Patterns in programming1

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

Patterns in programming3 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 JUnit try { methodThatThrowsExceptions(); fail(…); } catch (SomeException ex) { /* ignore */ }

Patterns in programming4 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.

Patterns in programming5 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.

Patterns in programming6 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

Patterns in programming7 Singleton Idea –Exactly 1 instance of a class –A single global point of access for all clients Example class 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

Patterns in programming8 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

Patterns in programming9 Structural patterns Composite –Compose objects into tree structures. Decorator –Attach additional responsibilities to an object dynamically.

Patterns in programming10 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!

Patterns in programming11 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

Patterns in programming12 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.

Patterns in programming13 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.

Patterns in programming14 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

Patterns in programming15 Template method Idea –Encapsulate part of an algorithm in an abstract method implemented by subclasses. Example abstract 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)‏

Patterns in programming16 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 2 nd edition, Wiley 2002 Freeman & Freeman Head first design patterns, O’Reilly 2004 Joshua Kerievsky Refactoring to Patterns, Addison Wesley Professional –Includes a catalog of patterns atterncatalog.htm atterncatalog.htm