Design patterns tutorials

Design Patterns Tutorials

Editor: Nguyễn Đức Minh Khôi
@Feb 2012, HCMC University of Technology
Email: nguyenducminhkhoi@gmail.com

Content
Intro to Design Patterns

Creational Patterns (A)

Structural Patterns (B)

Behavioral Patterns (C)

Other stuffs and conclusion
2/7/2012 Design Pattern Tutorials 2

• Object Oriented (OO) basics
– Abstraction
– Encapsulation
– Inheritance
– Polymorphism


• OO Principles
– Encapsulate what varies
– Favor composition over inheritance
– Program to interfaces, not implementations
– Strive for loosely coupled design between objects
and interact
– Only talk to your friends


• OO Principles (cont.)
– Classes should be open for extension but closed
for modification
– Depend on abstraction. Do not depend on
concrete classes
– Don’t call us, we’ll call you
– A class should have one reason to change


• OO rules:
– A class A extends (inherits) a class B
(A: subclass, B: superclass)
– An interface extends an interface
– A class implements an interface
– Inheritance -> IS relationship
– Composition -> HAS relationship


• What is Pattern?
– Pattern is a solution to a problem in a context
– Context: the situation in which the pattern apply
– Problem: goal you are trying to achieve in this
context (any constraints that occur in that context)
– Solution: what you are after – general design that
anyone can apply which resolves the goal and set
of constraints


Intro to Design
Patterns
List of Design Patterns
and their relationship


Overview of Patterns


Content





Creational Patterns

Singleton Abstract Factory

Creational Patterns involve object instantiation
and all provide a way to decouple a client from the
objects it needs to instantiate


Factory Method Pattern
• Intent: defines an interface for creating an object,
but let subclasses decide which class to instantiate.
Factory Method let a class defer instantiation to
subclasses.
• Use the Factory Method pattern when:
– a class can't anticipate the class of objects it must create.
– a class wants its subclasses to specify the objects it
creates.
– classes delegate responsibility to one of several helper
subclasses, and you want to localize the knowledge of
which helper subclass is the delegate.


Factory Method Pattern (cont.)


Factory Method Pattern (cont.)
• Notes:
– Also known as Virtual Constructor
– Encapsulate objects creation
– Rely on inheritance, object creation is delegated to
subclasses with implement the factory method to
create objects.
– Promote loose coupling by reducing the dependency
of your application to concrete classes
– Are powerful technique for coding abstraction, not
concrete classes

Abstract Factory Pattern
• Intent: Provides an interface for creating families of
related or dependent objects without specifying
their concrete classes
• Use the Abstract Factory pattern when:
– a system should be independent of how its products are
created, composed, and represented.
– a system should be configured with one of multiple
families of products.
– a family of related product objects is designed to be used
together, and you need to enforce this constraint.
– you want to provide a class library of products, and you
want to reveal just their interfaces, not their
implementations.


Abstract Factory Pattern (cont.)


Abstract Factory Pattern (cont.)
• Notes:
– Also known as KIT
– Rely on composition, object creation is
implemented in methods exposed in the factory
interface
– Others like Factory Method!


Singleton Pattern
• Intents: Ensure a class only has one instance, and
provide a global point of access to it.
• Use the Singleton pattern when
– there must be exactly one instance of a class, and it must
be accessible to clients from a well-known access point.
– when the sole instance should be extensible by
subclassing, and clients should be able to use an extended
instance without modifying their code.
– Be careful with multithread!


Singleton Pattern (cont.)


Content





Structural Patterns

Decorator Adapter Façade Composite Proxy

Structural Patterns let you compose classes or
objects into larger structures


Decorator Pattern
• Intent: Attach additional responsibilities to an object
dynamically. Decorators provide a flexible alternative
to subclassing for extending functionality.
• Use Decorator:
– to add responsibilities to individual objects dynamically
and transparently, that is, without affecting other objects.
– for responsibilities that can be withdrawn.
– when extension by subclassing is impractical. Sometimes a
large number of independent extensions are possible and
would produce an explosion of subclasses to support every
combination. Or a class definition may be hidden or
otherwise unavailable for subclassing.


Decorator Pattern (cont.)


Decorator Pattern (cont.)
• Notes:
– Also known as Wrapper
– Decorator classes are the same type as the
components they decorate, either through inheritance
or interface implementation.
– You can wrap a component with any number of
decorators.
– Decorators can result in many small objects in our
design, and overuse can be complex.
– Decorators are typically transparent to the client of
the component

Adapter Pattern
• Intent: convert the interface of a class into another
interface clients expect. Adapter lets classes work
together that couldn't otherwise because of
incompatible interfaces.
• Use the Adapter pattern when:
– you want to use an existing class, and its interface does not
match the one you need.
– you want to create a reusable class that cooperates with
unrelated or unforeseen classes, that is, classes that don't
necessarily have compatible interfaces.
– (object adapter only) you need to use several existing
subclasses, but it's impractical to adapt their interface by
subclassing every one. An object adapter can adapt the
interface of its parent class.

