Design Patterns – Details
Part 2
Gang of Four (GoF) Patterns
GRASP Patterns
More
Sources:
Chapter 6: Using Design Patterns, and
Chapter 30 (parts) Designing the Logical Architecture with Patterns
Craig Larman’s Text: Applying UML and Patterns, Chaps: 16, 17, 22, & 23.
WWW, personal notes, and more.
General Design Pattern
© Lethbridge/Laganière 2001
Chapter 6: Using design patterns
2
1. The Abstraction-Occurrence Pattern
• Context:
— Often found in class diagrams that form part of the system
domain model.
—Often in a domain model you find a set of related objects
(occurrences).
—The members of such a set share common information but also
differ from each other in important ways. (Sports cars ….)
• Problem:
—What is the best way to represent such sets of occurrences in a
class diagram? Use the commonality, yet represent the
differences!
• Forces:
—You want to represent the members of each set of occurrences
without duplicating the common information.
- Cannot have this!
—Want to maximize the flexibility of the system too.
© Lethbridge/Laganière 2001
Chapter 6: Using design patterns
3
Abstraction-Occurrence
• Solution:
«Abstraction»
Remember (Java): an
Interface can have
NO implementations;
may at most have
constants;
Abstract Class has at
least one abstract
method; can have
declarations.
TVSeries
*
*
seriesName
producer
Title
name
author
isbn
publicationDate
libOfCongress
«Occurrence»
Episode
number
title
storySynopsis
*
LibraryItem
barCodeNumber
Note: create an abstraction containing common data to all occurrences.
This is the “abstraction”. Then create the “occurrence” class that represents instances
(occurrences) of the abstraction. Realtionship is 1:*
ForeignSportsCar { CarMake; CountryofOrigin…} Auto { Model, Style, Cost..}
Note (as we shall see) this is not inheritance.
© Lethbridge/Laganière 2001
Chapter 6: Using design patterns
4
Abstraction-Occurrence
Antipatterns:
LibraryItem
nam e
author
isb n
pub licatio nDate
lib OfCo ngres s
barCod eNumbe r
Single class: Bad because
info would have to be
duplicated in each
occurrence – and same
info would be in each
occurrence!
© Lethbridge/Laganière 2001
LibraryItem
Title
nam e
author
isb n
pub licatio nDate
lib OfCo ngres s
nam e
author
isb n
pub licatio nDate
lib OfCo ngres s
barCod eNumbe r
LibraryItem
GulliversTravels
MobyDick
Here, separate subclass for each title.
All other information would have to be
duplicated in each instance.
Also, want to be able to add new books
without programming new classes!
barCod eNumbe r
Problem here is making the
abstract class a super class
of the occurrence.
Attributes would be
inherited, of course, but
data would be lost! We’d
have to fill in name, author
… for each occurrence!
Chapter 6: Using design patterns
5
Gang of Four (GoF) Patterns
Singleton
Observer
Delegation
Adaptor
Façade
Proxy
© Lethbridge/Laganière 2001
Chapter 6: Using design patterns
6
2. The Singleton Pattern (Gang of Four)
• Context:
—It is very common to find classes for which only one
instance should exist (singleton)
- Examples: a Main Window; Company or University class.
• Problem:
—How do you ensure that it is never possible to create
more than one instance of a singleton class?
• Forces:
—The use of a public constructor cannot guarantee that
no more than one instance will be created.
—The singleton instance must also be accessible to all
classes that require it
© Lethbridge/Laganière 2001
Chapter 6: Using design patterns
7
Singleton
• Solution:
«Singleton»
theInstance
getInstance
Have a private class variable,
possibly called, ‘theInstance.’
This stores the instance.
Company
Then have a public class method
(static method) possibly called,
‘getInstance.’
theCompany
if (theCompany==null)
theCompany= new Company();
Company «private»
getInstance
return theCompany;
First time method is called, it creates
a single instance and stores it in
theInstance.. Subsequent calls simply
return theInstance.
