Classes

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Classes

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Position (x,y) Fuel in tank. We will implement these ... A mutator that sets both the x and y positions at the same time. A means to 'use' the Car's fuel ... – PowerPoint PPT presentation

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Title: Classes

1
Classes

Chapter 6
Spring 2007
CS 101
Aaron Bloomfield

2
The Car class
3
More on classes vs. objects
4
A new example creating a Car class

What properties does a car have in the real
world?
Color
Position (x,y)
Fuel in tank
We will implement these properties in our Car
class
public class Car
private Color color
private int xpos
private int ypos
private int fuel
//...

5
Cars instance variables

public class Car
private Color color
private int xpos
private int ypos
private int fuel
//...

6
Instance variables and attributes

Default initialization
If the variable is within a method, Java does NOT
initialize it
If the variable is within a class, Java
initializes it as follows
Numeric instance variables initialized to 0
Logical instance variables initialized to false
Object instance variables initialized to null

7
Car behaviors or methods

What can a car do? And what can you do to a car?
Move it
Change its x and y positions
Change its color
Fill it up with fuel
For our computer simulation, what else do we want
the Car class to do?
Create a new Car
Plot itself on the screen
Each of these behaviors will be written as a
method

8
Creating a new car

To create a new Car, we call
Car c new Car()
Notice this looks like a method
You are calling a special method called a
constructor
A constructor is used to create (or construct) an
object
It sets the instance variables to initial values
The constructor
public Car()
fuel 1000
color Color.BLUE

9
Constructors

public Car()
fuel 1000
color Color.BLUE

No return type!
EXACT same name as class
For now, all constructors are public
10
Our Car class so far

public class Car
private Color color
private int xpos
private int ypos
private int fuel
public Car()
fuel 1000
color Color.BLUE

public class Car
private Color color
Color.BLUE
private int xpos
private int ypos
private int fuel 1000
public Car()

11
Our Car class so far

public class Car
private Color color
Color.BLUE
private int xpos 0
private int ypos 0
private int fuel 1000
public Car()
Called the default constructor
The default constructor has no parameters
If you dont include one, Java will SOMETIMES put
one there automatically

12
Alien Song

AlienSong.mpg

13
Another constructor

Another constructor
public Car (Color c, int x, int y, int f)
color c
xpos x
ypos y
fuel f
This constructor takes in four parameters
The instance variables in the object are set to
those parameters
This is called a specific constructor
An constructor you provide that takes in
parameters is called a specific constructor

14
Our Car class so far

public class Car
private Color color
Color.BLUE
private int xpos 0
private int ypos 0
private int fuel 1000
public Car()
public Car (Color c, int x, int y, int f)
color c
xpos x
ypos y
fuel f

15
Using our Car class

Now we can use both our constructors
Car c1 new Car()
Car c2 new Car (Color.BLACK, 1, 2, 500)

16
So what does private mean?
Note that its a different class!

Consider the following code
public class CarSimulation
public static void main (String args)
Car c new Car()
System.out.println (c.fuel)
Recall that fuel is a private instance variable
in the Car class
Private means that code outside the class CANNOT
access the variable
For either reading or writing
Java will not compile the above code
If fuel were public, the above code would work

17
End of lecture on 19 March 2007
18
So how do we get the fuel of a Car?

Via accessor methods in the Car class
public int getFuel()
return fuel
public Color getColor()
return color
As these methods are within the Car class, they
can read the private instance variables
As the methods are public, anybody can call them

public int getYPos() return ypos public
int getXPos() return xpos
19
So how do we set the fuel of a Car?

Via mutator methods in the Car class
public void setFuel (int f)
fuel f
public void setColor (Color c)
color c
As these methods are within the Car class, they
can read the private instance variables
As the methods are public, anybody can call them

public void setXPos (int x) xpos
x public void setYPos (int y) ypos y
20
Why use all this?

