CS110 Lecture 18
Tuesday, April 6, 2004
• Announcements
– hw8 due Thursday, April 8
– pass/fail, withdraw deadline Thursday, April 8
• Agenda
– Questions
– Juno
– JFile system internals
Lecture 18
1
Juno
classes
Lecture 18
2
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
3
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
4
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
5
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
6
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
7
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());
}
Lecture 18
8
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
9
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
10
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
11
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
12
JFile system uses two trees
Java class hierarchy
Juno Directory and TextFile hierarchy
\
class Object
users\
eb\
class JFile
backup\
Foo.java
class Directory
class TextFile
memo.txt
bill\
Lecture 18
13
JFile (easy part)
• private fields for
String name
User owner
Date createDate
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
14
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
15
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
16
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
17
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
18
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
19
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
Lecture 18
20
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
21
cd
• Syntax
cd
cd
cd
cd
cd
# change to home directory
foo # change to subdirectory foo
.
# stay where you are
..
# change to parent of current directory
..\bar\whatever # not supported in Juno
Lecture 18
22
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 );
Lecture 18
23
JFile getSuffix
\
• A unix/linux tradition
users\
appends a / when listing
the name of a Directory
eb\
• We want JFiles to behave
backup\
this way, but to use the
Foo.java
windows \ instead
memo.txt
• Ask a JFile to tell you its suffix
bill\
by sending it a getSuffix message
• getSuffix is abstract in JFile.java
Lecture 18
24
“\” 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
25
dir (windows)
full path name in shell prompt
directories first, including . and ..
file, directory and character counts, including . and ..
Lecture 18
26
list (Juno)
full path name in shell prompt
mars:\users\eb> list
Directory of \users\eb
04/05/2004 08:34 AM
04/05/2004 08:34 AM
04/05/2004 08:31 AM
04/05/2004 08:33 AM
04/05/2004 08:32 AM
2 File(s)
3 Dir(s)
2
13
17
30
.
..
backup\
Foo.java
memo.txt
bytes
file, directory and character counts, including . and ..
Lecture 18
27
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
28
Boxes and arrows
• coming soon
Lecture 18
29
Formatting
• Dates
• Numbers
• I18N
Lecture 18
30