A private Constructor, which ensures
no other class will be able to create
an instance of the singleton class is
needed.
© Lethbridge/Laganière 2001
Here, Company class may embody several
important characteristics of the Company
(operations and attributes).
The public class method getInstance() makes
this instance globally accessible.
Note: effectively, the Singleton instance is
effectively a global variable. Minimize these.
Chapter 6: Using design patterns
8
Controversy
There is criticism of the use of the singleton pattern, as
some consider it an anti-pattern, judging that it is
overused, introduces unnecessary restrictions in
situations where a sole instance of a class is not actually
required, and introduces global state into an application
The Abstract Factory, Builder, and Prototype patterns can
use Singletons in their implementation.
Façade objects are often Singletons because only one
Façade object is required.
© Lethbridge/Laganière 2001
Chapter 6: Using design patterns
9
3. The Observer Pattern (Gang of Four)
• Second in the gang-of-four patterns (Singleton was the first)
• This is another VERY popular one.
• Context:
—When you have a two-way association is created between two
classes, the code for the classes becomes inseparable.
—If you want to reuse one class, then you also have to reuse the
other. There is a dependency.
• Problem:
—How do you reduce the interconnection between classes,
especially between classes that belong to different modules or
subsystems?
• Forces:
—You want to maximize the flexibility of the system to the
greatest extent possible
© Lethbridge/Laganière 2001
Chapter 6: Using design patterns
10
Observer
• Example:
Java has an Observer interface and an
Observable class. This is a specific
implementation of this pattern.
«Observable»
*
*
addObserver
notifyObservers
Consider: a ‘forecast’ requires a lot of
computations. Once done, it ‘notifies’
all interested instances.
«interface»
«Observer»
update
«ConcreteObservable»
«ConcreteObserver»
Just a class…
Forecaster is thus an observable object.
Observable
*
* «interface»
Observer
One observer object might be an interface
object responsible for displaying weather
forecast; another might be dependent on
weather information to plan a schedule..
Forecaster
Observers are
notified when a new
prediction is ready
WeatherView er
Observer pattern in widely used to
structure software cleanly into relatively
independent modules. It is the basis of the
MVC architecture.
© Lethbridge/Laganière 2001
Chapter 6: Using design patterns
11
Observer
• Solution:
So, what do we do? We
create an abstract class we
will call <<Observable>>
that maintains a collection
of <<Observer>> instances.
<<abstract>>
«Observable»
*
addObserver
notifyObservers
*
«interface»
«Observer»
update <<abstract>>
<<Observable>> class is
«ConcreteObserver»
«ConcreteObservable»
very simple; it merely has
a mechanism to add and
remove observers as well as a method, notifyObservers, that sends an update message to
each <<Observer>>.
*
Any application class can declare itself to be a subclass of the <<Observable>> class.
In Java, we call these ‘listeners.’
So, the listener (concrete observable) implements addObserver and notifyObservers.
© Lethbridge/Laganière 2001
Chapter 6: Using design patterns
12
Observer
The Observer Pattern (aka Dependents, Publish/Subscribe) is
a software design pattern in which an object, called the
subject (Observable here), maintains a list of its dependents,
called observers, and notifies them automatically of any state
changes, usually by calling one of their methods, update in the
previous slide.
Mainly used to implement distributed event handling systems.
Observer is also a key part in the familiar MVC architectural
pattern.
© Lethbridge/Laganière 2001
Chapter 6: Using design patterns
13
Observer
The essence of the Observer Pattern is to “define a one
to many dependency between objects so that when one
object changes state, all of its dependents are notified
and updated automatically.
<<observable>>
earlier
© Lethbridge/Laganière 2001
Chapter 6: Using design patterns
14
4. The Delegation Pattern
• Context:
—You are designing a method in a class
—You realize that another class has a method which provides the
required service
—Inheritance is not appropriate (e.g. because the ‘isa’ rule does
not apply
• Problem:
—How can you most effectively make use of a method that
already exists in the other class?
