CS242 Advanced Programming Concepts in Java

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CS242Advanced Programming Concepts in Java

• 11/08/07
• Intro to Networking

Prof. Searleman jets_at_clarkson.edu
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Outline

• Design Patterns
• Composite
• Decorator
• Networking in Java (java.net)
• Reading Java Tutorial, custom networking
• HW5 posted, now due 11/15/07
• Exam2 Tues, Nov 27, 700 pm, SC 362
• practice problems

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Design Patterns

• cf OODP Chapter 5
• Iterator
• Observer
• Strategy
• Composite
• Decorator

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Strategy Design Pattern

• idea allows you to select one of several
  algorithms dynamically
• example using a layout manager

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Composite Design Pattern

• idea a component may be an individual object or
  a collection of objects
• example group of buttons arranged in a panel

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Decorator Pattern

• idea provides a way to enhance the behavior of a
  class
• example scroll bar

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Example invoice application

• an invoice is composed of a list of line items,
  each of which has a description and a price
• some line items are a group of related
  individual items (i.e. they are composite items)
• Use the Composite, Decorator, Observer Iterator
  patterns
• cf. OODP, pp. 200 - 212

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/ A line item in an invoice. / public
interface LineItem / Gets the price of
this line item. / double getPrice()
/ Gets the description of this line item./
String toString()
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/ A product with a price and description.
/ public class Product implements LineItem
/ Constructs a product. / public
Product(String desc, double price)
this.description desc this.price
price public double getPrice()
return price public String toString()
return description private String
description private double price
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/ A bundle of line items that is again a line
item./ public class Bundle implements LineItem
/ Constructs a bundle with no items. /
public Bundle() items new
ArrayListltLineItemgt() / Adds an item to
the bundle. / public void add(LineItem
item) items.add(item) public double
getPrice() double price 0 for
(LineItem item items) price
item.getPrice() return price
public String toString() / appends the
descriptions / private
ArrayListltLineItemgt items
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/ A decorator for an item that applies a
discount. / public class DiscountedItem
implements LineItem public
DiscountedItem(LineItem item, double discount)
this.item item this.discount
discount public double getPrice()
return item.getPrice() (1 - discount /
100) public String toString()
return item.toString() " (Discount "
discount ")" private LineItem
item private double discount
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/ An invoice for a sale, consisting of line
items. / public class Invoice private
ArrayListltLineItemgt items private
ArrayListltChangeListenergt listeners public
Invoice() items new ArrayListltLineItemgt
() listeners new ArrayListltChangeListene
rgt() public void addItem(LineItem
item) items.add(item) // Notify
all observers of the change to the invoice
ChangeEvent event new ChangeEvent(this)
for (ChangeListener listener listeners)
listener.stateChanged(event)
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/ public class Invoice (cont). / / Adds a
change listener to the invoice. / public void
addChangeListener(ChangeListener listener)
listeners.add(listener)
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/ public class Invoice (cont). / / Returns
an iterator that iterates through the items.
/ public IteratorltLineItemgt getItems()
return new IteratorltLineItemgt() public
boolean hasNext() return current lt
items.size() public LineItem next()
return items.get(current) public void
remove() throw new
UnsupportedOperationException()
private int current 0
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Networking in Java

• socket-based code
• programmer responsible for low-level details
• high-level code layered on top of sockets
• e.g. RMI, CORBA, EJB, JDBC

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Network layers
Definition TCP (Transmission Control Protocol)
is a connection-based protocol that provides a
reliable flow of data between two computers.
Definition UDP (User Datagram Protocol) is a
protocol that sends independent packets of data,
called datagrams, from one computer to another
with no guarantees about arrival. UDP is not
connection-based like TCP.
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TCP/UDP

• TCP is reliable, point-to-point
• Example phone conversation
• (http, ftp, telnet are services which require
  TCP)
• UDP is best attempt at sending datagram
• Example postal mail
• (clock service can use UDP)

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Client-Server
1. Connect to polaris 2. Request service.
How? Different port numbers are associated
with different services

lt 255 public apps 255..1023 marketed apps gt
1023 unregulated
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Client-Server

• client requests a service via a port
• server responds by establishing a
• connection (or refusing it)

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Example time of day service

• National Institute of Standards Technology in
  Boulder, CO
• Host time-A.timefreq.bldrdoc.gov
• Port 13
• To contact a server using TCP/IP
• open a socket
• A socket is a software abstraction for
  communication

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Client-side using TCP

• To contact a server
• Socket skt new Socket(ltservergt,ltportgt)
• If a time-out is desired
• skt.setSoTimeout(10000) // 10 seconds
• Use getInputStream() to associate the socket with
  an input stream
• Read from the input stream

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• public class SocketTest
• public static void main(String args)
• String hostName time-A.timefreq.bldrdoc.gov
• try
• Socket s new Socket(hostName, 13)
• BufferedReader is
• new BufferedReader(
• new InputStreamReader(
• s.getInputStream() ))
• String remoteTime is.readLine()
• System.out.println(Time on hostName
• is remoteTime)
• catch // errors

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• public class Daytime
• public static final short TIME_PORT 13
• // argument to main specifies a hostname
• // (if absent, then use localhost)
• public static void main(String args)
• String hostName
• hostName (args.length 0)? localhost
  args0

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• try
• Socket skt new Socket(hostName, TIME_PORT)
• BufferedReader is
• new BufferedReader(
• new InputStreamReader(skt.getInputStrea
  m()))
• String time is.readLine()
• System.out.println(Time on hostName
• is time)
• catch (IOException e)
• System.err.println(e)

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• public static void main(String argv)
• String server_name
• (argv.length() 1)? argv0
  localhost
• int tcp_port 80
• try
• Socket skt new Socket(server_name,
  tcp_port)
• System.out.println( connected to
  server_name)
• / read and write here /
• skt.close()
• catch (UnknownHostException uhe)
• System.err.println(server_name Unknown
  host.)
• catch (NoRouteToHostException nre)
• System.err.println(server_name
  Unreachable.)
• catch (ConnectException ce)
• System.err.println(server_name
  Connection refused.)
• catch (IOException ioe)
• System.err.println(server_name
  ioe.getMessage())