Design Patterns - University of Waterloo · Design challenge: the observers need ... Very simple...

Design Patterns

SE464Derek Rayside

images from NetObjectives.com& Wikipedia

modes of normal composition

fitness for future

Creational Structural Behavioural

Abstract FactoryBuilder

Factory MethodPrototypeSingleton

AdapterBridge

CompositeDecoratorFacade

FlyweightProxy

Chain of ResponsibilityCommandInterpreter

IteratorMediatorMementoObserver

StateStrategy

Template MethodVisitor


Singleton

AdapterBridge

Composite

Facade

CommandInterpreter

Observer

Strategy

Visitor


Singleton

AdapterBridge

Facade

Command

Observer

Strategy


Singleton

AdapterBridge

Observer

Strategy

Observer (Publish/Subscribe)

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


Design challenge: the observers need to know more


Design challenge: the observers need to know more

Solution 1: add parameters to notify method

public interface Observer {public notify(String acct, double amt, String emailAddr, String overdraftAcct);}public Audit implements Observer {public void notify(String auditedAccount, double overdrawnAmount, String ignoreEmailAddr, String ignoreOverdraftAcct) {// write info to log, take other appropriate action}}


Complications:Need to add new parameters in all existing ListenersMay be sending unused data to Listeners


Design challenge: the observers need to know more Solution 2: callbacks


Complications:May reveal too much information to Listeners

Solution: pass a Msg object that encapsulates the Account information instead of passing Account object by reference

Each listener will be blocked until previous listeners are done (in single-threaded situations) The state of the objects passed to Listeners might change in concurrent applications

Observer (Publish/Subscribe)Design challenge: the observers need to know moreSolution 3: reify the event

Observer (Publish/Subscribe)Design challenge: the observers need to know moreSolution 4: new Observer interface

Very simpleHow to distinguish between the old (legacy) interface and the new Observer interface

ex. The new interface extends old interface and adds two new methods.

Strategy

Vary the algorithm independently of the clients who use it.

Strategy

Vary the algorithm independently of the clients who use it. 1. Who chooses the strategy?

2. Are strategy objects mutable? Examples:

Strategy used to sort a list of numbers - if known the list is almost sorted, use merge sort; otherwise use quicksort

Strategy

Who choosesthe strategy?

a. clientb. contextc. config file (not pictured)

Strategy

Mutable Strategy Objectseasier to return more complex resultsneed to be instantiated for each use

Stateless Strategy Objectsreusablere-entrantsimpler API usage rulescan be Singleton

Singleton

Ensure a class has only one instance, and provide a global point of access to it.

Singleton

Advantagesconveniencecontrolled accessreduced namespacecan substitute alternativesmore flexible than static methods

Issues global variables

make testing harder synchronization may reduce parallelism memory leaks

initialization

class-loaders vs VMs distributed systems may hinder re-use

Singleton: traditional implementation

public class Singleton { private static final Singleton INSTANCE = new Singleton();

// Private constructor prevents external instantiation private Singleton() {}

public static Singleton getInstance() { return INSTANCE; }}

Singleton: safer initialization [Pugh]

public class Singleton { // Private constructor prevents external instantiation private Singleton() {}

/** * SingletonHolder is loaded on the first execution * of Singleton.getInstance() or the first access to * SingletonHolder.INSTANCE, not before. */ private static class SingletonHolder { static final Singleton INSTANCE = new Singleton(); }

public static Singleton getInstance() { return SingletonHolder.INSTANCE; }}

Adapter (Wrapper)

Convert the interface of a class into another interface clients expect.

Adapter

Adapter

To consider:1. Large delta between local & foreign => facade2. Exceptions?3. Instantiation?4. Does the adapter need to add functionality?

example: SWT and Swing.5. Stateful? Probably shouldn't be.

Bridge

Decouple an abstraction from its implementation so that the two can vary independently.

Adapter vs Bridge

Adapterconnection unforseeninterfaces already exist

Bridgeconnection forseenneed to be able to subclass both abstraction and implementation

Other patterns you need to know

Lectures so far & Lab1 visitorinterpreteriteratorcomposite

Reading & Future Lecturefacade [cf Bowman]command

Comprehension Questions

What are some ways of adapting an existing Observer class when more information is needed by a type of event? What are some pros and cons of these methods?Name some benefits and downsides of mutable and stateless Strategy objects.What are some issues with the Singleton pattern?Name some uses of the Strategy, Adapter, Singleton, Bridge, and Observer patterns. In GUI Framework, when a button is pressed, an OnButtonPressed event is received. What type of pattern is being used?

Answer: Observer Design Pattern is used. All the Observers (or Listeners in Java) are notified.

Comprehension Questions

Why is it generally considered bad practice to have mutable Singleton classes?What are some differences between the Adapter and the Bridge design pattern? Give an example of both to support your answer.In Java, switching between different layouts using a context makes use of what design pattern?

Answer: Strategy Design Pattern

Design Patterns - University of Waterloo · Design challenge: the observers need ... Very simple...

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