Design Patterns SE464 Derek Rayside images from NetObjectives.com & Wikipedia

modes of normal composition fitness for future

CreationalStructuralBehavioural Abstract Factory Builder Factory Method Prototype Singleton Adapter Bridge Composite Decorator Facade Flyweight Proxy Chain of Responsibility Command Interpreter Iterator Mediator Memento Observer State Strategy Template Method Visitor

CreationalStructuralBehavioural Singleton Adapter Bridge Composite Facade Command Interpreter Observer Strategy Visitor

CreationalStructuralBehavioural Singleton Adapter Bridge Facade Command Observer Strategy

CreationalStructuralBehavioural Singleton Adapter Bridge 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.

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.

Observer (Publish/Subscribe) Design challenge: the observers need to know more

Observer (Publish/Subscribe) 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 Addr, String overdraftAcct); } public Audit implements Observer { public void notify(String auditedAccount, double overdrawnAmount, String ignore Addr, String ignoreOverdraftAcct) { // write info to log, take other appropriate action } }

Observer (Publish/Subscribe) Complications: Need to add new parameters in all existing Listeners May be sending unused data to Listeners

Observer (Publish/Subscribe) Design challenge: the observers need to know more Solution 2: callbacks

Observer (Publish/Subscribe) Complications: May reveal too much information to Listeners o 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 more Solution 3: reify the event

Observer (Publish/Subscribe) Design challenge: the observers need to know more Solution 4: new Observer interface Very simple How to distinguish between the old (legacy) interface and the new Observer interface o 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.

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 chooses the strategy? a. client b. context c. config file (not pictured)

Strategy Mutable Strategy Objects easier to return more complex results need to be instantiated for each use Stateless Strategy Objects reusable re-entrant simpler API usage rules can be Singleton

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

Singleton Advantages convenience controlled access reduced namespace can substitute alternatives more flexible than static methods Issues global variables o 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

To consider: 1.Large delta between local & foreign => facade 2.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.

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

Adapter vs Bridge Adapter connection unforseen interfaces already exist Bridge connection forseen need to be able to subclass both abstraction and implementation

Other patterns you need to know Lectures so far & Lab1 visitor interpreter iterator composite Reading & Future Lecture facade [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? o 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? o Answer: Strategy Design Pattern