choose a pattern for a problem

Choose a patternChoose a patternfor a problem!for a problem!

Agile Saturday VIAgile Saturday VI

Anton KeksAnton Keksanton@codeborne.comanton@codeborne.com

The Increasing ComplexityThe Increasing Complexity

Time

Complexity of software systems

New programming paradigmsHigher level of abstraction

HW Assembler Structural (objects+functions?) CPUs Procedural <OOP> ...

What are design patterns?What are design patterns?

● Anywhere in the world you can find recurring patterns

● Design patterns

– are building blocks of a system's architecture

– are recurring solutions to design problems that you see over

– identify and specify abstractions that are above the level of single classes and instances

Pattern classificationPattern classification

The GOF book defines 3 major types of patterns:– Creational patterns – substitute the new keyword. This

gives your program more flexibility in deciding which objects need to be created for a given case

– Structural patterns - compose groups of objects into larger structures, such as complex user interfaces or accounting data

– Behavioral patterns - define the communication between objects and how the flow is controlled in a complex program.

DecoratorDecorator

AdapterAdapter

CompositeComposite

FactoryFactory

BuilderBuilder

PrototypePrototype

StrategyStrategy

StateState

ObserverObserver

IteratorIterator

Software Design: The GoodSoftware Design: The Good

● Loose coupling● Immutability● Encapsulation● Short code blocks● Appropriate typing● Duplication avoidance● Simplicity● Consistent terminology ● Clear naming

● Loose coupling● Immutability● Encapsulation● Short code blocks● Appropriate typing● Duplication avoidance● Simplicity● Consistent terminology ● Clear naming

Bad, bad, bad!Bad, bad, bad!

Tight coupling

Mutable data structures

Tramp data

Monster functions/methods

String/Integer overuse

Copy-Paste

Over-engineering

Mixture of terms

Runtime surprises

Let's get to business!Let's get to business!

● Next slides describe problems– What is given

(existing code)– What we want to achieve

(using a pattern)

● As usual, the code is on github: http://github.com/angryziber/patterns

Problem 1: PizzaProblem 1: Pizza

● We have:– A pizzeria– Multitude of options for pizzas: name, toppings,

thickness, size, delivery address, etc● We want:

– Allow users to “order” custom pizzas– Disallow changing the order once the pizza is

already in the oven– Keep the ordering code readable

Problem 2: UnzipperProblem 2: Unzipper

● We have: – unzip() method– It is able to decompress a single compressed

stream● We need:

– Support multiple-volume archives– Compressed data is split into multiple floppies

Problem 3: MapsProblem 3: Maps

● We have:– Map data of entire Tallinn with all the streets,

buildings, etc– The MapVisualizer class that can draw the full map

on the screen● We want:

– User to see only a section of the map at any given time

– User to be able to move around the map

Problem 4: CD PlayerProblem 4: CD Player

● We have:– A CD player with buttons play, stop, next, eject

● We want:– Play to switch the player on– Play again to pause playback, again to resume– Stop to turn off the player– Eject to open the disk tray (if no disk – no playback

possible)– etc, etc, etc

Problem 5: Vending MachineProblem 5: Vending Machine

● We have:– A vending machine that can sell any small enough

items● We want:

– To deploy several such machines selling bottles of pepsi, sushi boxes, and Hintai magazines

– Each machine to sell items of one type

Problem 6: SpreadsheetProblem 6: Spreadsheet

● We have:– A spreadsheet-type program– Containing a table for entering data and formulas

● We want:– The program to support charts– The charts to be updated automatically if user

changes the data in the table– To sell the chart add-on separately for only $99.99

Problem 7: BattlefieldProblem 7: Battlefield

● We have:– A tank– Several types of bullets– Lots of enemies around, e.g. airplanes, other

tanks, guys with Molotov-coctails● We want:

– To survive– To kill 'em all

Problem 8: TCP over DNSProblem 8: TCP over DNS

● We have:– A paid Wi-Fi network (e.g. in an airport)– Lots of spare time, but no credit card

● We want:– To be able to read emails and surf Facebook– (Most likely, during our next trip)

● Background info:– Paid Wi-Fi usually resolves DNS just fine– DNS packets can contain any payload

Thank you!Thank you!

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