Strategy Pattern

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Strategy Pattern

• By SmartBoard Team

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Agenda

• Scenario Writing UML
• Discuss with each teams UML
• What if
• BMVV design
• Strategy pattern
• Applied with BMVV design
• More exercise..

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Scenario

• Act yourself as Car company owner
• Car component will consist of
• Engine Type
• Fuel used
• Nitrous (Additional boost)

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Scenario (2)

• From its components, we can calculate
• the fuel burn rate. So, it should include method
  getBurnFuel(double distance).
• It comes from Engines power Nitrous power (if
  used)
• Each car can used only
• One type of engine
• One type of Fuel
• One type of Nitrous
• You need some program to
• show fuel burn rate for each car
• Car components can be changed in the future.
• Every team, please write UML Class diagram to
  according to the requirement above

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Writing UML 10-15 min

• Take your time

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Times up.

• Lets see your UML diagram

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What if..

• More engine type is added
• More fuel type is added
• More nitrous type is added
• Some car cant use some engine
• No more benzene ??
• Can you design support this changes?

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Lets record your problem
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BMVV Car company design
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BMVV Car company design 2
protected double getBurnFuel(double
distance) if(engine.equalsIgnoreCase("1500CC")
fuel.equalsIgnoreCase("Benzene"))
double consume distance0.054 consume
consume18/17 return consume else
if(engine.equalsIgnoreCase("1500CC")
fuel.equalsIgnoreCase("Diesel")) ..
else if(engine.equalsIgnoreCase("1500CC")
fuel.equalsIgnoreCase("Gasohol")) ..
else if(engine.equalsIgnoreCase("1900CC")
fuel.equalsIgnoreCase("Benzene")) .
.. .
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Problems occurs when

• If 4500cc engine are added
• No more benzene used.
• Super Gasohol is invented.
• If each case has more conditions.

else if(engine.equalsIgnoreCase(4500CC)
fuel.equalsIgnoreCase(Benzene)) else
if(engine.equalsIgnoreCase(4500CC)
fuel.equalsIgnoreCase(Diesel)) else
if(engine.equalsIgnoreCase(4500CC) Another
fuel type ) . . .
else if(engine.equalsIgnoreCase(1500CC)
fuel.equalsIgnoreCase(SuperGasohol))
else if(engine.equalsIgnoreCase(1900CC)
fuel.equalsIgnoreCase(SuperGasohol))
else if(engine.equalsIgnoreCase(4500CC)
fuel.equalsIgnoreCase(SuperGasohol))
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Unable to encapsulates data

• According to this design, if you company need to
  outsource your work

Confidential

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Strategy patterns

• Define a family of algorithms, encapsulate, and
  make them interchangeable. Strategy lets the
  algorithm vary independently from clients that
  use it.

http//en.wikipedia.org/wiki/Strategy_pattern
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Thai military communication network
Every message will be encrypted.
Communication between a station and an unite
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Old cipher machine model
Break encapsulation rule

• Conditions
• decryptA
• decryptB
• decryptC
• None

(RTA,2)
Machine
1500 lines
SUB
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New cipher machine model
Alternative subclass
Object.decrypt(RTA)
ltinterfacegt
(RTA, object)
decryptB
Machine
decryptA
Eliminate conditions
Family of algorithms
A choice of implementations
None
SUB
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New cipher machine model

• Decrypt A and B can use signature only a string.
  Object.decrypt(String)
• But Decrypt C need more parameters.
  Object.decrypt(String, int)
• So method signature should have two parameters.
  Object.decrypt(String, int)
• Ex . ObjectA.decrypt(RTA, Null)
  ObjectB.decrypt(RTA, Null)
  ObjectC.decrypt(RTA, 366)

Communication overhead
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Consequences

• Families of related algorithms.
• An alternative to subclassing.
• Strategies eliminate conditional statements.
• A choice of implementations.
• Clients must be aware of different strategies.
• Communication overhead between Strategy and
  Context.
• Increased number of objects.

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Another example Sort

• Assume that, you need to write a sort program
• Therere many kinds of sort..
• Strategy is what we group the many algorithms
  that do the same things and make it
  interchangeable at run-time

Ascending Sort
SortStrategies
Bubble Sort
Selection Sort
Descending Sort
Result 1
Result 2
Input 1
Input 2
Heap Sort
Quick Sort
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Another example Sort 2
Change the algorithm at runtime
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Apply strategy with BMVV design

• Indicates the problem..

Hard to maintain
Move it to the new class
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Apply strategy with BMVV design
Need getBurnFuel() from different factor
So as Nitrous

• Re-designing

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Apply strategy with BMVV design
If 4500cc engine is added?
Engine4500ccBenzene
Engine4500ccDiesel
Engine4500ccGasohol

• Overall design

Easier when doing the maintenance !!!
If no more benzene?
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Apply strategy with BMVV design

• How it works at runtime

It will call BurnFuel(double) according to its
engine
Jazz is changing its engine
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An action adventure game
Start
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Solutions of exercise
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Q/A
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References

• www.viewimages.com
• www.thailandstrategy.com
• http//www.maxituning.pl/archiwum/0412/pics/techni
  ka/nos2.jpg
• www.mysticcobra.net/system/
• http//jpfamily.net/Coolpix/NHRA/slides/
• http//www.robinstewart.com/products/graphics/gs_i
  con.gif
• www.freewarebox.com/screenshot_2476_clock.html
• http//tv.pcworld.hu/apix/0612/Data20thief.png
• http//www.offthemarkcartoons.com/cartoons/