CS110 Lecture 7
February 17, 2004
• Announcements
– hw3 due Thursday
• Agenda
–
–
–
–
–
–
questions
hw3 tips
getters and setters – information hiding
delegation
Shapes application
boxes and arrows
Lecture 7
1
hw3
•
•
•
•
Practice new Java vocabulary (Lens.java)
Improve TextFile class
Draw box-and-arrow pictures
Explore the Java API
Lecture 7
2
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
Lecture 7
3
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
Lecture 7
4
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 King’s Breakfast”,
http://ingeb.org/songs/thekingb.html
Lecture 7
5
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(contents+text);
is the same as
76 setContents(contents+text);
(don’t forget that it is a message: this is implicit)
Lecture 7
6
String tricks
• ("hello, " + "world").
equals("hello, world")
• + concatenates Strings
• remember to send equals message ,
don’t test with ==
• Java can sometimes guess what you mean,
converting a number to a String:
("balance: $" + 100).
equals("balance: $100")
Lecture 7
7
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 +
+
+
++++++++++++++++++++++
Lecture 7
8
Shapes classes
• Particular shapes:
– horizontal line: class HLine
– box: class Box
– VLine, Frame, Triangle (hw4)
• Shapes are clients for Screen
– Use Screen API (javadoc)
– Don’t look at source code
• TestShapes is a test driver (client)
for HLine and Box
Lecture 7
9
Screen API (javadoc)
Lecture 7
10
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
Lecture 7
11
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 ...
Lecture 7
12
Boxes and Arrows
• Draw a picture of a variable - box with narrow
border, showing name and type
type
name:
• If type is primitive, show value inside box
int
accountNumber:
2
• If type is a class then value is a reference to an
object ...
Lecture 7
13
Boxes and Arrows
• Draw a picture of an object - box with thick
border, showing type, containing fields
(which are just variables)
HLine
int
length: 3
char
paintChar:
‘x’
• The object’s methods are not part of this picture!
Lecture 7
14
Boxes and Arrows
HLine h0 = new HLine(3,‘x’);
HLine h1;
HLine
HLine
h0:
HLine
h1: null
int
length: 3
char
paintChar:
‘x’
• Value of h0 is a reference to (arrow to) an HLine
object (which wouldn’t fit into the h0 box in any
case)
• Value of h1 is null (reserved
word but not a keyword)
Lecture 7
15
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 it’s
the same HLine
HLine
HLine
h0:
HLine
h1:
int
length: 9
char
paintChar:
Lecture 7
‘x’
16
Reference values
h0 = new HLine(5,‘y’);
HLine
HLine
h0:
int
length: 5
char
paintChar:
‘y’
HLine
HLine
h1:
int
length: 9
char
paintChar:
‘x’
Lecture 7
17
Reference values
h0 = h1;
HLine
h0:
HLine
Now no variable refers
to this HLine - it’s ready
for garbage collection
int
length: 5
char
paintChar:
‘y’
HLine
HLine
h1:
int
length: 9
char
paintChar:
‘x’
Lecture 7
18
Bank main - Boxes and Arrows
Bank
javaBank:
Bank
“Engulf and Devour”
BankAccount
String
bankName:
int
balance:
999
BankAccount
account1:
BankAccount
account2:
Terminal
BankAccount
int
200
balance:
200
Terminal
atm:
Lecture 7
19
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 +
+
+
++++++++++++++++++++++
Lecture 7
20
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)
Lecture 7
0 1 2
+ + + +
0 +
1 +
2 + &
+ + + +
3 4
+ + +
+
G
+
+
+ + +21
for loop
start
test
step
for (int i = 0; i < 5; i=i+1) {
System.out.println(2*i + 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
Lecture 7
22
for loop
• HLine paintOn() method (lines 47,48)
for ( int i = 0; i < length; i++ ){
s.paintAt( x+i , y, paintChar );
}
• Counts from i = 0 to i = length-1,
executing what’s in the body each time
–
–
–
–
i=0: ask Screen s to put paintChar at (x,y)
i=1: ask Screen s to put paintChar at (x+1,y)
i=2: ask Screen s to put paintChar at (x+2,y)
and so on …
at (x+length-1,y)
Lecture 7
23
for loop
for ( int i = 0; i < length; i++ ){
s.paintAt( x+i , y, paintChar );
}
• Variable i is declared inside for statement
• Surround body with braces {...}for safety
• i++ is short for i = i+1 (or i += 1)
• Can do the same job other ways:
for (int col=x+len-1; col >=x; col-- ){
s.paintAt( col , y, paintChar );
}
Lecture 7
24
for and while
• while can replace for:
start
int i = 0;
while (i < 3) {
System.out.println(i);
i = i + 1;
}
• for can replace while:
boolean more = true;
while ( more ) {
// do something
more = ask();
test
for(int i=0;i<3;i++){
//ditto
}
step
note empty
start
step
for( ; ask(); ) {
// do something
}
}
• For loop advantages:
– fewer lines, control all on one line, elegant, idiomatic
Lecture 7
25
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
Lecture 7
26