Classes

When to Create

Cory discusses many situations where it is appropriate to create a new class:

• New concept
• Low cohesion
• Promote reuse
• Reduce complexity
• Clarify parameters

He explains the benefits of creating a new class in each of these situations

Cohesion

This clip explains the concept of high cohesion, which was first introduced by Tom De Marco in 1979.

Cory contrasts this with some of the magnet classes that were discussed in the Naming module.

As an example we see that a magnet Vehicle class is much better split out into three classes:

• Vehicle
• VehicleMaintenance
• VehicleFinance

Cory stresses that you should start with a good, specific class name, and that usually leads to higher cohesion.

When is a Class too Small?

Signs that a class is too small are:

1. Inappropriate intimacy
2. Feature Envy
3. Too many pieces

Steve Smith’s Refactoring Fundamentals course goes into a lot more detail in these areas.

Cory adds that it is very rare for classes to be too small, so be more concerned about classes that are too big.

Primitive Obsession

Primitive obsession is using primitive types such as strings and integers instead of encapsulating model classes.

Cory explains 4 advantages of creating a new class here.

Also see Mark Seemann’s From Primitive Obsession to Domain Modelling

Principle of Proximity

This is all about making code easy to read from the top of the file downwards, and Cory discusses keeping related actions together.

Outline Rule

Collapsed code should read like an outline. This is a tricky concept to grasp initially, so there is a demonstration later in this course.