CS110 Lecture 18 Tuesday, April 6, 2004

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CS110 Lecture 18Tuesday, April 6, 2004

• Announcements
• hw8 due Thursday, April 8
• pass/fail, withdraw deadline Thursday, April 8
• Agenda
• Questions
• Juno
• JFile system internals

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Juno classes
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Shell

• shell is standard computer science terminology
  for an operating systems command line interface
• Windows Command Prompt is a shell
• xemacs gives you access to the same shell
• Juno has a shell, presented to a user after she
  logs in

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Shell object

• Constructor sets some fields
• the Juno system that created this Shell (37)
  (like issuing Bank in BankAccount)
• the User and the console (38, 39)
• the current Directory (the Users home) (40)
• Then invokes CLIShell (command line interface)
  which works just like LoginInterpreter
• get an input line from the user (50)
• invoke this Shells interpret method
• done when interpret returns false for moreWork
  (user has typed logout)

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Shell interpret method (60)

• Create a StringTokenizer for the input line,
  after throwing away Juno comments ( )
• First token is the commandName (66)
• If its logout, then done (return false)
• Replace if else if with dispatch table
• (70,71) look up commandObject in command table
  (commandName String is key)
• (76) send commandObject a doIt() message
• Polymorphism!

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Polymorphism poly (many) morph (shape)

• ShellCommandTable.java
• maintains a list of (abstract) ShellCommand
  objects
• client retrieves them and sends them messages
• without knowing what kinds of JFiles they are!
• Client refers to objects of type Parent that are
  really instances of a Child extending Parent
• Powerful design tool - ignorance is bliss

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abstract class ShellCommand

• Documentation managed here
• helpString and argstring fields (19, 20)
• initialized by protected constructor (31, 32)
• doIt() method (54)
• abstract public void doIt (
  StringTokenizer args, Shell sh )
• doIt is passed the rest of the text on the Juno
  command line, and the Shell its acting for
• Each concrete ShellCommand implements its own
  doIt() - polymorphism

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Designing a ShellCommand object

• MkdirCommand extends ShellCommand (18)
• Constructor (24)
• super invokes ShellCommand constructor, telling
  it help string and argument string for mkdir
• implement abstract method doIt (37)
• next token on line is the name of the Directory
  to be made
• tell Directory constructor the name, owner,
  parent
• public void doIt( StringTokenizer args, Shell sh
  )
• String filename args.nextToken()
• new Directory(filename, sh.getUser(),
  sh.getDot())

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ShellCommandTable

• Juno constructor creates a ShellCommandTable
  (Juno.java line 52)
• ShellCommandTable.java
• declare and initialize a TreeMap (line 23)
• constructor (line 31) invokes fillTable (line 69)
• fillTable creates one of each concrete
  ShellCommand objects, invokes install (line 61)
  to put it in the table
• client (a Shell) invokes lookup (43), which wraps
  Map get method (and does the cast)

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How the dispatch table works (reprise)

• In CLIShell loop
• get first token on the line commandName
• lookup commandObject with commandName key
• send doIt() message
• Each particular ShellCommand extends the abstract
  ShellCommand class, implementing doIt() in its
  own way
• Polymorphism at work

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How LoginInterpreter interpret works (reprise)

• get first token on the line
• use if - else if - else if logic
• if exit return false! // leave loop in
  CLIlogin
• if register // create account for new
  user
• if help // give help
• else // input is a username

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Dispatch table vs if-else if-else if

• To add new commands just add a table entry
• Command semantics separate from syntax
• Lots of design overhead, hard to understand
• Good for large command sets that will grow
  (Juno shell commands)

• To add new commands must edit the main loop
• Command semantics and syntax in same place
• Quick and dirty, easy to understand and code
• Good for small command sets that stay put
  (Juno login loop)

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JFile system uses two trees
Java class hierarchy
Juno Directory and TextFile hierarchy

\ users\ eb\ backup\
Foo.java memo.txt bill\
class Object
class JFile
class Directory
class TextFile
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JFile (easy part)

• private fields for
• String name Date createDate
• User owner Date modDate
• getters and setters as appropriate
• abstract getSize method since each child must
  provide its own implementation
• number of JFiles in a Directory
• number of characters in a TextFile

