Template method

PBA WEB – BEWP 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

PBA WEB – BEWP 3

How to make a pizza

// From a class Pizza

MakePizza()

{

CreateDough();

RolloutDough();

if (MustAddTomatoSauce()) AddTomatoSauce();

AddToppings();

if (MustAddCheese()) AddCheese();

Bake();

Cut();

PutInBox();

}

PBA WEB – BEWP 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 ”level of generic-ness”

PBA WEB – BEWP 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

PBA WEB – BEWP 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

PBA WEB – BEWP 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

PBA WEB – BEWP 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

PBA WEB – BEWP 9

Method categorisation

Method Type Definition Override

Template Method

Defines generic skeleton for algorithm

No

Concrete Operation

Generic algorithm step No

Hook Default – but overridable – algorithm step

May

Primitive Operation

Specialised algorithm step

Must

PBA WEB – BEWP 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

PBA WEB – BEWP 11

Specialised pizza

public class HawaiiPizzaNoCheese : Pizza

{

// Hook (overrides default behavior)

public override bool MustAddCheese() { return false;}

public override void AddToppings() // Primitive operation

{

AddPineAppleSlices();

AddShrimps();

AddShreddedHam();

}

}

PBA WEB – BEWP 12

Template method pattern

AbstractClassTemplateMethod()…PrimitiveOpA()PrimitiveOpB()PrimitiveOpC()

Notice this is not an interface!

ConcreteClassPrimitiveOpA()PrimitiveOpB()PrimitiveOpC()

TemplateMethod() is never overwritten!

PBA WEB – BEWP 13

Template method pattern

Pizza myPizza = new CopenhagenSpecialPizza();

...

myPizza.MakePizza();

// Notice that Pizza myPizza = new Pizza()

// will fail!

PBA WEB – BEWP 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

PBA WEB – BEWP 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

PBA WEB – BEWP 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