Talk: A design pattern goes to the supermarket By: Kaya Weers Given on various conferences and Meetups

1. A design pattern
goes to the supermarket
Kaya Weers

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First, an important question..

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So… Design Patterns?
Design patterns don’t give you code.
They provide general, reusable solutions for the common
problems in software design
Blueprints that you can customize.

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A bit of history
• Book by the Gang of Four (1994)
• 23 patterns, in 3 categories
• Ways to solve problems
• Same vocabulary

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Let me introduce you:
Bobby
Billie

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On a Saturday afternoon..

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When..

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She said:

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So, what’s the pattern?
• Mom wants groceries
• She writes her request on a note: what needs to be
done by who.
• Little sister’s tasks is to bring the note to the table
• Bobby knows how to handle the request on the note

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Command pattern
• Encapsulates request as a stand-alone object
• Decouple trigger from the content request
• Invoker doesn’t need to know content of request

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Command pattern
(client) (invoker) (Command)
(receiver)
Mom Little Sister Note
DoGroceries
Note
Bobby
interface
setCommand(note) execute()
execute()
doGroceries()

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* constructors omitted for readability *

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Usage in the software world
• Queue or schedule
• Reversible operations

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10 minutes later..

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They cannot find the coffee

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“Sigh…“

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So, what’s the pattern?
• Bobby asks employee when the coffee will be back in
stock
• Employee doesn’t know at that moment
• Employee doesn’t want customers to ask him again
• Customers can sign up to be notified as soon as the
product is available

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Observer pattern
• One-to-many relationship
• When the state from an object changes,
all dependencies are notified
• Loose coupling

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Observer pattern
(subject)
(Observers)
StockUpdate
Bobby
Customer 1
Customer 2
register(observer)

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Observer pattern (Observers)
StockUpdate
Bobby
Customer 1
Customer 2
notifyObservers()
update()
update()
update()
(subject)

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* constructors omitted for readability *

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Usage in the software world
• Dynamic composition of objects at runtime
• No control over who gets information

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The next product..
.. is also a challenge

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But, the text is in..
Spanish!

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Luckily, it’s 2024

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So, what’s the pattern?
• Billie wants to read the ingredients
• But Billie can’t read Spanish
• Translation app converts the Spanish words to
English words
• Billie can read English

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Adapter pattern
• Lets classes work together that couldn’t otherwise,
because of incompatible interfaces
• The adapter converts an interface into an interface
that the client expects
• More generic interface

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Adapter pattern
(client) (client interface)
(adapter)
Billie Product
Spanish
Product
Product
Translator
getIngredients()
getIngredients
InSpanish()
getIngredients()
(service)
interface

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* constructors omitted for readability *

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Usage in the software world
• Library logic separate à easy to change
• No unwanted logic all over codebase

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Last on their list..

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They go to the bakery

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They enter the French bakery

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So, what’s the pattern?
• Order supermarket bakery: ‘bread’
• Order French bakery: ‘bread’
• Different result: different variants of bread
• Creation differs per location

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Factory method pattern
• Interface for creating objects in superclass
• Object creation in subclass, can alter the type of
objects
• Easy to extend: add new creator subclasses

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Factory method pattern
Bread
WholeGrain
SlicedBread
interface
Baguette
Bakery
Supermarket
Bakery
createBread()
createBread()
abstract class
French
Bakery
createBread()
factory method

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* constructors omitted for readability *

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Usage in the software world
• Don’t know all the types beforehand
• Extending library

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The smell of the bakery
.. made them hungry.

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The baker prepares the sandwiches

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The baker follows steps

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So, what’s the pattern?
• Bobby and Billie order two different sandwiches
• The baker creates them with different ingredients
• Most of the steps are the same, easy for baker
• Some steps are optional, like the topping

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Template method pattern
• Superclass defines a skeleton
• Subclasses have specific implementation for steps
• Superclass can contain (default) implementation

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Template method pattern
Sandwich
Chicken
Sandwich
prepare()
addFirstLayer()
addFirstLayer()
abstract class
Vegetable
Sandwich
addFirstLayer()
template method

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* constructors omitted for readability *

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Usage in the software world
• Fewer changes when skeleton changes
• Force structure

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With both their shopping bags and
bellies full..
they go home

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They drop the bag at the floor

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5 min later..

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So, what’s the pattern?
• Mom gives 1 instruction: “put away groceries”
• Bobby and Billie put everything in the right place
• Mom gives no specific instructions
• Mom doesn’t need to know about steps taken

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Facade pattern
• Simplified interface for client
• Client doesn’t need to know about subsystem
• Removes complexity from the interface

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Facade pattern
Bobby
Freezer
putAwayGroceries()
Cabinet Refrigerator Pantry
(client)
Mom
(facade)

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* constructors omitted for readability *

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Usage in the software world
• Home automation
• Multiple facades for complex subsystem

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Summary
Command Observer Adapter
Facade
Template method
Factory method

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Going to the supermarket..
will never be the same again

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THANK YOU
@KayaWeers
/KayaWeers