Structural Patterns

• Address issues concerned with the way in which classes and objects are organized.

• Offers effective ways of using object-oriented constructs such as inheritance, composition and aggregation to satisfy particular requirements.

       

Structural Patterns

• Adapter• Bridge• Decorator• Façade• Flyweight• Proxy• Composite

Adapter Pattern

• Consider a situation where there is a 3 and ½ inch floppy disk and a 5 and ½ inch drive.

• The disk cannot fit into the drive. • The task:Cater to the client disk without

changing either the disk or the drive. • Thus in order to make these two incompatible

interfaces work together, the interface of the drive must be converted to an interface that the disk expects.

• The Adapter pattern concept is used to make two unrelated interfaces work together.

Adapter Pattern

• In our example, we make the drive output some contents that are copied on to the disk.

• The Drive is declared to be an interface with one method send_output() that tries to send output to the disk.

• Since both are incompatible, the Adapter class acts as an intermediary.

• It has a reference to the disk and it also implements the Drive interface.

• It gives the output of the drive to the disk in a manner that the disk understands.

Bridge Pattern

• It decouples an abstraction from its implementation so both can vary independently.

• E.g. Establishing the mileage of cars on highways and city roads.

• The MarutiCar class is the base class. • Maruti800 class extends MarutiCar class• There is a class Road with a method

display_Mileage(). • The classes Highway and cityRoad extend the

Road class and implements the display_Mileage() operation in their own manner.

Bridge Pattern

• In Bridge pattern parlance, MarutiCar is the abstraction, the Maruti800 class is the refined abstraction, the Road is the implementer, Highway and CityRoad are the concrete implementers.

• The abstraction and the implementation are not coupled with each other.

• A new type of car say MarutiZen could be added as another refined abstraction or a new type of Road say Suburban could be added as another concrete implementer of the existing refined abstraction.

Decorator Pattern

• It is used to add extra behaviour to specific objects.

• E.g.A small construction firm initially sold flats and shops.

• Later the firm had a drop in sales of upper floor flats so they decided to rent them.

• To simulate this scenario, there is a class ConstructionProject that has an attribute area and a display method to display information about the shop or flat.

• Two classes i.e. Flat and Shop inherit from ConstrctionProject. Flat has the attributes flatId, floorNo, noOfBedrooms and presenceOfBalcony.

• Shop has its own attributes i.e. shopId and own.

Decorator Pattern

• The problem before us is how to make the Flat class leasable?

• Use a Decorator class that will have a constructor to initialize the type of library item to be decorated with extra behaviour.

• The actual decorator i.e. Leaseable class will extend from the Decorator class, use its constructor and will have a method leaseFlat() to decorate the Flat class.

• If the shop has to be decorated then another class can extend from the Decorator class and add the appropriate behaviour.

Facade Pattern

• In the case of large programs, a large number of classes/subsystems can crop up with the result that it becomes difficult to ascertain the flow of the program.

• There can be many subsystems, each with its own complicated interface.

• It is convenient to have a single class that serves as an intermediary between the client program and the classes/subsystems.

• This intermediary will provide a simplified interface to all these classes/subsystems.

• This intermediary is the façade that hides the complexity of

the structure from the client program.

Facade Pattern

• E.g. Problem is to select a cricket player based on his performance in the matches of the previous season.

• The performance is rated on the basis of 3 factors i.e. the runs scored, the wickets taken and the boundaries saved during fielding.

• The data about runs, wickets and saved boundaries is retrieved from the subsystems namely RunsScored, WicketsTaken and BoundariesSaved.

• Player class that stores the personal information of the players.So instead of the client program checking data of players by dealing with each of the subsystems separately, there is a façade class i.e. PlayerSelection that does this task and provides an answer whether a particular player

can be selected or not.

Flyweight Pattern

• It is useful if there are a small number of objects that are needed a large number of times.

• Instead of creating the all the objects all the time, certain objects can be reused wherever possible.

• E.g.Problem of constructing 10 rectangles filled with various colours.

• The created objects are stored in a HashMap. • The task: Check whether a DrawRectangle

object of a certain area has already been created. If it has been created then it is retrieved from the HashMap ands used else a new object is instantiated.

Flyweight Pattern

• The RectangleFactory class whose job is to manufacture DrawRectangle objects achieves this task.

• In the example, the code is deliberately written in such a manner that out of 10 rectangles half of them have the same area but different colours.

• So in reality only 5 DrawRectangle objects are created and each of them is used twice.

• This problem must be seen in the perspective of saving memory space if a thousand objects need to be built and only half of them are actually constructed and reused.

Proxy Pattern

• It is a pattern that separates the interface from the implementation.

• The proxy object controls access to the real object. • There are three classes called Client, RealSubject and Proxy

and one interface called Subject. • The client always operates on the interface "Subject". • The actual operations in the interface are implemented by

RealSubject class.• They are also "implemented" by the "Proxy" class in the

sense that the Proxy forwards any calls it receives to the RealSubject class.

• So, through the interface, the client class will work against a Proxy object and so the Proxy will control access to the RealSubject object.

Proxy Pattern

• E.g. There are 30 computers in a lab and only a computer to which a printer is attached.

• Only persons with administrative privileges can take printouts. S• uppose a user wants to take a printout and so he invokes the

print function. • A Proxy object can first evaluate whether the user has

administrative privileges. • If he doesn’t have the necessary privileges then he should be

disallowed. • If he has the necessary privileges, then the Proxy can delegate

the job to the actual Printer. • So, in this case, the RealSubject is the Printer. • While checking the authorization, the Proxy can itself

communicate with some authorization object to decide whether

the access is allowed or not.

Proxy Pattern

• A Proxy can be used to enhance performance and increase efficiency.

• Suppose if RealSubject object is too expensive to create because it has to be fetched from some other system.

• So a system can instantiate a cheap Proxy. • When an operation is called in the Proxy, it will

confirm whether the RealSubject object is instantiated or not.

• If it has been instantiated, then the request is forwarded to it.

• Else it is instantiated by the Proxy and then the requests is passed on to it.

Composite Pattern

• It is primarily used to indicate whole-part relationships and let the client program treat both whole object and part objects in the same manner.

• E.g. Finding the cost to a private college of an Employee. • It is calculated as the sum of the cost(salary) of the

Employee and all Employees under him. • In the college, the highest authority is the principal. • He has 2 employees i.e. Vice principal of the Arts section

and the Vice Principal of the Science section.• The Vice Principal of the Arts section has the Heads of

various departments(Philosophy, Sociology and Logic) under him.

• The Vice Principal of the Science section has the Heads Physics, Chemistry and Biology under him.

Composite Pattern

• Cost incurred by the college in employing the Principal is the sum total of the salaries of the Principal, the Vice Principals and the Heads of various departments.

• Whereas, the cost incurred by the private college in employing the head of Logic department is the head’s own salary.

• The point is to have a single method to calculate the cost of an employee to the college irrespective of whether the employee is primitive (does not have anybody under him) or composite.

Composite Pattern

• In the implementation, the class Employee has a designation, salary and an ArrayList to record details about employees working under him.

• It also has getSalaries() method which is implemented in such a manner so as to work for both types of objects whether composite or primitive.