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
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–
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Teaching evaluations, department questionnaire
Questions
Final project
Files
Persistence
Lecture 24
1
Exam 2 grade histogram
number of students
7
6
F
D
B
C
A
5
4
3
2
1
0
0-20 21-30 31-40 41-50 51-60 61-70 71-80 81-90
grade range
Lecture 24
2
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
Lecture 17, JOI pp 143-151
Lecture 24
3
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
Lecture 17
Lecture 24
4
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 17
Lecture 24
5
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
Lecture 24
6
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
Lecture 24
7
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
Lecture 24
8
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
Lecture 24
9
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
Lecture 24
10
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
Lecture 24
11
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
Lecture 24
12
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
Lecture 24
13
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
Lecture 24
14
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
Lecture 24
15
Streams/filters
data coming in
program
data going out
• 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
Lecture 24
16
*Stream
classes
Lecture 24
17
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
Lecture 24
18
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
Lecture 24
19
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
Lecture 24
20
Bank (version 9)
public class Bank
implements Serializable
• java Bank –f bankFileName
• if (bankFileName == null) {
theBank = new Bank( bankName );
}
else {
theBank = readBank
( bankName, bankFileName );
}
Lecture 24
21
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;
Lecture 24
22
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
Lecture 24
23
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
Lecture 24
24