Understanding Software Layering and Object Composition

Partitioning and Layering Fundamentals

The Basic Problem Change is a fact of life Requirements Technologies Bug Fixes Software Must Adapt

Solution: Software Layers Reduce software coupling Minimize the consequences of change Focused Unit Tests to verify change Example of Code Refactoring

Fundamental Concepts Difference between a type and an object Complex type Composition Interface

Type Complex Type class Employee { string name; void Pay() . } Data Behavior Simple Type

Object Employee emp = new Employee();

Object Composition class Auto { Engine engine; Wheel[4] wheels; } void Drive() { };

Object composition introduces dependencies class Engine { string manufacturer; int horsepower; class Wheel { string manufacturer; float tirePressure; } void Start(){ }; void Stop(){ }; void Turn(){ }; }

Write your applications so that the dependencies of one type on another are eliminated or minimized.

Make types dependent on type's behavior, not its implementation.

Unit tests verify that a type's behavior is correct.

Interfaces Interfaces describe behavior, not implementation interface IEngine { void Start(); void Stop(); } interface IWheel { void Turn(); }

Rewritten Engine, Wheel Classes class Engine : IEngine { string manufacturer; int horsepower; class Wheel : IWheel { string manufacturer float tirePressure; } } void Turn(); void Start () { } void Stop() { }

Interface Composition class Auto { IEngine engine; IWheel[4] wheels; } void Drive() { }

Interfaces Hide Implementation class Diesel: IEngine { string manufacturer; int horsepower; class WankelEngine : IEngine { string manufacturer; int rotationSpeed; } } void Start () { } void Stop() { } void Start () { } void Stop() { }

Coupling Preserve Essential Coupling Essential Semantics Remove Inessential Coupling Programming Artifacts

Electrical Analogy Wall socket interface removes the inessential coupling due to the physical shape of plugs and appliances An interface cannot remove the essential behavioral coupling of voltage and amperage of standard current

Complexity vs. Flexibility Interfaces add a level of indirection Put interfaces along application fault lines Hard to refactor out of a bad design

Interfaces vs. Inheritance Favor interface over object composition Interface Composition vs. Inheritance? class RacingCar : HighPerformanceCar : Auto Static Definition Need to Understand Base Class Behavior

"Inheritance Breaks Encapsulation" class HighPerformanceCar { virtual void Start() { TurnIgnition(); Press GasPedal(); } } class RacingCar : HighPerformanceCar { }

Interfaces Avoid Inheriting Implementation Restrict Inessential Coupling Make Interfaces Easy to Modify

Design Patterns Minimize Dependencies in Implementation Use Design Patterns Electrical Analogy Design to work with 110 or 220 volts? Use Transformer Pattern Flexibility even with Essential Coupling

Summary Reduce coupling and dependencies of complex types Use Interface Based Design Use Composition rather than Implementation Inheritance Write unit tests to validate behavior