• Forces:
—You want to minimize development cost by reusing methods;
also reduce coupling between classes. Too, methods should be
near the data, and the existing class with the method might be
the more appropriate place for the existing method. So, …
© Lethbridge/Laganière 2001
Chapter 6: Using design patterns
15
Delegation
• Solution:
«Delegator»
delegatingMethod
«Delegate»
method
delegatingMethod()
{
delegate.method();
}
Create a method in a Delegator class that only calls a method in a neighboring Delegate class.
In this way, we are reusing the method for which Delegate has responsibility.
By neighboring class, we mean that the Delegate has an association with the Delegator class.
© Lethbridge/Laganière 2001
Chapter 6: Using design patterns
16
Delegation
• Solution:
Stack
push
pop
isEmpty
LinkedList
addFirst
addLast
addAfter
removeFirst
removeLast
delete
isEmpty
push()
{
list.addFirst();
}
Here we can see that Stack operations push, pop, and isEmpty can readily use
existing methods from Linked List class – addFirst, addLast, and isEmpty.
Thus, we have the above relationship.
© Lethbridge/Laganière 2001
Chapter 6: Using design patterns
17
Delegation (continued)
Normally, the association already exists, and association
need only be unidirectional. We may sometimes need to
create a new association, if the additional complexity
doesn’t increase overall complexity too much.
Kind of a ‘selective inheritance.’
© Lethbridge/Laganière 2001
Chapter 6: Using design patterns
18
Delegation
Antipatterns – This is not good:
• Overuse generalization and inherit the method that is to
be reused instead of creating a single method in the
«Delegator» that does nothing other than call a method
in the «Delegate
• Access non-neighboring classes
© Lethbridge/Laganière 2001
Chapter 6: Using design patterns
19
5. The Adapter Pattern
• Context:
—You are building an inheritance hierarchy and want to
incorporate it into an existing class.
—The reused class is also often already part of its own inheritance
hierarchy.
• Problem:
—How to obtain the power of polymorphism when reusing a class
whose methods
- have the same function
- but not the same signature
as the other methods in the hierarchy?
• Forces:
—You do not have access to multiple inheritance or you do not
want to use it.
© Lethbridge/Laganière 2001
Chapter 6: Using design patterns
20
Adapter
• Solution:
«Superclass»
polymorphicMethod
polymorphicMethod()
{
return
adaptee.adaptedMethod();
}
«Adapter»
«Adaptee»
adaptedMethod
We don’t want to directly incorporate the reused class into our inheritance hierarchy.
Better: Use an <<Adapter>> class that is connected via association to the reused class. (Adaptee)
The polymorphic methods of <<Adapter>> delegate to methods of <<Adaptee>>, and delegate
method in <<Adaptee>> may / may not have same name as delegating polymorphic method.
Anything else in <<Adapter>> will be unaware that it is indirectly using the facilities of an
instance of <<Adaptee>>.
© Lethbridge/Laganière 2001
Chapter 6: Using design patterns
21
Adapter
Example:
volume()
{
return
adaptee.calcVolume();
}
ThreeDShape
volume
Sphere
Torus
TimsTorus
calcVolume
We want to create a hierarchy of three-dimensional shapes, and we’d like to use an existing
implementation of calcVolume in TimsTorus. (TimsTorus is already in its own hierarchy)
We cannot modify the code in TimsTorus because it is being used by others, and thus cannot
make it a subclass of ThreeDShape.
So, we develop Torus (an <<Adapter>>) such that its instances have a link to the instance of
TimsTorus and delegate all operations to TimsTorus.
Adapters are sometimes called Wrappers. Java wrapper classes Integer, Float, Double, etc
are adapters for the Java primitive types.
© Lethbridge/Laganière 2001
Chapter 6: Using design patterns
22
Adapter (Internet)
The adapter pattern (often referred to as the wrapper pattern or simply
a wrapper) is a design pattern that translates one interface for a class into a
compatible interface.
