1	CS110 Lecture 3 February 2, 2004 Announcements hw1 part 1 very poorly proofread! hw1 part 2 - electronic collection tonight: memo.txt and Bank.java hw2 available soon (probably tomorrow) java and XEmacs available from lab PCs to burn onto a CD class ends at 12:45, not 12:30 (your loss last time) read ahead in JOI Chapter 2 Agenda questions software development cycle – emacs as an IDE messages and methods flow control: messages, while, if else
2	emacs is a programmer’s editor compile: ctrl-x ctrl-m loop through compiler errors: ctrl-x ` (backquote) run programs: ctrl-x ctrl-r prettyprint: tab, java ® indent if a line doesn’t indent to where you think it should, look for a previous error – missing ; or unbalanced {} or () buffers and windows learn from XEmacs and XEmacs/Java tutorials
3	Compiling in emacs
4	Run the program ctrl-x ctrl-r opens a shell buffer
5	Buffers and frames c-x c-m and c-x c-r split emacs frame, show compilation or shell in other half c-x 1 unsplits frame, shows only buffer containing cursor Buffers pulldown menu allows you to switch between buffers (keyboard: ctrl-x b ) Use File ® Open in New Frame to create a second frame (not Open in New Window) If both frames show same buffer, typing in one is visible in the other
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7	Prettyprinting Whitespace (blank, tab, newline) conventions one statement per line (most lines end with ;) use blank lines wisely indent if statements and while loops line up {} braces TAB in emacs will indent a line properly Follow conventions in code you modify We charge for prettyprinting errors in hw!
8	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
9	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
10	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
11	Message invoking a method
12	Message invoking a method
13	Message invoking a method
14	Exam question Where are messages passed in the following lines of code? In each case identify the object sending the message the name of the message the name and type of the object receiving the message the name and type of the parameters (the information sent along with the message) Practice on the code you read every day Quiz your friends
15	Java flow control Flow control: which line executes next? Command java Foo starts execution in main method in class Foo (files Foo.java, Foo.class) Java executes statements in order (statement ends with ‘;’, usually one per line) Sequential order changes when a message is sent, invoking a method elsewhere code reaches end of a loop body ( e.g. while) if - else logic skips statements
16	a:\> java Bank 138 public static void main( String[] args ) 139 { 140 Bank javaBank = new Bank( "Engulf and Devour" ); 141 javaBank.open(); 142 } Program starts at line 140 (first in Bank main method), creating a new Bank object Line 141 sends that Bank object a message asking it to open itself Execution jumps to the code in Bank’s open method When that method is done we’re at the end of main, program is done, control returns to shell (shell prompt)
17	open method in class Bank 57 public void open() 58 { 59 atm.println( "Welcome to " + bankName ); 60 boolean bankIsOpen = true; 61 while ( bankIsOpen ) { 62 BankAccount account = this.whichAccount(); 63 if ( account == null ) { bankIsOpen = false; 65 } 66 else { 67 this.process…ForAccount(account); 68 } 69 } 70 atm.println( "Goodbye from " + bankName ); 71 }
18	while: Bank.java lines 58-64 Line 60: variable bankIsOpen has type boolean: value may be true or false At line 61, while tests value of bankIsOpen go to line 62 if bankIsOpen == true go to line 70 if bankIsOpen == false Value of bankIsOpen might change from true to false in lines 63, 64 Loop body is code between brace { on line 61 and brace } on line 69 Note how indentation (prettyprinting) helps you see the loop body
19	if: Bank.java lines 63-68 Line 62 gets value for variable account of type BankAccount Line 63 tests the value of that variable if account == null // no account execute line 64 skip to line 69 (then back to 61 and then to 70) else (that is, otherwise) skip line 64,65 execute line 67 continue at line 69 (then back to 61, then to 62) Note how indentation (prettyprinting) helps you see the logic
20	Talking about programs Add these words to your active vocabulary: I use them in class. You use them when you ask questions, talk to classmates, in your memo, and in your exam field, type, value, state, behavior, message, method, program, object, syntax, semantics, model, simulate, architecture, design, file, class, implementation, user interface, compile, edit, run, test, error, source code, flow control, loop, convention, prettyprint, ...