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

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