An adapter allows classes to work together that normally could not because of
incompatible interfaces, by providing its interface to clients while using the
original interface.
The adapter translates calls to its interface into calls to the original interface,
and the amount of code necessary to do this is typically small.
There are two types of adapter patterns:
Object Adapter pattern and
Class Adapter pattern.
© Lethbridge/Laganière 2001
Chapter 6: Using design patterns
23
Object Adapter Pattern
In this type of adapter pattern, the adapter contains an
instance of the class it wraps (Adaptee) so it can call the
method, methodB(). In this situation, the adapter makes
calls to the instance of the Adaptor via
adaptor.methodA(), which invokes adaptee.methodB().
Above is The object adapter pattern expressed in UML. The
adapter hides the adaptee's interface from the client.
© Lethbridge/Laganière 2001
Chapter 6: Using design patterns
24
Class Adapter Pattern
This type of adapter uses multiple polymorphic interfaces to achieve its goal.
The adapter is created by implementing or inheriting both the interface that is
expected and the interface that is pre-existing.
It is typical for the expected interface to be created as a pure interface class,
especially in languages such as Java that do not support multiple
inheritance.
Adaptor inherits the interface and
implements the interface (see method1()
© Lethbridge/Laganière 2001
Chapter 6: Using design patterns
25
6. The Façade Pattern
• Context:
—Often, an application contains several complex packages.
—A programmer working with such packages has to manipulate
many different classes
• Problem:
—How do you simplify the view that programmers have of a
complex package?
• Forces:
—It is difficult for a programmer to understand and use an entire
subsystem
—If several different application classes call methods of the
complex package, then any modifications made to the package
will necessitate a complete review of all these classes.
© Lethbridge/Laganière 2001
Chapter 6: Using design patterns
26
Façade Pattern (GofF) For Subsystem Packages:
• Here, the most common pattern of access is Façade, a GoF design pattern.
• We have a public façade object defining the services for the subsystem, and clients
collaborate with the façade, not internal subsystem components.
• The façade should not normally expose many low-level operations. Rather, it exposes
a small number of high-level operations – the coarse-grained services.
• A façade does not normally do its own work. Rather, it is consolidator or mediator to the
underlying subsystem objects, which do the work.
• The façade is a wrapper and single point of access into the ‘rules engine’ of a subsystem.
• Other packages do not see the implementation of the subsystem, as it is
hidden behind the façade.
© Lethbridge/Laganière 2001
Chapter 6: Using design patterns
27
Façade
Facade pattern: software engineering design pattern commonly used with OOP.
A facade is an object that provides a simplified interface to a larger body of code,
such as a subsystem or a class library.
Below: The façade class abstracts Packages 1,2, and 3 from the rest of the
application.
Clients use the façade pattern to access resources from the packages.
They are going to ‘doStuff’ with the objects c1, c2, and c3
© Lethbridge/Laganière 2001
Chapter 6: Using design patterns
28
Façade
• Another view:
«Facade»
«PackageClass1»
«PackageClass2»
«PackageClass3»
We create a special class called a <<Façade>>, which simplifies* the use of the package.
The Façade class simply contains a simplified set of public methods.
*
Result is the creation of a class containing the most often used / needed facilities in the
package. Thus other client elements desiring services of this complex
package can avail themselves of the Façade class, where it is likely what
services they want are simpler to acquire.
Since we are talking about a complex package, other clients that need some of the
complexities of the package can access these services as they normally would – via the
public interfaces of these other classes.
© Lethbridge/Laganière 2001
Chapter 6: Using design patterns
29
Façade
• Example:
Airline
*
RegularFlight
<<façade>>
findFlight
makeBooking
deleteBooking
*
Person
Airline reservation systems have many classes and methods.
Some design elements, like subsystems, may need to interact with the airline system but
don’t want to be exposed to any changes that might be made to it (via dependencies).