These methods are called a get/set pair
Used with private variables
Well see why one uses these later in this slide
set
Our Car so far

21
Back to our specific constructor

public class Car
private Color color
Color.BLUE
private int xpos 0
private int ypos 0
private int fuel 1000
public Car (Color c,
int x, int y, int f)
color c
xpos x
ypos y
fuel f

public class Car private Color color
Color.BLUE private int xpos 0 private
int ypos 0 private int fuel 1000 public
Car (Color c, int x, int y, int f)
setColor (c) setXPos (x) setYPos
(y) setFuel (f)
22
Back to our specific constructor

Using the mutator methods (i.e. the set
methods) is the preferred way to modify instance
variables in a constructor
Well see why later

23
Todays demotivators
24
So whats left to add to our Car class?

What else we should add
A mutator that sets both the x and y positions at
the same time
A means to use the Cars fuel
A method to paint itself on the screen
Lets do the first
public void setPos (int x, int y)
setXPos (x)
setYPos (y)
Notice that it calls the mutator methods

25
Using the Cars fuel

Whenever the Car moves, it should burn some of
the fuel
For each pixel it moves, it uses one unit of fuel
We could make this more realistic, but this is
simpler
Math.abs() returns the absolute value

public void setXPos (int x) xpos
x public void setYPos (int y) ypos
y
public void setXPos (int x) fuel -
Math.abs (getXPos()-x) xpos x public
void setYPos (int y) fuel -
Math.abs (getYPos()-y) ypos y
26
Setting both positions at once

public void setPos (int x, int y)
setXPos(x)
setYPos(y)
Notice that to modify the instance variables, the
mutator methods are used

27
Drawing the Car

The simple way to have the Car draw itself
public void paint (Graphics g)
g.setColor (color)
g.fillRect (xpos-50, ypos-100,
100, 200)
This draws a single rectangle that is 100 by 200
pixels in size
Lets use constants for the cars height and
width...

28
Drawing the Car

A better version
private final int CAR_WIDTH 100
private final int CAR_HEIGHT 200
public void paint (Graphics g)
g.setColor (color)
g.fillRect (getXPos()-CAR_WIDTH/2,
getYPos()-CAR_HEIGHT/2,
CAR_WIDTH, CAR_HEIGHT)
This makes it easier to change the car size
We could have made the car size instance
variables and set them via mutators
Lets add tires!

29
Drawing the Car

private final int CAR_WIDTH 100
private final int CAR_HEIGHT 200
private final int TIRE_WIDTH 20
private final int TIRE_HEIGHT 40
private final int TIRE_OFFSET 20
public void paint (Graphics g)
g.setColor (color)
g.fillRect (getXPos()-CAR_WIDTH/2,
getYPos()-CAR_HEIGHT/2,
CAR_WIDTH, CAR_HEIGHT)
// Draw the tires
g.setColor (Color.BLACK)
g.fillRect (getXPos()-(CAR_WIDTH/2TIRE_WIDTH
),
getYPos()-(CAR_HEIGHT/2-TIRE_OFFSET),
TIRE_WIDTH, TIRE_HEIGHT)
g.fillRect (getXPos()-(CAR_WIDTH/2TIRE_WIDTH
),
getYPos()(CAR_HEIGHT/2-TIRE_OFFSET-TIRE_HEI
GHT),
TIRE_WIDTH, TIRE_HEIGHT)

Dont worry about this just know that it draws
four tires
30
What happens when the car runs out of fuel?

We could do a number of things
Not allow the car to move anymore
Print out a message saying, fill me up!
Well color the car red
Well insert the following code at the beginning
of the paint() method
if ( fuel
color Color.RED

31
Drawing the Car

private final int CAR_WIDTH 100
private final int CAR_HEIGHT 200
private final int TIRE_WIDTH 20
private final int TIRE_HEIGHT 40
private final int TIRE_OFFSET 20
public void paint (Graphics g)
if ( fuel
color Color.RED
g.setColor (color)
g.fillRect (getXPos()-CAR_WIDTH/2,
getYPos()-CAR_HEIGHT/2,
CAR_WIDTH, CAR_HEIGHT)
// Draw the tires
g.setColor (Color.BLACK)
g.fillRect (getXPos()-(CAR_WIDTH/2TIRE_WIDTH
),
getYPos()-(CAR_HEIGHT/2-TIRE_OFFSET),
TIRE_WIDTH, TIRE_HEIGHT)

32
Our car in action

CarGUI.java

33
How well do you feel you understand the Car class?

Very well! This stuff is easy!
Fairly well with a little review, Ill be good
Okay. Its not great, but its not horrible,
either
Not well. Im kinda confused
Not at all. Im soooooo lost.

