1	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
2	Juno classes
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
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”)
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!
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
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
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, parent public void doIt( StringTokenizer args, Shell sh ) { String filename = args.nextToken(); new Directory(filename, sh.getUser(), sh.getDot()); }
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)
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
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
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)
13	JFile system uses two trees
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
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
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
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 );
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
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)
20	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
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
22	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
23	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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25	"mars:\users\eb>" 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 2 backup\ 04/05/2004 08:33 AM 13 Foo.java 04/05/2004 08:32 AM 17 memo.txt 2 File(s) 30 bytes 3 Dir(s)
26	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
27	“\” 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
28	getPathName (JFile line 77) public String getPathName() { if (this.isRoot()) { return separator; } if (parent.isRoot()) { return separator + getName(); } return parent.getPathName() + separator + getName(); }
29	Formatting Dates Numbers I18N
30	Boxes and arrows coming soon