So, we can define and Airline class to be a <<Façade>> that contains methods (access) to
the most important query and booking operations in other classes in the airline system.
Thus any dependency arising from this relation is only on the Façade class.
Here, above, via the Airline façade class, we have access to two other classes from other
likely complex packages
© Lethbridge/Laganière 2001
Chapter 6: Using design patterns
30
7. The Proxy Pattern
• Context:
—Often, it is time-consuming and complicated to create instances of
a class (heavyweight classes).
—There is a time delay and a complex mechanism involved in
creating the object in memory. Large classes must be loaded from
a database to support usage.
• Problem:
—How to reduce the need to create instances of a heavyweight class?
—Other objects may need to refer to or use instances of heavyweight
classes.
—Many domain classes are heavyweight classes; also common for
the collection classes used to implement associations (like
ArrayList or Vector) to be heavyweight classes too.
—So, how can we reduce the need to create instances of heavyweight
classes and/or to load large numbers of them from a database or
server when not all of then will be needed?
© Lethbridge/Laganière 2001
Chapter 6: Using design patterns
31
Proxy
• Forces:
—We want all the objects in a domain model to be available for
programs to use when they execute a system’s various
responsibilities.
—It is also important for many objects to persist from run to run of
the same program and it is impractical for all the objects to be
loaded into memory when a program starts.
—It would be much better to do the programming as if all objects
were already loaded into memory.
- Some programmers concerned with loading and storing the objects
- Some programmers concerned with implementing the responsibilities
of the domain model.
• A proxy, in its most general form, is a class functioning as an interface to
something else.
• The proxy could interface to anything: a network connection, a large object in
memory, a file, or some other resource that is expensive or impossible to
duplicate.
© Lethbridge/Laganière 2001
Chapter 6: Using design patterns
32
Proxy
In situations where multiple copies of a complex object must exist, the proxy
pattern can be adapted to incorporate the lightweight pattern in order
to reduce the application's memory footprint.
Typically, one instance of the complex object and multiple proxy objects are
created, all of which contain a reference to a single original complex object.
Any operations performed on the proxies are forwarded to the original object.
Once all instances of the proxy are out of scope, the complex object's memory
may be deallocated.
© Lethbridge/Laganière 2001
Chapter 6: Using design patterns
33
Proxy
«interface»
• Solution:
«Client»
«ClassIF»
*
*
«Proxy»
«HeavyWeight»
Create a simpler version of the heavyweight class, which we call a Proxy, which has the same
interface as the heavyweight class so programmers can declare variables without caring about whether
the Proxy or its Heavyweight version will be put in the variable.
The Proxy object is really only a placeholder, and the operations in the Proxy delegate the operation to
the HeavyWeight.
If/when needed, the Proxy can obtain the real heavyweight object. Further, the proxy only needs to
obtain the heavyweight one time and thus make it available in memory to others who use the proxy.
Some proxies may have implementations of a limited number of operations that can be performed
without the effort of loading the Heavyweight object.
© Lethbridge/Laganière 2001
Chapter 6: Using design patterns
34
Proxy - Example
«interface»
The list elements will
be loaded into local
memory only when
needed.
ListIF
Example:
ListProxy
PersistentList
«interface»
Student
StudentProxy
PersistentStudent
Here we have a variable that is to contain a List. This variable would, however, actually contain a
ListProxy, since it would be expensive to load an entire list of objects into memory, and the list might
not actually be needed. However, as soon as an operation accesses the list, the ListProxy might at that
point create an instance of PersistentList.
On the other hand, the ListProxy might be able to answer certain queries, such as the number of elements
in the list, without going to the effort of loading the PersistentList.
Imagine that the PersistentList is actually a list of students. These objects might also be
proxies – in this case, instances of StudentProxy. Again, instances of PersistentStudent
would only be loaded when necessary.
© Lethbridge/Laganière 2001
Chapter 6: Using design patterns
35