34
Cubic Tragedy

Cubic_tragedy_m640.mov

35
Miscellaneous Stuff
36
What Im not expecting you to know yet

What the static keyword means
And why the main() method is
And why other methods are not
Why you should always call the mutator methods,
instead of setting the field directly
Just know that its a good programming practice,
and follow it
Well see why later
Why instance variables are supposed to be private
Just know that its a good programming practice,
and follow it
Again, well see why soon

37
Terminology

An attribute of a class can be called
Instance variable or class variable
Well see the difference later
Static variable (or static field)
Synonymous with class variable
Field
Generally means either type
Variable
Also means either type
Attribute
Property
Argh!
I will generally use the terms variable or field
when I am not differentiating between the two
And instance variable/field and class
variable/field when I am

38
The main() method

Consider a class with many methods
public class WhereToStart
public static void foo (int x)
// ...
public static void bar ()
// ...
public static void main (String args)
// ...
Where does Java start executing the program?
Always at the beginning of the main() method!

39
Running a class without a main() method

Consider the Car class
It had no main() method!
The main() method was in the CarSimulation (or
CarGUI) class
So lets try running it

40
Program Demo

Car.java

41
Variable initialization

A local variable is NOT initialized to a default
value
This is any variable declared within a method
Or within a block within a method
This is pretty stupid, in my opinion
Parameters are initialized to whatever value they
are passed
Instance and class variables are initialized to
default values
Numbers to zero, booleans to false, references to
null
This means any field in a class
Either class variables or instance variables

42
How well do you feel you understand creating
classes so far?

Very well! This stuff is easy!
Fairly well with a little review, Ill be good
Okay. Its not great, but its not horrible,
either
Not well. Im kinda confused
Not at all. Im soooooo lost.

43
End of lecture on 21 March 2007
44
The Circle class

Introducing static-ness, visibilities, etc.

45
A Circle class

We are going to develop a Circle class
Perhaps for use in a graphics program
Why?
Partly to review creating classes
Go over some topics that were a bit fuzzy
Constructors and creating objects
Show why one uses the get/set methods instead of
directly modifying the instance variables
Discuss visibilities (public, private, etc.)
Discuss the static keyword

46
Circle class properties

What properties does a circle have?
Radius
PI 3.141592653589793234
Color (if plotting in a graphics program)
(x,y) location
These properties will become instance variables
We are only going to play with the first two
(radius and PI) in this example
Thus, we are ignoring the color and location

47
Our Circle class
Circle c new Circle() public class Circle
double radius double PI 3.1415926536
Note the radius field is not initialized by us
Note the fields are not static
Were ignoring the public for now
48
Accessing our Circle object

Any variable or method in an object can be
accessed by using a period
The period means follow the reference
Example System.in
Example System.out.println (c.radius)
Example c.PI 4

This is bad PI should have been declared
final (this will be done later)
49
Whats the output?

public class Circle
double radius
double PI 3.1415926536
public class CircleTest
public static void main (String args) int
x
Circle c new Circle()
System.out.println (x)
When a variable is declared as part of a method,
Java does not initialize it to a default value

Java will give a variable not initialized
error
50
Whats the output now?

public class Circle
double radius
double PI 3.1415926536
public class CircleTest
public static void main (String args) int
x
Circle c new Circle()
System.out.println (c.radius)
When a variable is declared as part of a class,
Java does initialize it to a default value

Java outputs 0.0!
51
Whats going on?

