The strategy pattern defines a family of algorithms, encapsulates each one, and makes them interchangeable. It allows an algorithm's behavior to vary independently from clients that use it. The strategy pattern can help manage complex conditional logic by replacing conditionals with object composition. Walkthrough examples demonstrate extracting an algorithm into strategy objects from a class to vary behaviors independently through composition rather than inheritance.

1. Strategy Pattern
ThePicture.getInstance(new TakeStrategy());

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3. What is it?
“Define a family of algorithms, encapsulate each one, and
make them interchangeable. Strategy lets the algorithm
vary independently from clients that use it.” Gang of Four

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5. Why would I use it?
Strategy lets the algorithm vary independently from clients that use it
Strategy pattern allows you to prefer composition over inheritance
Adhere’s to the Open/Closed principle of SOLID
Helps manage the complexity of having numerous variations of an
algorithm

6. Replacing conditional
logic with the strategy
pattern
if(designPattern) {
profit();
}

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8. Walkthrough
Conditional logic in a variant controls which of several
variants of a calculation are executed.
Create a strategy for each variant and make the method
delegate the calculation to a Strategy instance.

9. public double capital() {
if (expiry == null && maturity != null)
return commitment * duration() * riskFactor();
if (expiry != null && maturity == null) {
if (getUnusedPercentage() != 1) {
return commitment * getUnusedPercentage()
* duration() * riskFactor();
} else {
return (outstandingRiskAmount() * duration() * riskFactor())
+ unusedRiskAmount() * duration() * unusedRiskFactor();
}
}
return 0.0d;
}
public double capital() {
return capitalStrategy.capital(this);
}
public double capital(Loan loan) {
return loan.getCommitment()
* duration(loan)
* riskFactorFor(loan);
}

10. Loan
double capital() : double
Loan
double capital() : double
CapitalStrategy
capital(loan : Loan) : double
CapitalStrategyAdvisedLine
capital(loan : Loan) : double
CapitalStrategyRevolver
capital(loan : Loan) : double
CapitalStrategyTermLoan
capital(loan : Loan) : double
1

11. Create a Concrete Strategy
public class CapitalStrategy {
}

12. Delegate to your Concrete
Strategy
public double capital() {
return new CapitalStrategy().capital(this);
}
public double capital() {
return new CapitalStrategy().capital(expiry, maturity, commitment, duration(), riskFactor()...);
}
Pass the context as parameter
Pass the necessary data parameters

13. Pass the context as parameter
- Making context methods visible to obtain
information, breaking ‘information hiding’
+ When your class gets new public methods they’re
instantly available

14. Pass the necessary data
parameters
- All data will be passed to all strategies regardless if
it needs that data
+ Involves the least coupling between the context and
the strategy

15. Pass the context as
parameter
public class CapitalStrategy {
public double capital(Loan loan) {
if (loan.getExpiry() == null && loan.getMaturity() != null) {
return loan.getCommitment() * loan.duration() * riskFactorFor(loan);
}
if (loan.getExpiry() != null && loan.getMaturity() == null) {
if (loan.getUnusedPercentage() != 1) {
return loan.getCommitment() * loan.getUnusedPercentage()
* loan.duration() * riskFactorFor(loan);
} else {
return (loan.outstandingRiskAmount()
* loan.duration() * riskFactorFor(loan))
+ loan.unusedRiskAmount()
* loan.duration() * unusedRiskFactorFor(loan);
}
}
return 0d;
}
}
private double riskFactorFor(Loan loan) {
return 0;
}
private double unusedRiskFactorFor(Loan loan) {
return 0;
}

16. Pass the context as
parameterpublic class Loan {
….
public double capital() {
return new CapitalStrategy().capital(this);
}
double duration() {
return 0d;
}
int getUnusedPercentage() {
return new Random().nextInt(1);
}
double outstandingRiskAmount() {
return 0d;
}
double unusedRiskAmount() {
return 0d;
}
Date getExpiry() {
return expiry;
}
Date getMaturity() {
return maturity;
}
double getCommitment() {
return commitment;
}
}

17. Move functionality related to capital calc
public class CapitalStrategy {
public double capital(Loan loan) {
if (loan.getExpiry() == null && loan.getMaturity() != null) {
return loan.getCommitment() * duration() * riskFactorFor(loan);
}
if (loan.getExpiry() != null && loan.getMaturity() == null) {
if (getUnusedPercentage() != 1) {
return loan.getCommitment() * getUnusedPercentage()
* duration() * riskFactorFor(loan);
} else {
return (outstandingRiskAmount()
* duration() * riskFactorFor(loan))
+ unusedRiskAmount()
* duration() * unusedRiskFactorFor(loan);
}
}
return 0d;
}
...

