1. Introduction to Behavioral Patterns (3)
11/17/2018
Programming Design Patterns

2. Programming Design Patterns
Making Coffee and Tea
public class Coffee {
void prepareRecipe() {
boilWater ();
brewCoffeeGrinds();
pourInCup();
addSugarAndMilk(); }
// …
public class Tea {
void prepareRecipe() {
boilWater ();
steepTeaBag();
pourInCup();
addLemon(); }
// …
Programming Design Patterns
11/17/2018

3. Abstract the commonality (1)
Is there code duplication in the two classes?
Which two methods are the same?
Need to abstract the commonality into a base class
CLASSWORK
Define a base class CaffeineBeverage
force subclasses to implement the prepare() method
let subclasses reuse the base class implementation for
boilWater() and pourInCup()
Programming Design Patterns
11/17/2018

4. Abstract the commonality (2)
CaffeineBeverage: prepareRecipe()
abstract prepare()
boilWater() // shared
pourInCup() // shared
Coffee
prepare()
brewCoffee()
addSugar()
Tea
preprare()
steepTeaBags()
addLemon()
Programming Design Patterns
11/17/2018

5. Abstract the commonality (3)
Coffee
brewCoffee()
addSugar()
Tea
steepTeaBags()
addLemon()
Beverage (after further abstraction)
brew
addCondiments
Now design for the class for CaffeineBeverage
Programming Design Patterns
11/17/2018

6. Programming Design Patterns
Template Method
Defines the steps of an algorithm and
allows subclasses to provide the implementation for one or more steps
Programming Design Patterns
11/17/2018

7. Programming Design Patterns
CaffeineBeverage
Should the class be abstract?
Should the method prepare() be final?
Should some of the methods of the class be abstract?
Which pattern does this remind you of?
Homework: difference between this pattern and “factory method” pattern
Hint: one is a specialized case of the other
This is similar to the factory method pattern
Programming Design Patterns
11/17/2018

8. CaffeineBeverage class
public abstract class CaffeineBeverage {
public final void prepareRecipe() {
boilWater();
brew();
pourCup();
addCondiments(); }
protected abstract void brew();
protected abstract void addCondiments();
Programming Design Patterns
11/17/2018

9. Programming Design Patterns
Classwork
Write code for a “Tea” class with placeholder implementation (…) for the two methods
brew
addCondiments
Write driver code to call prepareRecipe() on a “Tea” instance
Programming Design Patterns
11/17/2018

10. Programming Design Patterns
Tea class
public class Tea extends CaffeineBeverage {
public void brew() { …}
public void addCondiments() { …} }
Programming Design Patterns
11/17/2018

11. Template Method Pattern: usage
Convert similar operations to a template.
Convert from many specialized operations to a generalized operation.
Refactor common behavior to simplify the code.
Programming Design Patterns
11/17/2018

12. Template Method Pattern: definition
Defines the skeleton of an algorithm in a method, deferring some steps to subclasses. Template method lets subclasses redefine certain steps of an algorithm without changing the algorithm’s structure.
Programming Design Patterns
11/17/2018

13. Template Method Pattern
What if a customer wants tea or coffee without condiments?
The pattern provides a hook
Programming Design Patterns
11/17/2018

14. Template Method Pattern: hook
public abstract class TemplateClass {
public final void templateMethod() {
primitiveOperation1();
primitiveOperation2();
if (hook()) {
concreteOperation(); }
protected abstract void primitiveOperation1();
protected abstract void primitiveOperation2();
final void concreteOperation();
protected boolean hook() { …}
Programming Design Patterns
11/17/2018

15. Template Method Pattern: hook (2)
public abstract class Beverage {
public final void templateMethod() {
primitiveOperation1();
primitiveOperation2();
if (customerWantsCondiments())
addCondiments(); }
protected abstract void addCondiments();
// subclasses can override this hook, if they want
protected boolean customerWantsCondiments() {
return true;
Programming Design Patterns
11/17/2018

16. Programming Design Patterns
Hollywood Principle
"Hollywood principle"
"Don't call us, we'll call you”
Clients of Tea/Coffee use the superclass abstraction.
reduces dependency between driver/client code and the subclasses
Homework
How is the “Hollywood Principle” different from “Dependency Inversion Principle”
Note that the parent calls the operations of the subclass, which is less common than child code referring to the parent's code
Programming Design Patterns
11/17/2018

17. Programming Design Patterns
Design Problem
A mortgage loan application consists of:
check bank balance, check credit score, check stocks, check client’s potential future earnings
An equity loan consists of:
check values of assets, check credit score, check stocks, check increments in salary in the next 5 years.
If the client is a bank employee then stocks need not be checked
Present your design for this problem
Programming Design Patterns
11/17/2018

18. Template Method Pattern (1)
You can vary behavior using a simple kind of inheritance
The idea is that most of an algorithm or procedure is fixed; the detailed behavior depends on calls to specific operations at certain locations
Template methods are a fundamental technique for code reuse
They are particularly important in class libraries, although its use is often hidden in the implementation and may not be seen in the API
Programming Design Patterns
11/17/2018