A (method) variable needs to be initialized
before it is used
Usually called a local variable
A instance variable is automatically initialized
by Java
All numbers are initialized to 0, booleans to
false, etc.
This is a bit counter-intuitive

52
Circle class behaviors

What do we want to do with (and to) our Circle
class?
Create circles
Modify circles (mutators)
Find out about our circles properties
(accessors)
Find the area of the circle
Plot it on the screen (or printer)
A few others
These will be implemented as methods

53
Calling the Circle constructor

To create a Circle object
Circle c1 new Circle()
This does four things
Creates the c1 reference
Creates the Circle object
Makes the c1 reference point to the Circle
object
Calls the constructor with noparameters (the
defaultconstructor)
The constructor is always the first method called
when creating (or constructing) an object

54
Calling the Circle constructor

To create a Circle object
Circle c1 new Circle(2.0)
This does four things
Creates the c1 reference
Creates the Circle object
Makes the c1 reference point to the Circle
object
Calls the constructor with 1double parameters
(the specificconstructor)
The constructor is always the first method called
when creating (or constructing) an object

55
Constructors

Remember, the purpose of the constructor is to
initialize the instance variables
PI is already set, so only radius needs setting
public Circle()
radius 1.0
public Circle (double r)
radius r

Note there is no return type for constructors
Note that the constructor name is the EXACT
same as the class name
Note that there are two methods with the same
name!
56
What happens in memory

Consider Circle c new Circle()
A double takes up 8 bytes in memory
Thus, a Circle object takes up 16 bytes of memory
As it contains two doubles

Shorthand representation
57
Circle class our class so far

public class Circle
double radius
double PI 3.1415926536
public Circle()
radius 1.0
public Circle (double r)
radius r

58
How well do you feel you understand the
constructors?

Very well! This stuff is easy!
Fairly well with a little review, Ill be good
Okay. Its not great, but its not horrible,
either
Not well. Im kinda confused
Not at all. Im soooooo lost.

59
New 2005 demotivatiors!
60
Circle class our class so far

public class Circle
double radius
double PI 3.1415926536
public Circle()
radius 1.0
public Circle (double r)
radius r

61
Consider the following code

public class CircleTest
public static void main (String args)
Circle c1 new Circle()
Circle c2 new Circle()
Circle c3 new Circle()
Circle c4 new Circle()

62
What happens in memory

There are 4 Circle objects in memory
Taking up a total of 416 64 bytes of memory

63
Consider the following code

public class CircleTest
public static void main (String args)
Circle c1 new Circle()
//...
Circle c1000000 new Circle()
public class CircleTest
public static void main (String args)
Vector v new Vector()
for ( int i 0 i
v.add (new Circle())

These programs create 1 million Circle objects!
64
What happens in memory

There are 1 million Circle objects in memory
Taking up a total of 1,000,00016 16 Mb of
memory

Note that the final PI field is repeated 1
million times
65
The use of static for fields

If a variable is static, then there is only ONE
of that variable for ALL the objects
That variable is shared by all the objects

Total memory usage 16 bytes (112 doubles)
Total memory usage 8 Mb 8 bytes
(1,000,00011,000,001 doubles)
Total memory usage 40 bytes (415 doubles)
c1000000
c4

3.1415926536
PI
66
More on static fields

What does the following print
Note that PI is not final
Circle c1 new Circle()
Circle c2 new Circle()
Circle c3 new Circle()
Circle c4 new Circle()
c1.PI 4.3
System.out.println (c2.PI)
It prints 4.3

Note you can refer to static fields
by object.variable
67
Even more on static fields

There is only one copy of a static field no
matter how many objects are declared in memory
Even if there are zero objects declared!
The one field is common to all the objects
Static variables are called class variables
As there is one such variable for all the objects
of the class
Whereas non-static variables are called instance
variables
Thus, you can refer to a static field by using
the class name
Circle.PI

68
Even even more on static fields

This program also prints 4.3
Circle c1 new Circle()
Circle c2 new Circle()
Circle c3 new Circle()
Circle c4 new Circle()
Circle.PI 4.3
System.out.println (c2.PI)

69
Even even even more on static fields

Weve seen static fields used with their class
names
System.in (type InputStream)
System.out (type OutputStream)
Math.PI (type double)
Integer.MAX_VALUE (type int)
Game.BOARD_X_COORD (in HW J6)

70
How well do you feel you understand static-ness?

Very well! This stuff is easy!
Fairly well with a little review, Ill be good
Okay. Its not great, but its not horrible,
either
Not well. Im kinda confused
Not at all. Im soooooo lost.

