CS110 Lecture 2
January 29, 2004
• Announcements
– hw1 part 1 – due right now
– hw1 part 2 – due Tuesday night
• Questions
• Agenda
– turnin
– Object oriented programming (bank simulation)
– XEmacs
Lecture 2
1
Questions
• Do ask in class when you
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have come with a question
are a little confused
think I have made a mistake
think others might learn from your question/insight
• Don’t ask
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just to show off
if you are completely lost (come see me instead)
if you haven’t read the book and hw and your email
what happened in a class you missed
after class about something in the class
Lecture 2
2
email etiquette (please)
• Ask (in email)
– after you have tried to solve your problem, but
before you’ve wasted 8 hours stuck in one place
– with enough detail so that I can help
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Don’t expect immediate feedback
I may cc question and answer to class
Do correspond with your friends
No junk mail
Lecture 2
3
Electronic hw submission
• turnin system: http://turnin.cs.umb.edu/
(available by Monday, perhaps sooner)
• Instructions: follow link on course web page
• Your user name (all lower case)
– up to 6 characters from last name
– balance to 8 characters from first name
• John Kennedy
• Clark Kent
• Wei Liu
 kennedjo
 kentclar
 liuwei
• Password: passwd (change it!)
Lecture 2
4
Object oriented programming
• The (software) world consists of objects
• Each object is an instance of a class
• An object has
– fields that describe what it looks like (its state)
– methods that describe how it behaves
• One object sends a message to another asking it
to use one of its methods to do some work
• Add italicized words to your vocabulary
• Illustrate these abstractions with bank example
in Java
Lecture 2
5
OOP in Java
• File BankAccount.java describes BankAccount objects
(instances of class BankAccount)
• BankAccount.java has two audiences
– people (programmers like us)
– the Java compiler
• Java files always start with comments
– a convention good programs honor
– not a rule in the Java language, but a rule for us
• Comments are for people to read: Java doesn’t care
// text up to end of line is a comment
/** special javadoc comment – more later */
/* old style comment– rare */
Lecture 2
6
BankAccount.java
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// joi/1/bank/BankAccount.java
//
File name, date, authors in // comments at top of file
//
// Copyright 2003 Bill Campbell and Ethan Bolker
/**
* A BankAccount object has a private field for
* this account's current balance, and public
* methods to return and change the balance.
*
Brief description of class instances in
* @see Bank
javadoc comment block /** … */
* @version 1
*/
public class BankAccount Official Java declaring the class
{
}
Java code between these braces describes fields and
methods of BankAccount objects
Lecture 2
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fields : what an object looks like
• Each field has a type, a name and a value
• BankAccount object picture
BankAccount
type of object
int
balance:
999
type of field
field name: value
• Code in BankAccount.java tells type (int, for
integer) and name (balance – good choice by
programmer)
17:
private int balance; // work in whole dollars
• Value (999) may change from time to time
Lecture 2
8
Bank object’s fields (picture)
Bank type of object
“Engulf and Devour”
BankAccount
String
bankName:
type of field
int
int
balance: 200
999
BankAccount
account1:
name of field
BankAccount type of object
BankAccount
account2:
Terminal
int
int
balance: 200
200
Terminal
atm:
Lecture 2
when value is another object,
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use an arrow
Bank object’s fields (code)
• From Bank.java, showing types and names of four fields
type
name
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private String bankName; // Bank’s name
private Terminal atm; // talks to customer
private BankAccount account1;
private BankAccount account2;
• Conventions:
– Class names always begin with an upper case letter
– Field names always begins with a lower case letter
• If type is a class then value is that kind of object
Lecture 2
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Messages
• Ask an object to work for you: send it a message
• Bank.java (line 116)
account.deposit(amount)
this Bank object sends a deposit message to
object account of type BankAccount
• Java syntax: object.message(info)
• syntax: what the program looks like on the page
• deposit method in BankAccount.java does
the work
Lecture 2
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methods : how an object behaves
• BankAccount has several methods:
• deposit (int amount) // line 47
– add amount to current balance, changing value of
balance field
• getBalance( ) // line 58
– tell whoever sent the message how much money
is in this account (value of balance field)
– balance does not change
– the empty parentheses tell us this method needs
no information from the sender to do its job
Lecture 2
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Messages (reprise)
• Ask an object to work for you: send it a message
• Bank.java (line 126)
atm.println(“sorry, . . . ”)
this Bank object sends a println message
to object atm of type Terminal asking it to
print a String on the screen
• Java syntax: object.message(info)
•
println(String something)
method in Terminal.java does the work
• Trust Terminal.java to do the right thing
Lecture 2
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Message invoking a method
Bank.java line 107
String command =
atm.readWord(“transaction: ”);
prompt = “transaction: ”
readWord (String prompt )
{
print prompt on screen
get first word user types
command = thatWord
return thatWord
}
somewhere in Terminal.java
execution flow: line 107 in Bank.java sends a readWord
message to a Terminal. Code for readWord method in
Terminal.java runs, then work resumes at line 104 in
Lecture 2
Bank.java
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Homework 1
• Part 1 (hard copy due right now)
– Play with Bank simulation
• Part 2 (collected electronically Tuesday night)
– Play with Bank simulation
– Improve the Bank simulation
– Write about your coding and testing
Lecture 2
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emacs is a programmer’s editor
tab, java  indent
ctrl-x ctrl-m
tools  compile  compile
• loop through compiler errors
ctrl-x ` (backquote)
tools  compile  next error
• run programs ctrl-x ctrl-r
tools  shell  shell
• learn from XEmacs/Java tutorial
• prettyprint
• compile
Lecture 2
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