Adapter Pattern (cont.)


Adapter Pattern (cont.)
• Notes:
– Implementing an adapter may require little work
or a great deal of work depending on the size and
complexity of the target interface
– Adapter Pattern has 2 forms: object and class
adapters. Class adapters require multiple
inheritance
– Adapter wraps an object to change its interface, a
decorator wraps an object to add new behaviors
and responsibilities


Facade Pattern
• Intent: Provide a unified interface to a set of
interfaces in a subsystem. Facade defines a higher-
level interface that makes the subsystem easier to
use.
• Use the Facade pattern when:
– you want to provide a simple interface to a complex
subsystem.
– there are many dependencies between clients and the
implementation classes of an abstraction. you want to
layer your subsystems.
– Use a facade to define an entry point to each subsystem
level. If subsystems are dependent, then you can simplify
the dependencies between them by making them
communicate with each other solely through their facades.

Facade Pattern (cont.)


Facade Pattern (cont.)
• Notes:
– A façade decouple a client from complex system
– Implementing a façade requires that we compose
the façade with its subsystem and use delegation
to perform the work of the façade.
– You can implement more than one façade for a
subsystem.
– A façade wraps a set of objects to simplify


Composite Pattern
• Intent: Compose objects into tree structures to
represent part-whole hierarchies. Composite lets
clients treat individual objects and compositions of
objects uniformly.
• Use the Composite pattern when:
– you want to represent part-whole hierarchies of objects.
– you want clients to be able to ignore the difference
between compositions of objects and individual objects.
Clients will treat all objects in the composite structure
uniformly.


Composite Pattern (cont.)


Composite Pattern (cont.)
• Notes:
– A composition provides a structure to hold both
individual (leaf nodes) objects and composites
– There are many design tradeoffs in implementing
Composite. You need to balance transparency and
safety with your needs


Proxy Pattern
• Intent: Provide a surrogate or placeholder for
another object to control access to it.
• Use the Composite pattern when:
– you want to represent part-whole hierarchies of objects.
– you want clients to be able to ignore the difference
between compositions of objects and individual objects.
Clients will treat all objects in the composite structure
uniformly.


Proxy Pattern (cont.)


Proxy Pattern (cont.)
• Notes:
– Also known as Surrogate
– The Decorator Pattern adds behavior to an object,
while a Proxy controls access
– Like any wrapper, proxies will increase the number
of classes and objects in your designs
– Types of proxy: Remote proxy, virtual proxy,
protection proxy


Content





Behavioral Patterns

Strategy Command Observer

Iterator
State Template

Behavioral Patterns: concerned with how classes and objects
interact and distribute responsibility

Command Pattern
• Intent: Encapsulate a request as an object, thereby
letting you parameterize clients with different
requests, queue or log requests, and support
undoable operations.
• Use the Command pattern when you want to:
– parameterize objects by an action to perform, as
MenuItem objects did above.
– specify, queue, and execute requests at different times.
– support undo. The Command's Execute operation can
store state for reversing its effects in the command itself.
– support logging changes so that they can be reapplied in
case of a system crash.
– structure a system around high-level operations built on
primitives operations.

Command Pattern (cont.)


Command Pattern (cont.)
• Notes:
– Also Known As Action, Transaction
– In practice, it is not uncommon for “smart”
Command objects to implement the request
themselves rather than delegating to a receiver.
– Commands may also be used to implement
logging and transactional systems.


Iterator Pattern
• Intent: Provide a way to access the elements of an
aggregate object sequentially without exposing its
underlying representation.
• Use the Iterator pattern:
– to access an aggregate object's contents without
exposing its internal representation.
– to support multiple traversals of aggregate objects.
– to provide a uniform interface for traversing different
aggregate structures (that is, to support polymorphic
iteration).

Iterator Pattern (cont.)


Iterator Pattern (cont.)
• Notes:
– Also known as Cursor
– When using an Iterator, we relieve the aggregate
of the responsibility of supporting operations for
traversing its data.
– An Iterator provides a common interface for
traversing the items of an aggregate, allowing you
to use polymorphism when writing code that
makes use of the items of the aggregate


Template Pattern
• Intent: define the skeleton of an algorithm in an
operation, deferring some steps to subclasses. Template
Method lets subclasses redefine certain steps of an
algorithm without changing the algorithm's structure.
• The Template Method pattern should be used:
– to implement the invariant parts of an algorithm once and leave
it up to subclasses to implement the behavior that can vary.
– when common behavior among subclasses should be factored
and localized in a common class to avoid code duplication.
– to control subclasses extensions. You can define a template
method that calls "hook" operations (see Consequences) at
specific points, thereby permitting extensions only at those
points.

Template Pattern (cont.)


