A Few Design Patterns

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Computing Fundamentals with Java ?Rick
Mercer Franklin, Beedle Associates, 2002 ISBN
1-887902-47-3
A Few Design Patterns A Few Exceptions A Little
Standard IO, and Persistent Objects
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Outline

• Object-Oriented Design Patterns
• Mediator Design Pattern
• Exception Handling
• try and catch blocks
• Standard Java I/O
• Decorator Design Pattern
• Java stream objects
• JScrollPane
• Persistence
• Serialization
• ObjectOutputStream
• ObjectInputStream

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OO Design Patterns

• Design Patterns Elements of Reusable
  Object-Oriented Software, Gamma et al.,
• referred to as the Gang of Four (GoF) book.
• Patterns
• facilitate communication of OO designs (names)
• provide tried and true design solutions
• examples of good OO design accepted by experts
• can be applied in different contexts

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OO Design Patterns

• Patterns reduce discovery costs and design
  complexity
• A Design Pattern is a solution to a recurring
  problem, occurring in different contexts, often
  balancing competing forces

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Pattern Names/Examples

• A few patterns some have been mentioned
• Mediator Coordinate activities between other
  objects
• Composite File system / Menus / JPanels /
  Portfolio
• Strategy Layout Managers / Decide what cards to
  play
• Iterator Iterator with next and hasNext
• Decorator Java IO Streams / JScrollPane

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The Mediator Pattern from GoF

• Intent
• Define an object that encapsulates how a set of
  objects interact. Mediator promotes loose
  coupling by keeping objects from referring to
  each other explicitly, and it lets you vary their
  interaction independently.
• Motivation
• OO design encourages the distribution of behavior
  among objects
• This results in many connections (associations)
  between objects
• Worst case Every object knows about each other
• Can't reuse objects as easily
• Can avoid these problems by encapsulating
  collective behavior in a separate mediator object

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Mediator

• Mediator
• A Mediator is responsible for controlling and
  coordinating the interactions of a group of
  objects
• The mediator serves as a go-between between other
  objects
• The mediator knows all objects, but the objects
  don't know each other
• See non-computer example of Mediator from our web
  site

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Examples

• The Disk Jockey (or WebPage or Radio Station)
  coordinates activities between several objects
• it has relationships with several objects
• The ATM System in C Sc 335 a conceptual model

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Another Example

• CourseScheduler in current project could do the
  work of adding and dropping courses
• Needs to talk to semester schedule and logged in
  student

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Riel's Heuristic 4.13

• A few of Arthur Riel's design heuristics
  (guidelines) support this design (use Mediator)
• A class must know what it contains, but the
  con-tained objects should not know who contains
  it
• OK for Account to know that it has a String, but
  String shouldn't be dependent on Account
• Can reuse String in other places
• Student and SemesterSchedule are not dependent on
  CourseScheduler

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Riel's Heuristic 4.14

• Objects that share lexical scope--those contained
  in the same containment--should not have uses
  relationships between them.
• Consider an ATM which contains a card reader and
  a display screen
• The card reader should not send a message to the
  display screen
• better reuse--could use the card reader at a door
  without taking along the display screen

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Violation of the heuristics increases complexity

• An ATM may contain
• card reader, display screen, and keypad
• The ATM should coordinate activities between
  these three objects
• We dont need to have additional collaborations

Automated Teller Machine
HaveACard?()
displayPIN()
Display
Card Reader
getPIN()
Keypad
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Course Reg System

• CourseScheduler may send messages to Student,
  StudentSchedule, SemesterSchedule
• These may exist independently
• Example A printer may print any StudentSchedule
  or the Semester schedule independently
• The SemesterSchedule may be built by a registrar
  independently of any StudentSchedule
• When add and drop rules change, which class will
  need to be modified? Will this make sense in
  2004?

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Things still needed for Course Registration

• Persistence objects
• Exception Handling
• Java I/O stream
• WindowListener

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Exceptions

• When programs run, exception events occur a 3rd
  thing that is sure in life, besides death and
  taxes
• Examples of "exceptional events" include
• Division by 0
• Attempt to open and it fails
• Array subscript out of bounds
• Trying to convert a string that is not a number
  into a number
• Open a file that does not exist
• Handle these with Java's exception handling

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Handling exceptional events

• Put code that "throws" exceptional event into a
  try block and add a catch block.
• try
• code that cause an exceptional event
  (throws an exception)
• catch(Exception anException)
• code that executes when the code in
  try above throws an exception
• If the code in the try block causes an
  except-ional event, control transfer to the catch
  block.