71
Hand Paintings
72
Back to our Circle class

public class Circle
double radius
final static double PI 3.1415926536
public Circle()
radius 1.0
public Circle (double r)
radius r
But it doesnt do much!

Note that PI is now final and static
73
Adding a method

public class Circle
double radius
final static double PI 3.1415926536
// Constructors...
double computeArea ()
return PIradiusradius

Note that a (non-static) method can use both
instance and class variables
74
Using that method

public class CircleTest
public static void main (String args)
Circle c new Circle()
c.radius 2.0
double area c.computeArea()
System.out.println (area)

Prints 12.566370614356
75
What happens when that method is called

public class Circle
double radius
final static double PI 3.1415926536
public Circle()
radius 1.0
// other constructor
double computeArea ()
return PIradiusradius
public class CircleTest
public static void main (String args)
Circle c new Circle()

76
Review of our Circle class

public class Circle
double radius
final static double PI 3.1415926536
public Circle()
public Circle (double r)
radius r
double computeArea ()
return PIradiusradius

Slight change from before
77
A note about methods/variable order

Within a method, a variable must be declared
before it is used
In a class, methods and variables can be declared
in any order
This is different than C

78
End of lecture on 23 March 2007
79
Review of static for fields

If a variable is static, then there is only ONE
of that variable for ALL the objects
That variable is shared by all the objects

Total memory usage 16 bytes (112 doubles)
Total memory usage 8 Mb 8 bytes
(1,000,00011,000,001 doubles)
Total memory usage 40 bytes (415 doubles)
c1000000
c4

3.1415926536
PI
80
Adding another method

double oneOverRadius()
return 1.0/radius
I couldnt think of a good reason to divide
something by the radius

81
What happens now?

Code in class CircleTests main() method
Circle c new Circle() // c.radius is now 0.0
System.out.println (c.oneOverRadius())
Java wont crash, but many other programming
languages (C and C, in particular) will
So well call this a crash for the sake of this
lecture
Java prints Infinity
Not what we wanted, though!

82
One way to fix this
Note that the radius variable is now initialized
to 1.0

public class Circle
double radius 1.0
final static double PI 3.1415926536
// Constructors...
double computeArea ()
return PIradiusradius
double oneOverRadius()
return 1.0/radius

83
Back to our program

This code will now run properly
Circle c new Circle() // c.radius 1.0
System.out.println (c.oneOverRadius())
But this code will crash
Circle c new Circle() // c.radius 1.0
c.radius 0.0
System.out.println (c.oneOverRadius())

84
Where the crash occurs

public class CircleTest
public static void main
(String args)
Circle c new Circle()
// c.radius 1.0
c.radius 0.0
System.out.println
(c.oneOverRadius())

public class Circle double radius
1.0 final static double PI 3.1415926536
double computeArea () return
PIradiusradius double oneOverRadius()
return 1.0/radius
Here is where the crash occurs
Here is the badly written code
85
Motivation for private fields

Problem We do not want people using our Circle
class to be able to modify the fields on their
own
Solution Dont allow other code to modify the
radius field
Give it private visibility
private means that only code within the class can
modify the field

86
One way to fix this
Note that the radius variable is now private

public class Circle
private double radius 1.0
final static double PI 3.1415926536
// Constructors...
double computeArea ()
return PIradiusradius
double oneOverRadius()
return 1.0/radius

87
Todays demotivators
88
Back to our program

This code will now not compile
Circle c new Circle() // c.radius 1.0
c.radius 0.0
System.out.println (c.oneOverRadius())
Java will give a compile-time error
radius has private access in Circle