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Testing JFile, Directory, TextFile

• JFile version 5 (before Juno) has static code for
  testing JFile and its subclasses
• We didnt study it
• Best way to test these classes as a part of Juno
  is to write Juno commands
• type to test newfile (done)
• cd and list to test mkdir (done for Juno 7)
• Read CdCommand
• hw9 improve the ListCommand we will provide

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JFiles in Juno

• Directory constructor signature
  (String directoryName, User owner,
  Directory parent)
• User constructor signature
  (String loginName, Directory home,
  String realName)
• Circular reference problem needs solving

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Set up Juno file system
(Juno.java 56-60)

• // create root directory
• slash new Directory( "", null, null )
• // create system administrator (a User)
• User root new User
  ( "root", slash, "Rick Martin" )
• // add system administrator to user table
• users.put( "root", root )
• // system administrator owns his home
• slash.setOwner(root)
• // create Directory for regular user homes
• userHomes new Directory ( "users", root,
  slash )

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Home Directory

• Each User has a home directory, in users, created
  when User is created
  (LoginInterpreter register method, line 100)
• Any user can read or write in any other users
  home directory

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Managing the JFile tree

• A Directory
• keeps a TreeMap of JFiles in its jfile field,
  keyed by name
• has methods to add and retrieve JFiles by name
• has a method that allows client to loop on
  contents
• A JFile has a parent field (line 37) in which it
  keeps a reference to the Directory it lives in
  (like BankAccount Bank)

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JFile constructor

• JFile.java, line 49
• protected visible to children, not public
• lines 51-52 are easy they initialize fields
• if (parent ! null) (line 53)
• parent.addJFile( name, this )
• if this JFile has a parent (not top of JFile
  tree) send message to parent to add this JFile
  (Directory or TextFile) to its TreeMap,
  with name as key. (Directory.java
  line 67)
• Careful parent directory ! parent class

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Current Directory

• When a Juno user logs in, the current Directory
  is her home Directory
• Convention (in the world of shells)
• . signifies the current Directory
• .. signifies its parent
• Juno Shell provides getDot and setDot methods

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cd

• Syntax
• cd change to home directory
• cd foo change to subdirectory foo
• cd . stay where you are
• cd .. change to parent of current directory
• cd ..\bar\whatever not supported in Juno

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doIt() in CdCommand class

• String dirname ""
• Directory d sh.getUser().getHome()
• if ( args.hasMoreTokens() )
• dirname args.nextToken()
• if (dirname.equals(".."))
• if (sh.getDot().isRoot())
• d sh.getDot() // no change
• else
• d sh.getDot().getParent()
• else if (dirname.equals("."))
• d sh.getDot() // no change
• else
• d (Directory)sh.getDot().
• retrieveJFile(dirname))
• sh.setDot( d )

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JFile getSuffix
\ users\ eb\ backup\
Foo.java memo.txt bill\

• A unix/linux tradition
  appends a / when listing
  the
  name of a Directory
• We want JFiles to behave
  this way, but to use the
  
  windows \ instead
• Ask a JFile to tell you its suffix
  by sending it a
  getSuffix message
• getSuffix is abstract in JFile.java

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\ vs /

• Windows uses one, Unix the other
• Java knows about both
• File.java (in the Java API) declares
  public static final String separator
• JFile.java declares
  public static String separator
  File.separator

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dir (windows)
full path name in shell prompt
directories first, including . and ..
file, directory and character counts, including .
and ..
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list (Juno)
full path name in shell prompt

• mars\users\eb list
• Directory of \users\eb
• 04/05/2004 0834 AM .
• 04/05/2004 0834 AM ..
• 04/05/2004 0831 AM 2 backup\
• 04/05/2004 0833 AM 13 Foo.java
• 04/05/2004 0832 AM 17 memo.txt
• 2 File(s) 30 bytes
• 3 Dir(s)

file, directory and character counts, including .
and ..
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getPathName (JFile line 77)

• public String getPathName()
• if (this.isRoot())
• return separator
• if (parent.isRoot())
• return separator getName()
• return parent.getPathName()
• separator getName()

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Boxes and arrows

• coming soon

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Formatting

• Dates
• Numbers
• I18N