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Example (a repeat)

• ParseDouble may be passed a string that does not
  represent a valid number.
• String numberAsString depositField.getText()
  
• double amount 0.0
• try
• // the next message may "throw an exception"
• amount Double.parseDouble(numberAsString)
• System.out.println("This message may not be
  sent")
• catch(NumberFormatException nfe)
• JOptionPane.showMessageDialog(null,
• "Not a valid deposit amount"
  numberAsString)

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parseDouble

• public static double parseDouble(String s)
• throws NumberFormatException
• Returns a number new represented by s
• Parameters s - the string to be parsed.
• Returns the double value represented by the
  string argument.
• Throws NumberFormatException - if the string
  does not represent a valid number, 1o0.0 for
  example.
• Many methods throw an exception
• Your methods could also

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A small piece of Java's large Exception hierarchy

• Code that throws RuntimeException need not be in
  a try block, All others must be "tried" (there is
  an alternative).

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Input/Output Streams

• Input is read through input stream objects
• Output is written through output stream objects
• A stream is a sequence of items read from some
  source or written to some destination
• Can do "standard" input (no TextReader)
• Can read text from and write text to files
• Or we can read and write entire objects
• This allows for persistent objects

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Persistent Objects

• A persistent object is one that stays around
  after a program terminates
• Can be used by other programs, or when the same
  program begins again
• Entire objects, even very big ones, can be
  written to a file on a disk and read from a file
  on a disk
• Use ObjectOutputStream and its writeObject method
• Use ObjectInputStream and its readObject method

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Write a list to disk

• //Simulate creation of a static (hard coded) list
• list.put("A", new BankAccount("A", 12.34))
• list.put("B", new BankAccount("B", 45.67))
• list.put("C", new BankAccount("C", 200.00))
• // Simulate changes to the objects in the list
• ((BankAccount)list.get("A")).deposit(9999999)
• ((BankAccount)list.get("C")).withdraw(200)
• try
• FileOutputStream bytesToDisk
• new FileOutputStream(fileName)
• ObjectOutputStream outFile
• new ObjectOutputStream(bytesToDisk)
  
• // outFile understands the writeObject message.
• // Make the object persist so it can be read
  later.
• outFile.writeObject(list)
• outFile.close() // Always close the output
  file!

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Serializable

• The objects must be Serializable
• ArrayList, String and many other Java classes do
  this already
• The primitive types and arrays are also
  persistent
• Classes you write will need the Serializable tag
• public class Account // can implement 2 or
  more
• implements Comparable,
  Serializable
• The instance variables must also be Serializable
  or transient, which means they are not written to
  the output stream or read from a stream

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A Serialized Object in Memory
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Read back in

• try
• // Open the stream
• FileInputStream diskToBytes
• new FileInputStream(fileName)
• ObjectInputStream inFile
• new ObjectInputStream(diskToBytes)
  
• // inFile understands the readObject message.
• // Simulate the program starting again with
  saved state.
• HashMap localCollection (HashMap)inFile.readOb
  ject()
• inFile.close() // Always close the output
  file!
• System.out.println(localCollection.get("A"))
• System.out.println(localCollection.get("B"))
• System.out.println(localCollection.get("C"))
• catch( ClassNotFoundException cnfe )
• System.out.println( "Wrong class of object "
  cnfe )
• catch(IOException ioe)

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Multiple catch blocks

• readObject may throw ClassNotFoundException
• One try block may have one to many catch blocks
  associated with it

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Decorator Pattern in Java

• InputStreamReader
• ... bridge from byte streams to character
  streams It reads bytes and translates them into
  characters using the specified character
  encoding. JavaTMAPI
• BufferedReader
• Read text from a character-input stream,
  buffering characters so as to provide for the
  efficient reading of characters, arrays, and
  lines. JavaTMAPI
• What programmer's can now code
• BufferedReader keyboard new BufferedReader(
• new InputStreamReader(System.in))

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Decorator Example
BufferedReader decorates InputStreamReader
BufferedReader readLine()
ready()
InputStreamReader read() close()
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Java streams

• With gt 60 streams in Java, you can create a wide
  variety of input and output streams
• this provides flexibility good
• it also adds complexity bad
• Flexibility made possible with inheritance and
  classes that accept many different classes that
  extend the parameter
• You can have an InputStream instance or any
  instance of a class that extends InputStream
• public InputStreamReader(InputStream in)

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One Constructor for many subclasses

• InputStream has these direct known subclasses
  ByteArrayInputStream, FileInputStream,
  FilterInputStream, InputStream,
  ObjectInputStream, PipedInputStream,
  SequenceInputStream, StringBufferInputStream
• System.in is an instance of InputStream
• Next we will see decorators for persistence
• FileInputStream decorated with ObjectInputStream
  to read objects (not text) from a file
• FileOutputStream decorated with
  ObjectOutputStream to write objects (not text) to
  a file

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WindowListener

• Check the API or WindowListener
• WindowClosing is the event you want to handle
• The other 6 methods could be do nothing
• You could extend WindowAdaptor