Template method

RHS – SOC 2 How to make a pizza If you order a pizza, the manufacturing of a pizza goes through certain steps We can write up a sort of algorithm for making a pizza

RHS – SOC 3 How to make a pizza // From a class Pizza makePizza() { createDough(); rolloutDough(); if (mustAddTomatoSauce()) addTomatoSauce(); addToppings(); if (mustAddCheese()) addCheese(); bake(); cut(); putInBox(); }

RHS – SOC 4 How to make a pizza This method for creating a pizza is generic; we always follow this algorithm However, not all of the individual steps are generic; they vary with the actual product We can categorise the methods, according to their ”genericity”

RHS – SOC 5 How to make a pizza makePizza() { createDough(); rolloutDough(); if (mustAddTomatoSauce()) addTomatoSauce(); addToppings(); if (mustAddCheese()) addCheese(); bake(); cut(); putInBox(); } Is a fully generic method for making a pizza

RHS – SOC 6 How to make a pizza makePizza() { createDough(); rolloutDough(); if (mustAddTomatoSauce()) addTomatoSauce(); addToppings(); if (mustAddCheese()) addCheese(); bake(); cut(); putInBox(); } The red steps are fully generic – they never vary

RHS – SOC 7 How to make a pizza makePizza() { createDough(); rolloutDough(); if (mustAddTomatoSauce()) addTomatoSauce(); addToppings(); if (mustAddCheese()) addCheese(); bake(); cut(); putInBox(); } The green steps may vary, but have a default behavior

RHS – SOC 8 How to make a pizza makePizza() { createDough(); rolloutDough(); if (mustAddTomatoSauce()) addTomatoSauce(); addToppings(); if (mustAddCheese()) addCheese(); bake(); cut(); putInBox(); } The brown step will vary, and has no default behavior

RHS – SOC 9 Method categorisation Method TypeDefinitionOverride Template Method Defines generic skeleton for algorithm No Concrete Operation Generic algorithm step No HookDefault – but overridable – algorithm step May Primitive Operation Specialised algorithm step Must

RHS – SOC 10 Specialised pizza How can we then make a specific pizza? We can let a subclass – e.g HawaiiPizza- NoCheese specialise the Pizza class What should this class implement..? Non-generic steps, i.e –Hooks –Primitive Operations

RHS – SOC 11 Specialised pizza public class HawaiiPizzaNoCheese extends Pizza { // Hook (overrides default behavior) public boolean mustAddCheese() { return false;} public void addToppings() // Primitive operation { addPineAppleSlices(); addShrimps(); addShreddedHam(); }

RHS – SOC 12 Template method pattern AbstractClass templateMethod() … primitiveOpA() primitiveOpB() primitiveOpC() Notice this is not an interface! ConcreteClass primitiveOpA() primitiveOpB() primitiveOpC() templateMethod() is never overwritten!

RHS – SOC 13 Template method pattern Pizza myPizza = new CopenhagenSpecialPizza();... myPizza.makePizza(); // Notice that Pizza myPizza = new Pizza() // will fail!

RHS – SOC 14 Template method pattern What have we achieved? –Program to an interface, not an implementation Clients will only know the AbstractClass interface –Encapsulate the aspects that vary Specialised algorithm steps are located in the specialised classes

RHS – SOC 15 Template method pattern What have we achieved? –Open for extension, closed for modification We can add new specialisations without modification of base –Don’t call us, we’ll call you Base class calls specialised steps, not vice versa The last principle is called the Hollywood Agent principle

RHS – SOC 16 Template method pattern Base MakePizza Spec. AddToppings() Base Spec. MakePizza CreateDough() Don’t call us, we’ll call you - High-level classes are in control - Low-level classes can be hooked onto a system - Low-level classes don’t call high-level classes directly

RHS – SOC 17 Exercises Download the NetBeans project TemplateMethodExample from the Website (go to Classes, Week 43) Examine the code; the class Pizza implements a template method makePizza for making a pizza. The template method contains concrete operations, hooks and primitive operations Recall that hooks may – and primitive operations must – be overrided in specialised classes. Two such classes have been added; HawaiiPizza and VegetarianPizza. Examine their implementation. A test of making concrete pizzas is found in Main. Try it out! Notice how you just call makePizza on the specialised class objects, in order to make a specific pizza Try adding a couple of additional specialised pizza classes to the project, and add tests of them to the test.