6'170 Recitation 7

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6.170 Recitation 7

• Asfandyar Qureshi

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Anonymous Classes Huh?

• Comparator myComparator new Comparator()
• public int compare(Object o1, Object o2)
  
• if ((o1 instanceof String) (o2
  instanceof String))
• String s1 (String)o1
• String s2 (String)o2
• return -s1.compareTo(s2)
• else
• return (o1.hashCode() -
  o2.hashCode())
• // java.util.Comparator is an interface
• TreeSet set new TreeSet(myComparator)
• set.add("a") set.add("b") set.add("c")
• System.out.println(set)

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Design Patterns Overview

• Design patterns are
• High level programming idioms
• Standard solutions to common problems.
• Useful design buzzword
• Examples
• Encapsulation and Data Hiding
• Subclassing and Interfaces (Inheritance)
• Exceptions
• Iterators

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Design Patterns Restraint

• When to use them?
• Often there are trade-offs (flexibility/performanc
  e)
• Dont apply them indiscriminately!
• Delay, get something basic first, then improve it
  once you understand it better.
• If youre design has a problem, consider patterns
  that address that problem.
• Design patterns and Understandability
• Decrease (add indirection, increase code size)
• Increase (modularity, ease description)

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Design Patterns

• Creational
• Factory
• Singleton
• Prototypes
• Structural
• Composite
• Adaptor
• Wrapper
• Proxy, Decorator, etc
• Behavioural
• Model / View (Observer)
• Blackboard
• Visitor

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Pattern Singleton

• Gaurantees that only one object of a particular
  class ever exists.
• class HeartSuit extends CardSuit
• private static final
• HeartSuit INSTANCE
• private HeartSuit()
• public static Hearts getInstance()
• if (INSTANCE null) INSTANCE new
  HeartSuit()
• return INSTANCE
• Why not make every method static?
• Most useful for large expensive objects
• Variant have x instances instead of 1 (think
  CardSuit)

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Pattern Adaptors

• Purpose convert the interface of a class into
  another interface that clients expect.
• Think of the driver classes from PS4 and PS5
• They used delegation can also use subclassing
• Can be necessary if you want to reuse code, but
  that code does not satisfy a required interface
• Simpler to write an adapter
• Harder to refactor code to conform to new
  interface
• Testing adaptors
• You still need to test the adaptor!
• Test with the adaptors specs

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Pattern Factory Methods

• Problem with Java constructors
• Returned object is always of the given type,
  never of a subtype.
• Factory methods try to solve this
• Construct and return an object that is either of
  the specified type OR of a subtype
• The intent of this pattern is to define an
  interface for creating an object
• Parameters (e.g. file contents) and subclassing
  can determine the exact type of the returned
  object

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Factory Methods Example

• class JPEGImage extends Image
• class GIFImage extends Image
• class SVGImage extends Image
• Image readImageFromFile(File filename)
• throws IOException

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Factory Objects Example

• Class JPEGFactory extends ImageFactory
• Image readImageFromFile(File filename)
• throws IOException
• Image localizeImage (Image image)
• class someGUIclass
• ImageFactory factory
• public someGUIclass()
• factory new JPEGFactory()
• // use factory to read images

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Pattern Observer

• The Observer pattern allows one or more objects
  (the observers) to watch another (the subject)
• Defines a publish / subscribe relationship
• Whenever something interesting happens at the
  subject, all the observers are notified.
• Event at the subject gets propagated to observers
• Key Decouples observer from subject
• Subject doesnt need to know anything special
  about observers.
• Observers take the initiative and subscribe to
  events from the subject.

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Pattern Observer
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Observers Java Support

• class Observable
• public void addObserver(Observer o)
• public void deleteObserver(Observer o)
• public void setChanged()
• public void notifyObservers()
  notifyObservers(null)
• public void notifyObservers(Object arg)
• Interface Observer
• public void update(Observable o, Object arg)

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Observers Java Support

• class Logger extends Observable
• public void log(String entry)
• entry getCurrentTimeString() entry
• logList.add(entry)
• setChanged()
• notifyObservers(entry)
• ...
• class EchoObserver implements Observable
• public void update(Observable o, Object arg)
• System.out.println(arg)
• Logger logger new Logger()
• logger.log(before we added the observer)
• logger.addObserver(new EchoObserver())
• logger.log(afterwards)

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Pattern Blackboard

• Generalizes observer pattern
• Multiple models multiple views
• Models publish data on the blackboard
• Views react if the data is relevant
• insert filters
• Example MIT mailing lists
• Anyone on csail-discuss_at_mit.edu?

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Blackboard

• class Blackboard
• public insertFilter (Filter msgFilter, Observer
  observer)
• filters.put(msgFilter, observer)
• public publishMessage(Message msg, Observable
  publisher)
• Iterator iter filters.values.iterator()
• while (iter.hasNext())
• Filter filter (Filter)iter.next()
• if (filter.interesting(msg))
• ((Observer)filters.get(filter)).update(pu
  blisher, msg)

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Paradigm Events

• Event driven programming
• Think of system in terms of events
• When an event occurs, do something, handle it
• Network data arrives
• Spreadsheet data changes (update charts)
• Someone clicks on a button
• Java (and many other GUI APIs) are structured
  around events and event handlers
• Youll see lots of observers/listeners in Swing
• More efficient to polling.
• Event driven programming is also powerful in many
  other contexts (e.g. efficiency see 6.824)

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Quick Overview PS6

• Two weeks to complete longer than others
• Overview
• Graph Building (read maps from TigerDB)
• Traffic Sources (inject cars into simulation)
• Traffic Monitors (monitor flow of cars)
• Programmatic Interface
• Graphical User Interface
• Youll have to learn Java Swing programming
• Documentation
• Miniature version of final project documentation

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PS6 Overview

• Premature Optimization
• Get it working first!
• Use profiling (locate hot-spots) and then
  optimize what you need to.
• Use the TimeProfiler class we provide you, or
  your favourite Java profiler
• Staff Provided code
• Database Reader
• GUI helper classes (drawing cars etc)
• Utility classes (e.g. TimeProfiler)

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PS6 Checkpoint!

• Binary checkpoint due by Friday
• Come find any TA or LA to check you off
• What you have to demonstrate
• Graph building code
• Ideally, it should be working
• But if you havent finished debugging it, you can
  still get checked off
• Design Overview, Schedule, Test Strategy
• Convince us that youve started to think about
  these issues doesnt have to be the final MDD