design patterns se464 derek rayside images from netobjectives.com & wikipedia
TRANSCRIPT
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
Creational Structural Behavioural
Singleton
AdapterBridge
Composite
Facade
CommandInterpreter
Observer
Strategy
Visitor
Creational Structural Behavioural
Singleton
AdapterBridge
Facade
Command
Observer
Strategy
Creational Structural Behavioural
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.
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 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}}
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 moreSolution 3: reify the event
Observer (Publish/Subscribe)
Design challenge: the observers need to know moreSolution 4: new Observer interface
• Very simple• How to distinguish between the old (legacy) interface and
the new Observer interfaceo 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 choosesthe strategy?
a. clientb. contextc. 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
Adapter
To consider:1. Large delta between local & foreign => facade– Exceptions?– Instantiation?– Does the adapter need to add functionality?
example: SWT and Swing.– 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