1. Summary prepared by Kirk Scott
Chapter 23 Strategy
Summary prepared by Kirk Scott

4. Design Patterns in Java Chapter 23 Strategy
Summary prepared by Kirk Scott

5. The Introduction Before the Introduction
In general, a single strategy might be thought of as an algorithm or an operation
In the context of the Strategy design pattern, the idea is that there are multiple approaches to doing something, depending on certain conditions or context
The Strategy design pattern, then, depends on picking the approach or picking the strategy

6. When there is more than one way to go about doing something, complexity can result
There are the implementations of the different strategies
There is also the code for choosing which strategy to use

7. The purpose of the Strategy design pattern is to separate the implementations of different strategies from each other
It also separates the code for picking the strategy from the strategy implementations

8. The pattern defines a single interface for all strategies
The separate strategies are implemented with a method of the same name in each of the classes implementing the interface

9. Which strategy is used will depend on what kind of object the method implementing the strategy is called on
The intent of the pattern is realized through an interface and depends on polymorphism and dynamic binding

10. Book Definition of Pattern
The intent of Strategy is to encapsulate alternative approaches, or strategies, in separate classes that each implement a common operation.

11. Modeling Strategies
Like with the previous chapters, and others, the book illustrates the Strategy design pattern in the following way:
It develops an example with multiple strategies that doesn’t use the Strategy design pattern
It then refactors the example using the Strategy design pattern

12. Example Scenario
When a potential customer calls in, interested in buying fireworks, there is software which will make a recommendation or suggestion
There are several different ways a recommendation can be made

13. Ways of recommending a purchase:
Recommend a particular firework that is being promoted
Use a piece of software, Rel8
Use another piece of software, LikeMyStuff
Use a default recommendation option

14. Promoted Firework There is nothing special about this option
If the company is promoting a firework, recommend it

15. Rel8 Rel8 relies on a customer’s already being registered
During registration the customer specifies preferences in entertainment and fireworks
Rel8 makes a suggestion based on the similarity of the customer to other customers (presumably suggesting something that similar customers have tended to buy)
If the customer isn’t registered, Rel8 can’t be used

16. LikeMyStuff
LikeMyStuff doesn’t rely on pre-registration, but it does rely on customer information
The idea is that it will make a recommendation based on a profile of recent purchases by the customer
If not enough data can be obtained to form the profile, then LikeMyStuff can’t be used

17. The Default Option This is the default:
If none of the previous options applies, then a firework is suggested at random

18. UML for the Scenario
The UML diagram on the following overhead shows the classes involved in the design as described so far
Appendix D on UML clarifies the notation:
“Use a dashed arrow between classes to show a dependency that does not use an object reference. For example, the Customer class relies on a static method from the LikeMyStuff recommendation engine.”

20. The getRecommended() Method
Viewing the scenario from the top down, what you have is this:
The Customer class has a getRecommended() method in it
This method consists of if/else code which chooses one of the strategies, whether to do a promotion, or to use Rel8, LikeMyStuff, or the default

21. Doing a Promotion
If there is a promotion underway, the first part of the logic of getRecommended() deals with that case
The logic for doing a promotion consists of looking up the contents of a file named strategy.dat in a directory named config
If there is such a file, its contents should look something like this: promote=JSquirrel

22. The basic idea is that if the data file is not empty, the firework it contains is returned
If its contents come up null you go on to the next option
Also, if the file read doesn’t work, you don’t do anything in the catch block, you just continue on to the other options

23. Using Rel8 The Rel8 class has a method advise()
getRecommended() wraps a call to advise() if the Rel8 strategy is selected
The call looks like this:
if(isRegistered())
return (Firework) Rel8.advise(this);
“this” is the customer, and Rel8 relies entirely on the information contained in the registered customer object

24. Using LikeMyStuff The LikeMyStuff class has a suggest() method
getRecommended() wraps a call to suggest() if the LikeMyStuff strategy is selected
The call looks like this:
if(spendingSince(cal.getTime()) > 1000)
return (Firework) LikeMyStuff.suggest(this);

25. spendingSince() is called on the implicit parameter, customer
cal in the parameter refers to an instance of Calendar
getTime() specifies a recent period of time
“this” is the customer, which is sent as a parameter to suggest()

26. suggest() relies on a database of recent purchases by that customer
The idea is that if the customer has recently spent $1,000, those purchases provide the basis for a recommendation

27. Doing the Default
The Firework class has a getRandom() method, so if all else fails, getRecommended() wraps a call to that method

28. The Code for getRecommended() in the Customer Class
The code for getRecommended() is shown on the following overheads
It is a collection of if statements.
It is unfortunate that it is not organized as a sequence of if/else if’s.

