Monday, December 7, 2015

Head Fist Design Pattern CH8: Template Method Pattern and Hollywood Principle

template method define the skeleton of algorithm in a method, 
for example making coffee has the follwoing steps: 
1- boil water, 2- brewCoffeeGrinds 3- pourInCup, 4- addSugarAndMilk.

making tea has the following steps:
1- boil water, 2- steepTeaBag, 3- pourInCup, 4- addlemon

ass you can see that the method can be abstracted in a template method

as you can see we defined the template method prepareRecipe() as final.

brew() method, represent brewCoffeeGrinds and steepTeaBag, is defined as abstract so it can be implemented by the subclass.

same for addCondiments

now you can implement the class like this

in addition in template method you can use something called hook methods

hook methods has an empty implementation in the base class, the subclass can decide if it wants to implement it or not

OO Principle
Hollyowood Principle: dont call me i will call you. 
you can see it here in Template Method, the super class is calling the sub class when it needs anything.

Template Method vs Strategy Pattern
Notice the difference between Template Method and Strategy Pattern, 
1- Strategy Pattern depends on composition, here we have inheritance
2- In Strategy Pattern you can change the algorithm on runtime 
3- Template Method defines just the skeleton of the algorithm, Strategy Pattern is responsible for the algorithm itself.

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