Template Pattern
• Notes:
– Hooks are methods that do nothing or default
behavior in the abstract class, but may be overridden
in the subclass
– To prevent subclasses from changing the algorithm in
the template method, declare the template method
as final
– The Strategy and Template Method Patterns both
encapsulate algorithms, one by inheritance and one
by composition
– The Factory Method is a specialization of Template
Method


Observer Pattern
• Intent: define a one-to-many dependency between objects so
that when one object changes state, all its dependents are
notified and updated automatically.
• Use the Observer pattern in any of the following situations:
– When an abstraction has two aspects, one dependent on the other.
Encapsulating these aspects in separate objects lets you vary and
reuse them independently.
– When a change to one object requires changing others, and you don't
know how many objects need to be changed.
– When an object should be able to notify other objects without making
assumptions about who these objects are. In other words, you don't
want these objects tightly coupled.


Observer Pattern (cont.)


Observer Pattern (cont.)
• Notes:
– Also known as Dependents, Publish-Subscribe
– You can push or pull data from the Observable
when using the pattern
– Don’t depend on a specific order of notification
for your Observers.
– Java has several implementations of the Observer
Pattern, including the general purpose
java.util.Observable


Strategy Pattern
• Intent: define a family of algorithms, encapsulate
each one, and make them interchangeable. Strategy
lets the algorithm vary independently from clients
that use it.
• Also known as Policy
• Use the Strategy pattern when:
– many related classes differ only in their behavior.
Strategies provide a way to configure a class with one of
many behaviors.
– you need different variants of an algorithm.
– an algorithm uses data that clients shouldn't know about.
Use the Strategy pattern to avoid exposing complex,
algorithm-specific data structures.
– Instead of many conditionals, move related conditional
2/7/2012 branches into their own Pattern Tutorials class.
Design Strategy 51

Strategy Pattern (cont.)


State Pattern
• Intent: allow an object to alter its behavior when its
internal state changes. The object will appear to change
its class.
• Use the State pattern in either of the following cases:
– An object's behavior depends on its state, and it must change its
behavior at run-time depending on that state.
– Operations have large, multipart conditional statements that
depend on the object's state. This state is usually represented
by one or more enumerated constants. Often, several
operations will contain this same conditional structure. The
State pattern puts each branch of the conditional in a separate
class. This lets you treat the object's state as an object in its own
right that can vary independently from other objects.


State Pattern (cont.)


State Pattern
• Notes:
– Also known as Objects for States
– The Context gets its behavior by delegating to the
current state object is composed with.
– The State and Strategy Patterns have the same
class diagram, but they differ in intent.
– Using the State Pattern will typically result in a
greater number of classes in your design
– State classes may be shared among Context
instances


Content





Pattern of pattern
• Pattern are often used together and combined
within the same design solution
• Compound pattern combines two or more
patterns into a solution that solves a recurring
or general problem
• The Model View Controller Pattern (MVC) is a
compound pattern consisting of the Observer,
Strategy and Composite patterns.


MVC Patterns


MVC Patterns (cont.)


Other Design Patterns
• Prototype (A) • Interpreter (C)
• Builder (A) • Mediator (C)
• Bridge (B) • Memento (C)
• Flyweight (B) • Visitor (C)
• Chain of responsibility (C)

Look up them more in the Head First
Design Pattern book for more details!


Test your
understanding!
Match each pattern
with its description


Conclusion
Purpose Design Pattern Aspect(s) That Can Vary

Creational Abstract Factory families of product objects

Factory Method subclass of object that is instantiated

Singleton the sole instance of a class

Structural Adapter interface to an object

Composite structure and composition of an object

Decorator responsibilities of an object without
subclassing

Facade interface to a subsystem

Proxy how an object is accessed; its location

Conclusion (cont.)
Purpose Design Pattern Aspect(s) That Can Vary

Command when and how a request is fulfilled

Iterator how an aggregate's elements are accessed,
traversed
Observer number of objects that depend on another
object; how the dependent objects stay up
Behavioral to date
State states of an object

Strategy an algorithm

Template steps of an algorithm
Method


Conclusion (cont.)
• Design Patterns (DP) aren’t set in stone, adapt
and tweak them to meet your needs
• Always use the simplest solution that meets
your needs, even if it doesn’t include a pattern
• Study DP catalogs to familiarize yourself with
patterns and the relationship among them
• Most patterns you encounter will be adaptations
of existing patterns, not new pattern


References
• Head First Design Patterns – by Elisabeth
Freeman, Eric Freeman, Bert Bates, Kathy
Sierra, Elisabeth Robson - O'Reilly Media
(2004)
• Design Patterns: Elements of Reusable
Object-Oriented Software – by Gang of Four -
Addison-Wesley Professional (1994)


The End of Design Patterns Tutorials
THANKS FOR LISTENING
Editor: Nguyễn Đức Minh Khôi
@Feb 2012, HCMC University of Technology
Email: nguyenducminhkhoi@gmail.com

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