29. The Code for getRecommended()
public Firework getRecommended() {
// if promoting a particular firework, return it
try
Properties p = new Properties();
p.load(ClassLoader.getSystemResourceAsStream(“config/strategy.dat”));
String promotedName = p.getProperty(“promote”);
if(promotedName != null)
Firework f = Firework.lookup(promotedName);
if(f != null)
return f; }

30. catch(Exception ignored){
// If resource is missing or it failed to load,
// fall through to the next approach. }
// if registered, compare to other customers
if(isRegistered())
return (Firework) Rel8.advise(this);

31. // check spending over the last year
Calendar cal = Calendar.getInstance();
cal.add(Calendar.YEAR, -1);
if(spendingSince(cal.getTime()) > 1000)
return (Firework) LikeMyStuff.suggest(this);
// oh well!
return Firework.getRandom(); }

32. What’s Wrong with the Initial Design
The book identifies two basic problems with the getRecommended() method as given:
It’s too long
It combines both selecting a strategy and executing it

33. This is actually one of the high points of the book
It explains that you know that the method is too long because you need to put comments in it
“Short methods are easy to understand, seldom need explanation…”

34. Comments Are Bad…
Finally, what every student always knew: Comments are bad…
More accurately, you might facetiously say that code which requires comments is bad.
The book doesn’t say that putting a comment at the beginning for the whole method is bad.
A useful observation might be that a method should be short and sweet enough that it doesn’t need internal commenting.

35. Refactoring to the Strategy Pattern
Applying the Strategy design pattern involves three things:
1. Creating an interface that defines the strategic operation
2. Writing classes that implement the interface and embody each of the different strategies
3. Refactoring the code to select and use an instance of the right strategy class

36. The Interface 1. The interface for this example will be named Advisor
The interface requires the implementation of a recommend() method
The recommend() method will take a customer as a parameter
It will return a firework
A UML diagram of the interface is given on the next overhead

38. The Implementing Classes
2. The next step is to write the classes that implement the interface and embody each of the different strategies
These classes will have to implement the recommend() method

39. The book does the refactoring in part with challenges
As usual, it’s easiest to just look at the solutions
The UML diagram on the following overhead shows:
A new Customer class making use of an Advisor interface
4 classes which implement the interface and embody the 4 strategies

40. Solution 23.1

41. The Implementing Classes
The PromotionAdvisor and RandomAdvisor class names should be self-explanatory
GroupAdvisor refers to the use of Rel8
ItemAdvisor refers to the use of LikeMyStuff
The implementations of the recommend() method for these classes will wrap a call to the static methods of Rel8 and LikeMyStuff
An expanded UML diagram for these two classes is given on the next overhead

43. Making Instances of the Implementing Classes
An interface can’t define static methods
An interface defines what the book calls “object methods”—methods that are called on objects
That means that client code will have to make instances of GroupAdvisor and ItemAdvisor
The recommend() method will be called on these objects

44. Only one instance each of GroupAdvisor and ItemAdvisor are needed
In the refactored design, these instances will be static objects in the Customer class
So the advisor objects will be “singleton like”
There won’t be an instance of each kind of advisor for each customer

45. Even though the recommend() method isn’t a static method, it more or less acts like one
If there is only one advisor object, then there is the one recommend() method that can be called on that object
The recommend() method does something for customers
But it does so by taking the customer as an explicit parameter rather than being called on the customer

46. Code for the recommend() Method in the GroupAdvisor Class
This is the recommend() method in the GroupAdvisor class:
public Firework recommend(Customer c) {
return (Firework) Rel8.advise(c); }
It wraps a call to the advise() method of Rel8
In essence, the call is adapted to the recommend() interface of Advisor

47. Code for the recommend() Method in the ItemAdvisor Class
The code for the recommend() method in the ItemAdvisor class is analogous.
The book doesn’t give it and it doesn’t even bother to give it as a challenge.
It should be straightforward to write that method.

48. Challenge 23.2
“In addition to Strategy, what pattern appears in the GroupAdvisor and ItemAdvisor classes?”
[The answer to this was given away in the last remark about the recommend() code in GroupAdvisor.]

49. Solution 23.2
“The GroupAdvisor and ItemAdvisor classes are instances of Adapter, providing the interface a client expects, using the services of a class with a different interface.”

