Patterns in programming 1

Patterns in programming

Patterns in programming 2

What are patterns?

• Answers to common design problems.

• A language used by developers– To discuss answers to design problems– Note: Actually more important to understand

situations than specific solutions• But knowing solutions also required

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Pattern categories

• Architectural patterns– High level: Affects a whole application or a major part of an

application.

• Design patterns– Intermediate level: Affects a few classes.

• Idioms– Low level: Affects a single class or part of a single class.– Closing a resource like a file or network connection

• try { … use resource … } finally { close resource }

– Testing an expected exception in JUnittry { methodThatThrowsExceptions(); fail(…); } catch (SomeException ex) { /* ignore */ }

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Design pattern categories

• Creational patterns– How to create objects when creation requires

decisions.

• Structural patterns– How classes and objects are composed to

form larger structures.

• Behavioral patterns– Algorithms and assignment of responsibilities

between objects.

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Creational patterns

• Factory method– Define an interface for creating an object, but let

subclasses decide which class to instantiate.

• Singleton– Ensures a class has only one instance, and provides

a global point of access to it.

• Object pool– Manages the reuse of objects when a type of object is

expensive to create or only a limited number of objects can be created.

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Factory method• Idea

– Can return an instance of the specified class, or one of its subclasses• A constructor can only “return” an instance of its own class

– Does not have to construct a new object• A constructor must create a new object

– Can have any name• A constructor must have the same name as the class

• Naming conventions• createXxx()• getXxx()

• Java API usage– JDBC

• Connection DriverManager.getConnection(…)• Statement Connection.createStatement(…)• ResultSet Statement.executeQuery(…)

– javax.net.SocketFactory / javax.net.ssl.SSLSocketFactory

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Singleton• Idea

– Exactly 1 instance of a class– A single global point of access for all clients

• Exampleclass A { private static A INSTANCE = new A(); // eager initialization public static A getInstance() { return INSTANCE; } private A() { … } // more methods}

• Variations– Lazy initialization

• The instance is not created before it is needed.• If it is never needed, it does not use resources. • getInstance method might need synchronization.

– Protected constructor• Makes it possible to subclass the singleton class

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Object pool

• Idea– Reuse of objects that are expensive to create

• Usages– Thread pool– Connection pool

• Connections to databases, networks, etc.

• Related patterns– Singleton

• The pool class is often a singleton, since the whole idea is to make all parts of an application share a single pool of objects.

• Java API usage– Thread pool

• Java.util.concurrent.Executors

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Structural patterns

• Composite– Compose objects into tree structures.

• Decorator– Attach additional responsibilities to an object

dynamically.

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Composite

• Idea– Recursively build composite objects from other objects.– Objects are composed at runtime using

• addXx methods to add a new sub-object• removeXx methods to remove a sub-object

• Object diagram looks like a tree– Composite element as root and interior nodes– Non-composite elements as leafs

• Usages– Swing / AWT

• Object of class Container contains other visual components.• JButton extends Container, i.e. we can add JButton’s on a JButton!

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Decorator

• Idea– Enhancing the service provided to the user of the class.– Extend the functionality of an object – transparent to its clients.

• Also known as– Wrapper

• Usages– Java.io

• BufferedReader decorates another reader

– Java.util.Collections• Synchronized wrappers• Immutable wrappers• ReverseOrder on natural order / comparator

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Behavioral patterns• Observer

– Define a one-to-many dependency between object so that when one object changes state, all its dependents are notified (and updated) automatically.

• Strategy– Define a family of algorithms, encapsulate each one,

and make them interchangeable at runtime.

• Template method– Define a skeleton of an algorithm, deferring some

steps to subclasses.

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Observer• Idea

– Decouples otherwise dependent classes.– Observers register with an observable class.– When something happens (like change of state) in the

observable, all observers are notified.• Usage

– Swing / AWT • Visual components send events to listeners.

– JavaBeans• Components sends property change events to listeners.

– Logging• Logger sends messages to handlers.

• Variations– Data is sent with the notification.– Notification does not contain data. Data is fetched from the

observable using get-methods.

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Strategy

• Idea– Encapsulate (part of) an algorithm in an interface.– The algorithm may be changed at runtime.

• Usages– Logging java.util.logging

• Handlers have a Filter and a Formatter• Handler.setFilter(Filter filter)• Handler.setFormatter(Formatter formatter)

• Related patterns– Template method

• Alternative to strategy

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Template method• Idea

– Encapsulate part of an algorithm in an abstract method implemented by subclasses.

• Exampleabstract class SuperClass { abstract protected T doPartOfSomething(); public void doSomething() { …doPartOfSomething(); … }}

class SubClass1 extends SuperClass { protected T doPartOfSomething() { … }}

• Usage– Often used in frameworks

• Users of the framework are supposed to extend the abstract classes.• Related patterns

– Strategy• Modifies the logic / algorithm of individual objects (specified at run-time)• Template method modifies the logic of an entire class (specified at compile-time)

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References• Gamma et al. Design Patterns,

Addison Wesley 1994– First book on design patterns.– Examples in C++ and Smalltalk.– Written by the so-called “Gang of Four”

(GoF)• Buschmann et al. Pattern-Oriented

Software Architecture, Volume 1, Wiley 1996

• Mark Grand Design Patterns in Java, Volume 1 2nd edition, Wiley 2002

• Freeman & Freeman Head first design patterns, O’Reilly 2004

• Joshua Kerievsky Refactoring to Patterns, Addison Wesley Professional 2005

• http://www.hillside.net/– Includes a catalog of patterns

http://www.hillside.net/patterns/onlinepatterncatalog.htm