CS110 Lecture 7 February 17, 2004

1
CS110 Lecture 7February 17, 2004

• Announcements
• hw3 due Thursday
• Agenda
• questions
• hw3 tips
• getters and setters information hiding
• delegation
• Shapes application
• boxes and arrows

2
hw3

• Practice new Java vocabulary (Lens.java)
• Improve TextFile class
• Draw box-and-arrow pictures
• Explore the Java API

3
getters and setters

• Good
• private String contents
• public String getContents()
• public void setContents
  (String contents)
• aTextFile.setContents(foo) in client class
• Watch naming conventions
• Bad (public access to field itself)
• public String contents
• aTextFile.contents foo in client class

4
getters and setters

• Hide implementation details from TextFile
  clients
• setContents(String contents) (line 51)
• sets value of field and
• changes modification date
• practice using this
• int getSize() (line 97)
• looks like a getter but
• there is no size field - code delegates the job

5
Delegation

• Pass along the message, asking some other object
  to do the work for you
• Important OO design pattern
• The King asked The Queen, and The
  Queen asked The Dairymaid "Could we have
  some butter for The Royal slice of bread?"
• A. A. Milne, The Kings Breakfast,
  http//ingeb.org/songs/thekingb.html

6
this

• Keyword for the object we are looking at
• Tricky - takes getting used to
• Settles ambiguity in variable names
• 40 this.contents contents
• declared on line 25 on line 37
• Send a message to yourself
• 76 this.setContents(contentstext)
• is the same as
• setContents(contentstext)
• (dont forget that it is a message this is
  implicit)

7
String tricks

• ("hello, " "world"). equals("hello,
  world")
• concatenates Strings
• remember to send equals message ,
  dont test with
• Java can sometimes guess what you mean,
  converting a number to a String
• ("balance " 100).
  equals("balance 100")

8
Shapes

• Character graphics on your terminal

• A 20x10 Screen with 3 HLines
• RRRRRRRRRR
• GGGGGGGGGGGGGGG
• BBBBBBBBBBBBBBB

• draw 3 Boxes (2 overlapping)
• RRRR
• RRRR
• RGGGGGGG
• GGGGGGG
• GGGGGGG GGGGGGG
• GGGGGGG GGGGGGG
• GGGGGGG
• GGGGGGG

9
Shapes classes

• Particular shapes
• horizontal line class HLine
• box class Box
• VLine, Frame, Triangle (hw4)
• Shapes are clients for Screen
• Use Screen API (javadoc)
• Dont look at source code
• TestShapes is a test driver (client)
  for HLine and Box

10
Screen API (javadoc)
11
TestShapes

• Client for Screen, HLine, Box, self documenting
• interesting code fragments
• 28-31 create a Screen, declare and create two
  HLines
• 32,33 paintOn message to HLine wants Screen
  and position as arguments ask the HLine to
  paint itself on a Screen - Screen is
  invisible still
• 34 creates an anonymous new HLine
  which is then asked to
  paint itself on the Screen
• 35 draw message to Screen gets Terminal as an
  argument ask the Screen to draw itself on a
  Terminal finally, everything is
  visible

12
Variables and Values (review)

• Variable named place to hold a value of a
  particular type
• Kinds of variables fields (instance variables),
  local variables in methods, parameters
• Variables must be declared before use
• Type is either
• primitive (int, char, boolean,...)
• reference to an instance (object) of some class
• Why reference to ? Draw pictures ...

13
Boxes and Arrows

• Draw a picture of a variable - box with narrow
  border, showing name and type
• If type is primitive, show value inside box
• If type is a class then value is a reference to
  an object ...

type
name
int
accountNumber
2
14
Boxes and Arrows

• Draw a picture of an object - box with thick
  border, showing type, containing fields
  (which are just variables)
• The objects methods are not part of this
  picture!

15
Boxes and Arrows

• HLine h0 new HLine(3,x)
• HLine h1

HLine
h0
HLine
h1
null

• Value of h0 is a reference to (arrow to) an HLine
  object (which wouldnt fit into the h0 box in any
  case)
• Value of h1 is null (reserved word but not a
  keyword)

16
How References Work

• h1 h0
• h0.setLength(9)
• Variables h0 and h1 refer to the same HLine
  instance
• The HLine referred to by h1 sees the change since
  its the same HLine

HLine
h0
HLine
h1
17
Reference values

• h0 new HLine(5,y)

HLine
h0
HLine
h1
18
Reference values
Now no variable refers to this HLine - its ready
for garbage collection

• h0 h1

HLine
h0
HLine
h1
19
Bank main - Boxes and Arrows
Bank
javaBank
Engulf and Devour
Bank
BankAccount
String
int
bankName
balance
999
BankAccount
account1
BankAccount
200
BankAccount
int
balance
account2
200
Terminal
Terminal
atm
20
Shapes

• Character graphics on your terminal

• A 20x10 Screen with 3 HLines
• RRRRRRRRRR
• GGGGGGGGGGGGGGG
• BBBBBBBBBBBBBBB

• draw 3 Boxes (2 overlapping)
• RRRR
• RRRR
• RGGGGGGG
• GGGGGGG
• GGGGGGG GGGGGGG
• GGGGGGG GGGGGGG
• GGGGGGG
• GGGGGGG

21
Counting

• 1,2,3,... (everyday, mathematics)
• 0,1,2,... (computer science)
• Screen models (x,y) coordinates
• y value increases as you read down
• (0,0) is upper left hand corner
• Each location holds one pixel a character
• Frame of s is not part of Screen
• 5 ? 3 Screen with
  G at position (3,1),
  at position (0,2)

0 1 2 3 4 0
1 G 2
22
for loop

• start test
  step
• for (int i 0 i lt 5 ii1)
• System.out.println(2i 1) body
• Prints 1, 3, 5, 7, 9 on successive lines
• do start
• if test is true
  do body
  
  do step
  
  go back and test again
• else loop is done, so do first line after body
• Use a for loop when you know how many repetitions
  you want (else use while loop)
• See ForDemo.java in JOI

23
for loop

• HLine paintOn() method (lines 47,48)
• for ( int i 0 i lt length i )
  s.paintAt( xi , y, paintChar )
  
• Counts from i 0 to i length-1, executing
  whats in the body each time
• i0 ask Screen s to put paintChar at (x,y)
• i1 ask Screen s to put paintChar at (x1,y)
• i2 ask Screen s to put paintChar at (x2,y)
• and so on at
  (xlength-1,y)

24
for loop

• for ( int i 0 i lt length i )
  s.paintAt( xi , y, paintChar )
  
• Variable i is declared inside for statement
• Surround body with braces ...for safety
• i is short for i i1 (or i 1)
• Can do the same job other ways
• for (int colxlen-1 col gtx col-- )
  s.paintAt( col , y, paintChar )
  

25
for and while

• while can replace for
• int i 0
• while (i lt 3) for(int i0ilt3i)
  
• System.out.println(i) //ditto
• i i 1
• for can replace while
• boolean more true for( ask() )
• while ( more ) // do something
• // do something
• more ask()
• For loop advantages
• fewer lines, control all on one line, elegant,
  idiomatic

test
start
step
note empty start step
26
Signatures

• HLine paintOn messages in HLine unit test (main)
• line 116 hline1.paintOn(screen)
• line 118 hline1.paintOn(screen, 0, 1)
• Two declarations for paintOn in HLine.java
• line 45 paintOn(Screen, int, int)
• line 58 paintOn(Screen)
• delegates work to first paintOn
• JVM uses shape of message to select method
• Signature method name types of parameters