. A week (or month or year) later you are swimming in spaghetti. Without a paddle.

Tangled inheritance is a problem because it leads to code duplication, the bane of the programmer's existence. It makes changes more difficult, because the strategies for solving a certain kind of problem are spread around. Finally, the resulting code is hard to understand. You can't just say, "This hierarchy here, it computes results." You have to say, "Well, it computes results, and there are subclasses for the tabular versions, and each of those has subclasses for each of the countries."

You can easily spot a single inheritance hierarchy that is doing two jobs. If every class at a certain level in the hierarchy has subclasses that begin with the same adjective, you probably are doing two jobs with one hierarchy.

Mechanics

  • Identify the different jobs being done by the hierarchy. Create a two-dimensional grid (or three- or four-dimensional, if your hierarchy is a real mess and you have some really cool graph paper) and label the axes with the different jobs. We assume two or more dimensions require repeated applications of this refactoring (one at a time, of course).
  • Decide which job is more important and is to be retained in the current hierarchy and which is to be moved to another hierarchy.
  • Use Extract Class at the common superclass to create an object for the subsidiary job and add an instance variable to hold this object.
  • Create subclasses of the extracted object for each of the subclasses in the original hierarchy. Initialize the instance variable created in the previous step to an instance of this subclass.
  • Use Move Method in each of the subclasses to move the behavior in the subclass to the extracted object.
  • When the subclass has no more code, eliminate it.
  • Continue until all the subclasses are gone. Look at the new hierarchy for possible further refactorings such as Pull Up Method or Pull Up Field.

Examples

Let's take the example of a tangled hierarchy (Figure 12.1).

Figure 12.1. A tangled hierarchy
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This hierarchy got the way it did because Deal was originally being used only to display a single deal. Then someone got the bright idea of displaying a table of deals. A little experiment with the quick subclass Active Deal shows you can indeed display a table with little work. Oh, you want tables of passive deals, too? No problem, another little subclass and away we go.

Two months later the table code has become complicated but there is no simple place to put it, time is pressing, the usual story. Now adding a new kind of deal is hard, because the deal logic is tangled with the presentation logic.

Following the recipe, the first step is to identify the jobs being done by the hierarchy. One job is capturing variation according to type of deal. Another job is capturing variation according to presentation style. So here's our grid:

Deal Active Deal Passive Deal
Tabular Deal

The next step tells us to decide which job is more important. The dealness of the object is far more important than the presentation style, so we leave Deal alone and extract the presentation style to its own hierarchy. Practically speaking, we should probably leave alone the job that has the most code associated with it, so there is less code to move.

The next step tells us to use Extract Class to create a presentation style (Figure 12.2).

Figure 12.2. Adding a presentation style
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The next step tells us to create subclasses of the extracted class or for each of the subclasses in the original hierarchy (Figure 12.3) and to initialize the instance variable to the appropriate subclass:

Figure 12.3. Adding subclasses of presentation style
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//ActiveDeal constructor
...
presentation = new SingleActivePresentationStyle();
...

You may well be saying, "Don't we have more classes now than we did before? How is this supposed to make my life better?" It is true that sometimes you have to take a step backward before you can take two steps forward. In cases such as this tangled hierarchy, the hierarchy of the extracted object can almost always be dramatically simplified once the object has been extracted. However, it is safer to take the refactoring one step at a time than to jump ten steps ahead to the already simplified design.

Now we use Move Method and Move Field to move the presentation-related methods and variables of the deal subclasses to the presentation style subclasses. We don't have a good way of simulating this with the example as drawn, so we ask you to imagine it happening. When we're done, though, there should be no code left in the classes Tabular Active Deal and Tabular Passive Deal, so we remove them (Figure 12.4).

Figure 12.4. The tabular subclasses of Deal have been removed
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Now that we've separated the two jobs, we can work to simplify each separately. When we've done this refactoring, we've always been able to dramatically simplify the extracted class and often further simplify the original object. The next move will get rid of the active-passive distinction in the presentation style in Figure 12.6.

Figure 12.6. Presentation differences can be handled with a couple of variables
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Even the distinction between single and tabular can be captured by the values of a few variables. You don't need subclasses at all (Figure 12.6).

Figure 12.5. The hierarchies are now separated
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