18. Extract Field
public class Loan {
private final Date expiry;
private final Date maturity;
private final double commitment;
private final CapitalStrategy strategy;
public Loan(Date expiry, Date maturity, double commitment) {
this.expiry = expiry;
this.maturity = maturity;
this.commitment = commitment;
this.strategy = new CapitalStrategy();
}
public double capital() {
return strategy.capital(this);
}
...
...
Date getExpiry() {
return expiry;
}
Date getMaturity() {
return maturity;
}
double getCommitment() {
return commitment;
}
}

19. Extract Parameter
public static Loan newTermLoad(Date expiry, Date maturity, double commitment) {
return new Loan(expiry, maturity, commitment, new CapitalStrategy());
}
public static Loan newRevolver(Date expiry, Date maturity, double commitment) {
return new Loan(expiry, maturity, commitment, new CapitalStrategy());
}
public Loan(Date expiry, Date maturity, double commitment, CapitalStrategy capitalStrategy) {
this.expiry = expiry;
this.maturity = maturity;
this.commitment = commitment;
this.strategy = capitalStrategy;
}

20. Replace conditional with Polymorphism
public class CapitalStrategyTermLoan extends CapitalStrategy {
public double capital(Loan loan) {
return loan.getCommitment() * duration() * riskFactorFor(loan);
}
@Override
protected double duration() {
return 0d; // specific calculation for term loans
}
}
public static Loan newTermLoad(Date expiry, Date maturity, double commitment) {
return new Loan(expiry, maturity, commitment, new CapitalStrategyTermLoan());
}

21. Replace conditional with Polymorphism
public static Loan newTermLoad(Date expiry, Date maturity, double commitment) {
return new Loan(expiry, maturity, commitment, new CapitalStrategyTermLoan());
}
public static Loan newRevolver(Date expiry, Date maturity, double commitment) {
return new Loan(expiry, maturity, commitment, new CapitalStrategyRevolver());
}
public static Loan newAdvisedLine(Date expiry, Date maturity, double commitment, int riskRating) {
if(riskRating > 3) return null;
return new Loan(expiry, maturity, commitment, new CapitalStrategyAdvisedLine());
}

22. CapitalStrategy changes
public abstract class CapitalStrategy {
public abstract double capital(Loan loan);
protected double duration() {
return 0d; // shared implementation
}
protected double riskFactorFor(Loan loan) {
return 0d; // shared implementation
}
}

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24. Benefits
+ Clarifies algorithms by decreasing or moving conditional
logic
+ Simplifies a class by moving variations of an algorithm
to a hierarchy
+ Enables one algorithm to be swapped for another at
runtime

25. Liabilities
- Complicates a design when simpler refactorings like
compose method are available
- Complicates how an algorithm obtains or receives data
from their context class

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27. Alternatives
Smaller changes
- Compose Method
- Decompose conditional
More suited changes
- Move embellishment to decorator
- Replace conditional with polymorphism
see: Refactoring to Patterns - Joshua Kerievsky

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29. Refactoring ‘towards’ a
pattern- Refactor to, towards or away
- Some patterns are all or nothing
- Aim is always for a better design
- Evaluate if the design has improved
- Pools of Insight
http://www.industriallogic.com/papers/khdraft.pdf

30. Further Reading
Refactoring - Martin Fowler
Refactoring to Patterns - Joshua Kerievsky
Head First Design Patterns - Elisabeth Freeman
Design Patterns - Gof
https://www.goodreads.com/genres/novoda-new-starts-
reading

31. throw new SlideIndexOutOfBoundsException();
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