Lecture 18 1

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

Lecture 18 2

Juno classes

Lecture 18 3

Shell• “shell” is standard computer science

terminology for an operating system’s 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

Lecture 18 4

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 User’s home) (40)

• Then invokes CLIShell (command line interface) which works just like LoginInterpreter– get an input line from the user (50)– invoke this Shell’s interpret method – done when interpret returns false for moreWork

(user has typed “logout”)

Lecture 18 5

Shell interpret method (60)• Create a StringTokenizer for the input line, after

throwing away Juno comments (# …)

• First token is the commandName (66)

• If it’s “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!

Lecture 18 6

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

Lecture 18 7

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 it’s acting for

• Each concrete ShellCommand implements its own doIt() - polymorphism

Lecture 18 8

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, parentpublic void doIt( StringTokenizer args, Shell sh )

{

String filename = args.nextToken();

new Directory(filename, sh.getUser(), sh.getDot());

}

Lecture 18 9

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)

Lecture 18 10

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

Lecture 18 11

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

Lecture 18 12

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)

Lecture 18 13

JFile system uses two trees

Java class hierarchy

class Object

class JFile

class Directory class TextFile

Juno Directory and TextFile hierarchy

\

users\

eb\

backup\

Foo.java

memo.txt

bill\

Lecture 18 14

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

Lecture 18 15

Testing JFile, Directory, TextFile

• JFile version 5 (before Juno) has static code for testing JFile and its subclasses

• We didn’t 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

Lecture 18 16

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

Lecture 18 17

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 );

Lecture 18 18

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 user’s home directory

Lecture 18 19

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)

Lecture 18 20

JFile constructor• JFile.java, line 49protected: visible to children, not public• lines 51-52 are easy: they initialize fieldsif (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

Lecture 18 21

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

Lecture 18 22

cd

• Syntaxcd # 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

Lecture 18 23

doIt() in CdCommand classString 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 );

Lecture 18 24

JFile getSuffix

• 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

\

users\

eb\

backup\

Foo.java

memo.txt

bill\

Lecture 18 25

“\” 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

Lecture 18 26

full path name in shell prompt

directories first, including . and ..

file, directory and character counts, including . and ..

dir (windows)

Lecture 18 27

mars:\users\eb> listDirectory of \users\eb04/05/2004 08:34 AM .04/05/2004 08:34 AM ..04/05/2004 08:31 AM 2 backup\04/05/2004 08:33 AM 13 Foo.java04/05/2004 08:32 AM 17 memo.txt 2 File(s) 30 bytes 3 Dir(s)

list (Juno)full path name in shell prompt

file, directory and character counts, including . and ..

Lecture 18 28

getPathName (JFile line 77)public String getPathName() { if (this.isRoot()) { return separator; } if (parent.isRoot()) { return separator + getName(); } return parent.getPathName() + separator + getName();}

Lecture 18 29

Boxes and arrows

• coming soon

Lecture 18 30

Formatting

• Dates

• Numbers

• I18N