Motivation:
At times different
components have significant similarities with a only a few steps in between
which might vary. Let’s take few real time examples like buying a flight ticket online. We might buy the ticket
from any website from anywhere most of the steps would be the same with some
variations.
- Login to Website (We will have different websites).
- Selecting the itinerary for the journey
- Search the ticket (Different websites will offer different carriers).
- Apply the Offer (Offers will vary depending on the website).
- Select the ticket
- Payment
If we
want to launch a new new website to book Air Tickets, Do I need to implement
all these methods? How about if you get some default template implementation
where you just need to plug few thing as per your requirements!!! Sounds
interesting. Watch for the Template Method Pattern discussed below.
Definition:
Defines the skeleton of an algorithm in a method, deferring some
steps to sub classes Template Method lets sub classes redefine certain steps of
an algorithm without changing the algorithms structure.
Structure:
The Template Method pattern is quite simple -
let's look at a class diagram representation:
The AbstractClass contains the templateMethod(),
which should be made final so that it cannot be overridden i.e. the order of
the steps cannot be modified. This template method calls other operations
available in order to run through the algorithm. Not all operations used by
this template method need to be made abstract, we can have concrete methods
with default implementation.
These concrete methods are that part of the
algorithm which remains common. The ConcreteClass implements all the
abstract operations required by the templateMethod in the parent class. There
can be many different Concrete Class implementations. Abstract methods are
that part of the algorithm which will vary.
Getting are Hands Dirty with
Coding !!!
We will implement the above Flight Ticket booking Algorithm with
help of Template Pattern.
public abstract class
AirTicketBookingTemplate {
public final void bookTicket()
{
loginToWebsite();
chooseItenary();
searchTicket();
applyOffers();
selectTheTicket();
payment();
}
void
chooseItenary() {
System.out.println("Select
Source/Destination/Date");
}
void
displayingAllTicktes() {
System.out.println("Displaying
All the Availale Tickets!!");
}
void
selectTheTicket() {
System.out.println("You
have selected the below Ticket!!");
}
void payment() {
System.out.println("Payment
Recieved !! Ticket Confimed.");
}
abstract void
loginToWebsite();
abstract void
searchTicket();
abstract void applyOffers();
}
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This is
the abstract class in which bookTicket act as a Template Method with some
concrete and abstract methods.
Now let
us have a look at our First Concrete Class:
public class MakeMyTrip extends
AirTicketBookingTemplate{
void
loginToWebsite() {
System.out.println("Welcome
to www.makemytrip.com !!!");
}
void
searchTicket() {
System.out.println("Searching
Jet Airways/Air India/Kingfisher/Indigo");
}
void applyOffers()
{
System.out.println("Current
Offer is Buy one Ticket Get Another Free !!!");
}
}
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Another
counterpart of MakeMyTrip which has really become popular these days is
Goibibo. Let’s have a look and check how will they serve us. The booking algorithm or process is almost the same but it offers better Offers and gives
us more number of choices to book the Ticket. Code is present below.
public class GoIBiBo extends
AirTicketBookingTemplate{
void
loginToWebsite() {
System.out.println("Welcome
to www.goibibo.com !!!");
}
void applyOffers()
{
System.out.println("Current
Offer is Rs 200 off on one way 400 on return !!!");
}
void
searchTicket() {
System.out.println("Searching
Sahara/Air India/Jet Lite/Go Air/SpiceJet/Jet Airways/Indigo");
}
}
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Oh…Finally
I will be able to the book the Flight Ticket
public static void main(String[]
args) {
MakeMyTrip makeMyTrip = new MakeMyTrip();
makeMyTrip.bookTicket();
System.out.println("-------------------");
GoIBiBo goIBiBo = new GoIBiBo();
goIBiBo.bookTicket();
}
Output:
Welcome to
www.makemytrip.com !!!
Select
Source/Destination/Date
Searching
Jet Airways/Air India/Kingfisher/Indigo
Current
Offer is Buy one Ticket Get Another Free !!!
You have
selected the below Ticket!!
Payment
Recieved !! Ticket Confimed.
-------------------
Welcome to www.goibibo.com
!!!
Select
Source/Destination/Date
Searching
Sahara/Air India/Jet Lite/Go Air/SpiceJet/Jet Airways/Indigo
Current
Offer is Rs 200 off on one way 400 on return !!!
You have
selected the below Ticket!!
Payment
Recieved !! Ticket Confimed.
|
Getting Hooked with the
Template
Hook is
a method that is declared in abstract class, but only given empty or default
implementation. This gives the subclasses the flexibility to modify or hook into the algorithm at
various points, if they wish. Subclasses are free to ignore the hook method if
not required.
Bigger Question here is when to use abstract
method and when to use hooks?
Use abstract methods when your subclass must provide the implementation. Choose
hook when it is optional for subclasses to implement it.
Let us
understand Hooks inTemplate Design Pattern with a simple diagram below:
We will
see how we can Hook in various features in our Daily Morning Walk.
We have
placed two Hooks in our Morning Walk, one is to for Music lovers and other is
for people who have house hold resposibilites as well. But let me tell you
these are not mandatory. We can still enjoy our morning walk without any of
these.
Let us
know enhance our Flight Booking Example with some Hooks:
Here we
have given a hook complimentaryMeal () which returns false by default i.e. the no free meals will be served on the
flight. But if a website prefers, it has all the right s to feed it’s hungry
passengers for free.
public abstract class
AirTicketBookingTemplate {
public final void bookTicket()
{
//All the previous code here
if(complimentaryMeal()){
chooseYourMeal();
}
payment();
}
abstract void chooseYourMeal();
boolean
complimentaryMeal(){
return false;
}
}
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public class GoIBiBo extends
AirTicketBookingTemplate{
boolean
complimentaryMeal() {
// This can
also be based on Off Season or some Statistical Data to more profit
return true;
}
void chooseYourMeal() {
System.out.println("Kindly
Select the Complimentary Meal");
}
}
Output:
All the lines
are same as above only the highlighted part is different
Kindly Select the Complimentary Meal
Payment
Recieved !! Ticket Confimed.
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Hollywood Priciple:
The Template Method pattern makes use of the Hollywood
Principle: Don't call us, we'll call you. We will cover this
topic in some other post.
When to Use Template Pattern?
· When behavior of an algorithm can vary, you let sub classes implement the behavior through overriding
· You want to avoid code duplication, implementing variations of
the algorithm in sub classes
·
You want to control the point that subclassing is allowed.
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