CS51 - Spring 2010 - Lecture 24

  • quiz

  • sending/receiving data externally
       - What type of data am I sending?
          - characters/strings
             - use Readers and Writers
          - built-in types
             - use DataInputStream/DataOutputStream
          - objects and or a mixtures of types
             - use ObjectInputStream/ObjectOutputStream
       - Buffered or unbuffered?
          - Unbuffered data sends immediately
          - Buffered data queues it up and then sends it in larger chunks
          - Often we'll want to buffer our data, but sometimes immediate action is necessary
          - For reader/writers
             - BufferedReader
             - PrintWriter
          - For streams
             - BufferedInputStream
             - BufferedOutputStream
       - All of the above take as a parameter to the constructor another Stream or Reader/Writer. So, to get things started:
          - If you're reading/writing from/to a file:
             - Characters/strings
                - FileReader/FileWriter
             - Stream
                - FileInputStream
                - FileOutputStream
          - If you're reading from the network or some other device, there will frequently be a getInputStream() or similar method.

  • look at DrawingPanelNetwork code
       - Sockets
          - a socket is a framework for bidirectional communication
          - sockets are specified by an IP address and a port
             - every computer has an IP address, think of it like a phone number
             - consists of 4 numbers 0 to 255 xxx.xxx.xxx.xxx
             - or, you can think of it as 4 bytes
             - think of the port, like an extension, it uniquely identifies what program/process on a computer another program is talking to
          - Looking up ip addresses
             - "nslookup <domain_name>"
             - nslookup www.cs.pomona.edu
          - the server only need to specify a port, which is the port it is listening on
          - "localhost", is a keyword for specifying the current computer
       - beAServer()
          - ServerSocket is used to wait for a connection from a client
             - note that we only need to specify the port
          - drawingSocket.accept()
          - another blocking operation
          - waits for the client to connect and gives us the socket for the connection
          - socket.getOutputStream()
          - uses writeToStream to transmit the data
       - beAClient()
          - creates a new socket using the ip and the port specified
          - socket.getInputStream()
          - can leverage readFromStream

  • Pictionary lab
       - Encouraged to work in partners
       - Two key challenges:
          - Understanding what is going on with the code provided. Spend time looking at the code and walking through the execution.
          - I/O and Stream processing
       - If you read the lab beforehand and have thought about it a bit, most people will finish in lab (I want you to be working on your test programs...)

  • show SimpleIM demo
       - only need client/server relationship to know who should contact who
       - how does this work for most IM clients?

  • what needs in happen in IM program?
       - establish a connection (should look similar to DrawingPanelNetwork)
          - from these we get a socket (multidirectional)
       - both the client and the server now should do the same thing
          - get reader/writers for both input and output
          - if we have text to write, send it out
          - if we receive text, display it
       - what type of I/O classes should we use (Streams or Reader/Writer)?

  • show SimpleIM code
       - SimpleIM.actionPerformed
          - what are the actions?
          - notice that we use the same class for both the server and the client
          - when the user enters text, we set the text for the message handler
       - IMMessageHandler
          - again, random port
             - http://en.wikipedia.org/wiki/TCP_and_UDP_port
             - http://www.iana.org/assignments/port-numbers
          - after establishing a connection, they do the same thing
          - use BufferedReader and PrintWriter because we're dealing with Strings/characters
             - also give us the ability to send a line at a time
             - use InputStreamReader to go from a stream to a reader
          - ready() returns true if there is data to read, false otherwise
          - see if we have text to send, if so, send it
          - just keep looping

  • Look at the mystery method in Mystery class
       - what does it do?

  • show HTMLLinkFinderBufferedReader demo

  • show HTMLLinkFinderBufferedReader code
       - getWebPage method
          - creates a new URL object
             - URL = uniform resource locator
          - url.openStream()
          - read the data using a BufferedReader
          - new String(buildpage), could also have used builldpage.toString()
          - throws MalformedURLException

  • telnet
       - telnet is a command line program with similar functionality to HTTPConnection
       - "telnet <host_name> <port>" will open a connection
       - standard web server port is 80
          - "telnet www.google.edu 80"
       - to fetch a web page once you have a connection open:
          GET <page_name> HTTP/1.1
       - a common page is "/index.html"
    •