Transcript Adapter and Façade Patterns

Adapter and Façade
Patterns
By Wode Ni and Leonard Bacon-Shone
Outline: What we’ll cover
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Façade Pattern
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Motivation
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Definition of a Façade
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Advantages of Façade Pattern
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The principle of least knowledge
Adapter Pattern
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Motivation
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Object Adapter
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Class Adapter
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Real World Adapter: Enumeration to Iterator
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Adapter vs Façade vs Decorator
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Implementation of Class adapter: C++ and Java
Façade Pattern: Motivation
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Suppose we have a complex system, but we want a sequence of actions to be
executed on one keystroke.
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Example: a Computer
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The procedure of starting a computer: After the power button is clicked. A
series of operation are done by the CPU, RAM, and HDD, and then the
operating system is booted.
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Do it individually or do it collectively?
http://www.techspot.com/news/52
073-infographic-what-happenswhen-your-computer-turns-on.html
Definition of Facade
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“A unified interface to a set
of interfaces in a subsystem.
Façade defines a higher-level
interface that makes the
subsystem easier to use.”
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Interface - does not only
refer to a Java interface.
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Real-world example:
Compiler.
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When you hit compile, or do
“javac”, a sequence of
instructions got executed in
side a complex java
compiler.
Advantages of having a Facade
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No need to change my way of turning a computer on!
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Regardless of the details of the computer: change of
CPU, OS, Motherboard etc.
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Decouple the client code(pressing the power button)
from any one subsystem(Hardware of the computer)!
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Code example
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Still have an option to gain control of individual
components.
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You can change the setting for your brand new Razor
mouse!
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The façade pattern leaves you the access to all the
classes
The principle of least knowledge
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Definition: Talk only to your immediate friends
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Guidelines: In a method of a class, only invoke methods that belong to
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The object that the class itself represents
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Objects passed in as a parameter to the method
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Any object the method creates or instantiates
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Any components of the object
How does the Façade pattern apply this knowledge?
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We are not breaking the rules!
Adapter Pattern: Motivation
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Existing code incompatible with other code you want to use
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To allow existing code to use other code, create an adapter
Imagine a person from Hong Kong trying to charge their phone
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Hong Kong people use a different socket to the US
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In order to charge their phone, they must use an adapter
Adapter Pattern allows for clients to use existing software to
connect to incompatible classes
Adapter Pattern: Definition
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“The Adapter Pattern converts the interface of a class into another interface
the client expects. Adapter lets classes work together that couldn’t otherwise
because of incompatible interfaces.” (p.243)
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Terms explained:
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Vendor class = adaptee = the US socket
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Client code = Hong Kong plug
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Adapter = HK to US converter
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Naming Convention in the book: Adaptee’s name + “Adapter”
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Plain English: We want the functionality of the adaptee via the language of
the target interface, which is not supported by the adaptee.
Object Adapter
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Object adapter uses composition to store the object of the adaptee and
implement the target interface (the interface used by the client)
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Because of polymorphism, all the subclasses of the adaptee can be adapted
by the object adapter
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Imagine a Chinese person traveling to the US, and wanting to speak like an
American
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Coding Example
Class Adapter
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Class adapter uses multiple inheritance
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Generally considered not possible in Java
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While it is possible to emulate multiple inheritance in Java 8, using the default
method in interfaces
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Discussed further at the end of the presentation
Class adapters inherit both the client and adaptee classes
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Class adapters mean that we don’t have to reimplement the entire interface
because of inheritance
Object Adapter
Class Adapter
Enumeration to Iterator
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Some legacy code may still use the Enumerator
interface, but a client who needs the functionality
of the legacy code has an existing system with an
Iterator
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In order to access the functionality of the legacy code,
an adapter is needed
Some methods are very easily adapted, e.g.
hasNext() to hasMoreElements()
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But what about a method that doesn’t exist in
Enumeration, like the remove() method?
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The best that can be done about non-existing related
methods is throw a runtime exception (fortunately,
the Iterator class already does this with
UnsupportedOperationException)
Adapter vs Façade vs Decorator
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Difference of intents:
Pattern
Intent
Adapter
Convert one interface to another
Facade
Makes an interface simpler
Decorator
Doesn’t alter the interface, but adds responsibility
谢谢
AmericanAdapter adapter = new AmericanAdapter(american);
adapter.xiexie();
Thank you!
Asked by ~everybody: implementation of
class adapter!
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C++ version:
multiple
inheritance. Even
two way adapter!
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Java approximation:
new feature in
Java 8 = default
methods in
interface