template method. dcs – swc 2 how to make a pizza if you order a pizza, the manufacturing of a...
TRANSCRIPT
Template method
DCS – SWC 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
DCS – SWC 3
How to make a pizza
// From a class Pizza
makePizza()
{
createDough();
rolloutDough();
if (mustAddTomatoSauce()) addTomatoSauce();
addToppings();
if (mustAddCheese()) addCheese();
bake();
cut();
putInBox();
}
DCS – SWC 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”
DCS – SWC 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
DCS – SWC 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
DCS – SWC 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
DCS – SWC 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
DCS – SWC 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
DCS – SWC 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
DCS – SWC 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();
}
}
DCS – SWC 12
Template method pattern
AbstractClasstemplateMethod()…primitiveOpA()primitiveOpB()primitiveOpC()
Notice this is not an interface!
ConcreteClassprimitiveOpA()primitiveOpB()primitiveOpC()
templateMethod() is never overwritten!
DCS – SWC 13
Template method pattern
Pizza myPizza = new CopenhagenSpecialPizza();
...
myPizza.makePizza();
// Notice that Pizza myPizza = new Pizza()
// will fail!
DCS – SWC 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
DCS – SWC 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
DCS – SWC 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