Bad Smells: Hosting the change

Some problems become more apparent when you try to change the code. Ideally, a change decision affects only a single location. When this does not happen, it is a sign of code duplication. Detecting these problems usually has other benefits such as facilitating the testability of the code.

Bad smells related to forced simultaneous switching between classes are:

• Divergent Change
• Shotgun Surgery
• Parallel Inheritance Hierarchies
• Combinatorial Explosion

Divergent Change

Symptoms

The same class needs to change for different reasons.

To do

• If the class finds an object and does something with it, let the client find the object and pass it to it or let the class return a value that the client uses.
• Use Extract Class to create separate classes for each decision.
• If several classes are sharing the same type of decisions, you can unify them into a new class with Extract Superclass or Extract Subclass. If necessary, these classes can form a layer.

Rewards

• Improves communication (by better expressing the intention).
• Improve the structure for future changes.

Shotgun Surgery

Symptoms

Making a change involves modifying several classes.

To do

• Identify the class that should contain the change group. It can be an existing class or you have to create it with Extract Class.
• Use Move Field and Move Method to bring the functionality to the chosen class. Once the remaining classes are simple enough, you can use Inline Class to remove them.

Rewards

• Reduces duplication.
• Improves communication.
• Improves maintainability (next changes will be more localized).

Parallel Inheritance Hierarchies

Symptoms

• Creating a new subclass in a hierarchy means creating a related class in another hierarchy.
• You find two hierarchies where the superclasses have the same prefixes (the names reflect the requirement to coordinate them). This is a special case of Shotgun Surgery.

To do

Use Move Field/Method to redistribute the features in a way that you can eliminate one of the hierarchies.

Rewards

• Reduces duplication.
• Improves communication.
• You can reduce the size.

Combinatorial Explosion

Symptoms

• Introducing a new class means introducing multiple classes in various places in a hierarchy.
• Each layer of the hierarchy uses a common set of words (informs style, mutability, etc.).

To do

• If things haven’t gone too far, you can use Replace Inheritance with Delegation (when creating the same interface for variations, you can use the Decorator pattern.
• If the situation has become too complex, major refactorings such as Tease Apart Inheritance are required.

Rewards

• Reduces duplication.
• Reduces the size.