50. Code for the recommend() Method in the PromotionAdvisor Class
A PromotionAdvisor class is also needed, with a recommend() method
On the one hand, promotion should be a simple case
On the other hand, the book puts a lot of detail into the implementation

51. Most of the logic of the original code is moved into the constructor for the new class
If a promotion is on, then the promoted instance variable of the class is initialized
In addition to the recommend() method, there is a hasItem() method which can be called to see whether a promoted item is available

52. The book’s implementation makes use of class loading logic
This requires try/catch blocks
The details of this technique will not be covered since they are extraneous to the design pattern
The code is shown on the following overheads

53. public class PromotionAdvisor implements Advisor{
private Firework promoted;
public PromotionAdvisor()
try
Properties p = new Properties();
p.load(ClassLoader.getSystemResourceAsStream("config/strategy.dat"));
String promotedFireworkName = p.getProperty("promote");
if (promotedFireworkName != null)
promoted = Firework.lookup(promotedFireworkName); }
catch (Exception ignored)
// Resource not found or failed to load
promoted = null;

54. public boolean hasItem(){
return promoted != null; }
public Firework recommend(Customer c)
return promoted;

55. Code for the recommend() Method in the RandomAdvisor Class
The RandomAdvisor class is simple
Its code is shown on the following overhead

56. public class RandomAdvisor implements Advisor{
public Firework recommend(Customer c)
return Firework.getRandom(); }

57. Refactoring the Customer Class to Use the Interface

58. Creating the Advisor Objects
A single instance of each kind of advisor is created in the new Customer2 class
For any given customer, recommend() may be called on one of these advisor objects
The code for the new customer class, Customer2, begins by creating these objects
This code is shown on the following overhead

59. private static PromotionAdvisor promotionAdvisor =
new PromotionAdvisor();
private static GroupAdvisor groupAdvisor =
new GroupAdvisor();
private static ItemAdvisor itemAdvisor =
new ItemAdvisor();
private static RandomAdvisor randomAdvisor =
new RandomAdvisor();

60. Implementing the getAdvisor() Method
Customer2 contains a method named getAdvisor() for picking which kind of advisor to use
The original design had if/else statements in which different strategies were called
In the new design you don’t eliminate if/else statements, but they appear in a different place

61. In the new design the if/else logic is implemented in the getAdvisor() method
getAdvisor() returns an advisor object
Polymorphism and dynamic binding determine which strategy is used when recommend() is called on the advisor object

62. How the getAdvisor() Method Works
Recall that getAdvisor() is in the Customer2 class
Its implementation includes a call to hasItem(), mentioned earlier to check for a promotion
Its implementation also includes calls to isRegistered() and isBigSpender() on the implicit customer parameter to determine if the advisor should be Rel8 or LikeMyStuff

63. getAdvisor() does lazy initialization of the advisor
The value of the advisor instance variable is only set at the time the getAdvisor() method is called
It’s not lazy construction because one instance of each kind of advisor has already been created at the top of Customer2
The code is shown on the following overhead

64. private Advisor getAdvisor(){
if (advisor == null)
if (promotionAdvisor.hasItem())
advisor = promotionAdvisor;
else if (isRegistered())
advisor = groupAdvisor;
else if (isBigSpender())
advisor = itemAdvisor;
else
advisor = randomAdvisor; }
return advisor;

65. The getRecommended() Method in Customer2
Challenge 23.3
“Write the new code for Customer.getRecommended().”
Comment mode on:
It may be helpful to step back and review where we’re at in the redesign
The UML for the redesign is repeated on the following overhead

67. We’ve got the 4 advisor classes
We’ve got the getAdvisor() method
The final ingredient of the new design is the getRecommended() method in the Customer2 class
This code should rely on the advisor for the customer, and a call to the recommend() method on that advisor
The code is shown on the following overhead

68. Solution 23.3 Your code should look something like:
public Firework getRecommended() {
return getAdvisor().recommend(this); }

69. The Benefit of the Design Pattern
It is apparent that some of the code in the application is simplified by the application of the design pattern
Polymorphism and dynamic binding determines which version of recommend() will be used depending on the type of advisor that was initialized earlier.

