1	CS110 Lecture 24 Thursday, April 29, 2004 Announcements exam answers, final project (WISE) posted final exam Thursday, May 20, 8:00 AM McCormack, Floor 01, Room 0608 (easier than last Tuesday’s!) Agenda Teaching evaluations, department questionnaire Questions Final project Files Persistence
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3	How the dispatch table works 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
4	How LoginInterpreter interpret works 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
5	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)
6	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
7	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
8	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)
9	WISE Final project due Thursday, May 13 Intermediate deliverables Tuesday, May 4,11 must show substantial progress – I will grade and return them promptly What classes will (might) you need? WISE (with main) Student, Course, Professor StudentList, CourseList, ProfessorList TimeOfDay Do not count on exact input/output formats yet
10	I/O programming I/O = input/output I/O is hard deals with real world beyond programmers control Output easier than input (programmer knows more) System.out.println() is straightforward Terminal readLine() wraps hard to use System.in java.io package provides lots of useful classes I/O programming may throw many Exceptions Even good tools are hard to use when topic is hard Count on borrowing from code that works
11	Copy Classic example dealing with file contents Write Windows command line copy in Java: > java Copy sourcefile targetfile main in Copy.java (pseudocode): open sourcefile for reading open targetfile for writing while (get stuff from sourcefile) write stuff to targetfile close both files
12	Copy1.java FileReader inStream = null; // 26, outside try FileWriter outStream = null; // 27, outside try try { inStream = new FileReader(args[0]); // 32 outStream = new FileWriter(args[1]); // 33 while (…) { // 36-38 copy loop } catch // various errors 40: faulty command line input - give usage message 44: source file not found (or not readable) target file not writeable 47: something went wrong in actual read/write
13	Keyword finally try { } catch() { { finally { code here runs whether or not try works } Copy1.java 53, 61: close files whether or not there was an error in processing (underlying OS may limit number of files you may have open) try (lines 51, 63) since even closing a file may throw an Exception
14	FileReader/FileWriter i/o int ch; // character read as an int (line 28) while ((ch = inStream.read()) != -1) { // 36 outStream.write(ch); } Java (and C) idiom: assignment statement x = y gets value of x , so (ch = inStream.read()) != EOF sends instream a read() message assigns returned int to variable ch compares that int to EOF, declared final static, used by read() to signal end of file result is true or false, so useful as while test
15	Copy2 using BufferedReader/Writer BufferedReader inStream = null;// lines 24, 25 BufferedReader outStream = null String line; try inStream = new BufferedReader ( new FileReader(argv[0])); outStream = ... while ((line = inStream.readLine()) != null) outStream.write( line ); outStream.newLine(); // no ‘\n’ in line BufferedReader/Writer handle whole lines (Strings) readLine returns null at EOF
16	Streams/filters data can be characters, Strings, bytes, objects,… Streams connect to file, terminal, String, net, … Always use same methods: read, write (polymorphism) Examples: copy: stream of characters, or of lines (Strings) Profile: stream of lines, program counts kinds TV: input stream from cable, output stream to screen
17	*Stream classes
18	Profile main in Profile.java (pseudocode): declare and initialize counters open Java source for reading while (get a line from source file) classify line, increment counters close source file print results
19	Persistence Bank and Juno should remember state between invocations read state from a file at startup write state back at exit Can imagine a text representation of the state Better: Java knows how to save whole Objects
20	Bank (version 9) Bank instance can be saved to a file java Bank –f bankFileName live demo … if –f bankFileName && file exists read Bank from that file else create new Bank() visit bank if –f bankFileName write Bank to that file
21	Bank (version 9) public class Bank implements Serializable java Bank –f bankFileName if (bankFileName == null) { theBank = new Bank( bankName ); } else { theBank = readBank ( bankName, bankFileName ); }
22	Read Bank instance from a file private static Bank readBank( String bankName, String bankFileName) { File file = new File( bankFileName ); if (!file.exists()) { return new Bank( bankName ); } ObjectInputStream inStream = null; try { inStream = new ObjectInputStream( new FileInputStream( file ) ); Bank bank = (Bank)inStream.readObject(); System.out.println( "Bank state read from file " + bankFileName); return bank; }
23	Why read/write only Bank? BankAccount and Month are also Serializable Bank box-and-arrow picture shows that all objects of all types are pointed to (indirectly) by arrows starting in the Bank private transient Terminal atm; Terminal not saved when Bank is saved
24	Persistence Juno version 10 is persistent new Java keyword implements, as in implements Serializable Serializable is an interface, not a class Find out about interfaces in cs210 (and a little bit next week) Java 1.5 does a cleaner job with persistence