Classes

Two Types of Classes

Procedural

Business logic or Service

• Stateless, singleton or static if possible

• Instantiated mostly by the DI framework - Spring, Java EE

• If stateful, create it from the code with new

Data structure

Entity, DTO (Data Transfer Object)

• Only data members

• No business logic

• Always instantiated as new - by the database layer or with new from the code

• Accessors/mutators are questionable - getters/setters

Rules

• One reason to exist, change

• "Small"

• No "God" class - "Sack", "Blob"

• Encapsulation (hiding, protection) decreases dependency - Other classes cannot depend on this

• Tight cohesion

Cohesion

• Implements the "one thing" rule

• Contains only dependent members

• Refactor to cohesive classes

• There will be many small classes - Like a toolbox with drawers

• Reduces amount, cost and risk of changes

Interfaces

• Create interfaces for service classes -but not always paranoidly for everything

• Prefer interfaces to parent classes

• Create marker interfaces

Class Smells

No meaningful name

• You cannot give a meaningful name - e.g. "Parent", "Common", "Processor", etc.

Unnecessary polymorphism

• Polymorphic class is not used as polymorphic - never declared to parent type - see later

Does more things - Low cohesion

• Many methods - "God" or "Blob" class

• Methods of a class can be split into distinct call chains - they should be in separate classes

• Test coverage is not visible or cheating

• Refactor to composition + Facade pattern

Example: Refactoring low cohesion