70. Comparing Strategy and State
In the book State and Strategy are presented one after the other
These patterns have some structural similarities and the book compares them
The similarities can be summarized by looking at the UML diagram for state again
It is shown on the following overhead

72. In both patterns there is a client that makes use of an interface/abstract class
Each implementing class/subclass contains a method of the same name
In the client, the method is called on an object and polymorphism/dynamic binding determines which version of the method is called

73. Just like the adapter, this structure will appear again in other patterns
Using the book’s terminology, the design patterns under consideration which share this structure are different patterns because their intents differ
In other words, what you accomplish by using the structure is not the same

74. It is quite apparent at the detailed level that getting a recommended firework in no way causes a customer to transition into a different state
However, touching the control button on the carousel door caused it to enter a different state
This important kind of difference isn’t apparent from looking at the overall structure at a macro level in a UML diagram

75. Comparing Strategy and Template Method (A Sideshow?)
This section is not critically important
There may be some worthwhile ideas presented, but it doesn’t help too much in focusing on what the strategy design pattern is
It’s really more focused on showing that the template design pattern is not the same as the strategy design pattern

76. The book compares the Strategy and Template methods
In this case the similarities that the book suggests might exist are not illustrated by a UML diagram
The authors are directly addressing the question of whether Strategy and Template can be viewed as having the same intent

77. Recall the first presentation of the Template Method design pattern
The template was a sort() method in the Arrays or Collections class
What was stubbed in was the pairwise comparison operation for elements of the data structure

78. The authors state that you might argue that changing the pairwise comparison step changes the strategy of sorting
They illustrate this idea by suggesting that sorting rockets by price rather than by thrust would support a different marketing strategy
Observe that the claim about strategy in this argument is about something in the problem domain, marketing, not in the code writing

79. Challenge 23.4
“Provide an argument that the Arrays.sort() method provides an example of Template Method or that it is an example of Strategy.”
Comment mode on:
It’s clear that the book thinks sorting is an example of template, not strategy, since sorting was covered in the template unit.
I agree with its reasoning, as presented in the second of the following arguments

80. Solution 23.4
“Is a reusable sort routine an example of Template Method or of Strategy?”
[See the following overheads.]

81. An Argument that Sorting is an Example of the Strategy Design Pattern
“Template Method, according to the original Design Patterns book, lets “subclasses” redefine certain steps of an algorithm.
But the Collections.sort() method doesn’t work with subclasses; it uses a Comparator instance.
Each instance of Comparator provides a new method and thus a new algorithm and a new strategy.
The sort() method is a good example of Strategy.

82. An Argument that Sorting is an Example of the Template Design Pattern
There are many sorting algorithms, but Collections.sort() uses only one (QuickSort).
Changing the algorithm would mean changing to, say, a heap sort or a bubble sort.
The intent of Strategy is to let you plug in different algorithms.
That doesn’t happen here.
The intent of Template Method is to let you plug a step into an algorithm.
That is precisely how the sort() method works.”

83. The book’s argument is good—but maybe not entirely clear.
The point is that at the macro level, a choice among strategies would mean a choice between different sorting algorithms
At the micro level, choosing between pairwise comparisons doesn’t change the overall strategy
It is an example of applying the template pattern

84. Another Example
The other example is based on cups and seeds in Togiz Kumalak
One side of a Togiz Kumalak board, consisting of 9 cups, is initialized with random seed counts and presented on the screen
This half of the board belongs to player whose turn it is
The player has to choose which cup to play

85. The application implements 3 different strategies:
Play a cup at random
Play the cup with the minimum number of seeds
Play the cup with the maximum number of seeds

86. The player is prompted to pick a strategy
The application then tells which cup to play based on that strategy
The application is illustrated in the screenshot on the following overhead

88. The code for this example application is not presented in the overheads
It is available on the course Web page in case you’re interested
You will find it is an extremely simple implementation of the strategy design pattern as presented in the book

89. UML for the Pattern
The UML diagram given earlier is repeated on the following overhead
It is specific to the fireworks problem, but otherwise it shows what the pattern is about
It might be possible to make this and other patterns clearer by including sequence diagrams
It would show how the getRecommended() method makes a call to the recommend() method

91. A UML Diagram for the Pattern from Lasater
Lasater’s UML diagram is given on the following overhead
It is useful, as usual, because it uses generic names for the classes in the pattern
This one is also useful because it shows that you could use an abstract class just as easily as you could use an interface when applying the pattern

93. Summary Multiple strategies may appear in a single method
This can be an unwieldy design
A solution is to implement each separate strategy as a method of the same name in different classes
The individual implementations will be simpler

94. The original design would have logic to pick one of the strategies
The new design would also contain such logic somewhere, but at least it is separated from the implementations of the strategies
The new logic will be based on which of the strategy classes is constructed
It would be reasonable to hold the strategy object as a reference in a client class

95. Then at the appropriate time, this object has the strategy method called on it
In other words, once the object choice is made, the method choice is accomplished by polymorphism and dynamic binding
The result of this kind of design is individual pieces of code that are smaller, simpler, and unified in purpose

96. Overall, the design should be clearer, with different strategies implemented in different classes
The cleaner the design, the less commenting may be needed…

97. The End