SOLID Design principles

'SOLID' Desgin Principles

Our aim is to make software more stable unlike ...

We write code that is …

Rigid

Fragile

Immobile

Making everyone's life miserable

Source: http://www.welaf.com/13500,miserable-life-of-a-small-cat.html

So WHY SOLID?

It helps us to write code that is ...

Loosely coupled

Highly cohesive

Easily composable

Reusable

SOLID

Coined by Robert C Martin

Not new, exsiting principles brought together

Single Responsibility Principle (SRP)

Single Responsibility Principle (SRP)

Open Closed Principle (OCP)

Single Responsibility Principle (SRP)

Open Closed Principle (OCP)

Liskov Substitution Principle (LSP)

Single Responsibility Principle (SRP)

Open Closed Principle (OCP)

Liskov Substitution Principle (LSP)

Interface Seggregation Principle (ISP)

Single Responsibility Principle (SRP)

Open Closed Principle (OCP)

Liskov Substitution Principle (LSP)

Interface Seggregation Principle (ISP)

Dependency Inversion Principle (DIP)

Single Responsibility Principle

Single Responsibility Principle

”There should be NEVER be more than ONE reason for a class to change”

Single Responsibility Principle

Rectangle-----------------------

Draw()Area()

Computationapplication

GUIapplication

GUI

What is the issue with this design?

Single Responsibility Principle

Rectangle does Computes the area Draws it on the GUI

How can we fix it?

Single Responsibility Principle

Rectangle-----------------------

Area()

Computationapplication

GUIapplication

GUI

GUIRectangle-----------------------

Draw()

INHERITS

Single Responsibility Principle

JDK follows this (java.awt package) Graphics2D – drawing shapes Shape – for representing the geometrical shapes

Some other examples: Task to download a file, parse it, and store it in a

database UserSetting- provide customisation feature, check

Access

Open Closed Principle

Open Closed Principle

”Modules must be OPEN for extension, CLOSED for modification”

Open Closed Principle

Coined by Bertrand Meyer

Open Closed Principle

How can you add new features without editing the existing module?

Open Closed Principle

Extend the existing modules!

Open Closed Principle

How is it even possible?

Open Closed Principle

Depend on abstractions, new features can be added by extending the abstractions.

Open Closed Principle

Examples-- Drawing shapes

- Loan approval process

Liskov Substitution Principle

Liskov Subsitution Principle

”Base classes instances must be replaceable by the sub class instances without any change in the

application”

Liskov Subsitution Principle

Lets go back to an earlier example of Drawing shape

Liskov Subsitution Principle

Other examples- - Bird, Flight and Non-Flight

- Loading of settings-Rectangle and Square

Interface Seggregation Principle

Inteface Seggregation Principle

”Clients should not depend upon the interfaces they do not use”

Inteface Seggregation Principle

How can we pollute the interfaces? OR

How do we end up creating Fat interfaces?

Inteface Seggregation Principle

Door, Time and TimedDoor example. This example also violates LSP

Inteface Seggregation Principle

Possible Solutions Use delegation- Adapter Pattern Use multiple inheirtance- Interfaces, Mixins

Dependency Inversion Principle

Dependency Inversion Principle

”High level modules should not depend on the low level details modules, instead both should depend

on abstractions”

Dependency Inversion Principle

Copy

Read Keyboard Write Printer

What's GOOD or BAD about this design?

Dependency Inversion Principle

Copy program is NOT REUSABLE Tightly bound to Keyboard and Printer

Read Keyboard and Write Printer are REUSABLE

Dependency Inversion Principle

Copy program shouldn't be dependent on the Low level Read/Write modules.

How can we correct this?

Dependency Inversion Principle

Copy

Keyboard Reader Print Writer

Reader Writer

Copy program now depends on Abstractions- Reader and Writer

Resources

Wikipedia – SOLID OO Design Robert C Martin articles. Jim Weirich SOLID Ruby talk at Ruby

Conference 2009. Rob Martin interview at Hansel Minutes. … and various other small articles,

presentations

Ideas for next learning sessions?

GoF Design Patterns One or two patterns each week with code samples

JVM Internals NoSQL Java and Concurrency Functional programming in Java