89
Back to our program

This code will also not compile
Circle c new Circle() // c.radius 1.0
System.out.println (c.radius)
Java will give the same compile-time error
radius has private access in Circle

90
The problem now

But now you cant modify a Circles radius!
Or find out what it is
Solution Use a get/set methods in Circle
A mutator method
void setRadius (double r)
radius r
An accessor method
double getRadius ()
return radius

91
Our Circle class so far

public class Circle
private double radius 1.0
final static double PI 3.1415926536
// Constructors...
double computeArea ()
return PIradiusradius
double oneOverRadius()
return 1.0/radius
void setRadius (double r)
radius r
double getRadius ()

92
Using the get/set methods

public class CircleTest
public static void main
(String args)
Circle c new Circle()
c.setRadius (1.0)
System.out.println
(c.computeArea())
System.out.println
(c.getRadius())

public class Circle private double radius
1.0 final static double PI
3.1415926536 double computeArea ()
return PIradiusradius double
oneOverRadius() return 1.0/radius
void setRadius (double r) radius r
double getRadius () return radius

Here a method is invoked
Here the change to radius occurs
93
Wait! Another problem!

public class CircleTest
public static void main (String args)
Circle c new Circle()
c.setRadius (0.0)
System.out.println (c.oneOverRadius())

Here is the problem now
94
This problem is easily fixed

Change the setRadius() method to the following
void setRadius (double r)
if ( r 0.0 )
radius r
else
radius 1.0
Now there is (almost) no way for code outside the
Circle class to change the radius to zero
This is the purpose of mutators
To prevent changing the fields to a bad value
Well see another motivation in a bit

95
Visibilities in Java

There are four visibilities
private Only code within the same class can
access the field or method
Note access means reading or writing the
field, or invoking the method
public Any code, anywhere, can access the field
or method
protected Used with inheritance
We wont get to that this semester
default Almost the same as public
This is the default (duh!)
Note that it cant be specified like the others
Also called package

96
A few notes on visibilities

You can NOT specify visibilities for method
variables
Any method variable can only be accessed within
that method
Think of it as public within the method (after
its defined) and private outside the method
You can also specify visibilities for methods and
classes
We will see this a bit in this course

97
How well do you feel you understand visibilities?

Very well! This stuff is easy!
Fairly well with a little review, Ill be good
Okay. Its not great, but its not horrible,
either
Not well. Im kinda confused
Not at all. Im soooooo lost.

98
DeCSS The program

include typedef unsigned int uint
char ctb512"33733b2663236b763e7e362b6e2e667bd39
3db0643034b96de9ed60b4e0e4\ 69b57175f82c787cf125a1
a528fca8ac21fd999d10049094190d898d001480840913d7d3
5246\ d2d65743c7c34256c2c6475dd9dd5044d0d4594dc9cd
4054c0c449559195180c989c11058185\
081c888c011d797df0247074f92da9ad20f4a0a429f53135b8
6c383cb165e1e568bce8ec61bb\ 3f3bba6e3a3ebf6befeb6a
beeaee6fb37773f2267276f723a7a322f6a2a627fb9f9b1a0e
9a9e\ 1f0b8f8b0a1e8a8e0f15d1d5584cd8dc5145c1c5485c
c8cc415bdfdb5a4edade5f4bcfcb4a5e\
cace4f539793120692961703878302168286071b7f7bfa2e7a
7eff2bafab2afeaaae2ff" typedef unsigned char
ucharuint tb0115,0,1,2,3,4,0,1,2,3,4uchar
FNULL uint lf0,lf1,outvoid ReadKey(uchar
key)int ichar hst3 hst20if(F\
NULL)Fmalloc(256)for(i0i2ihst1ctb2i1Fi\ strtol(hst,NULL,16)
out0lf0(key116)(key\ 3lf10x7)0x8uchar Cipher(int sw1,\ int sw2)int
i,a,b,x0,y0for(i0i2)(lf01
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99
DeCSS The shirt (and tie!)
100
DeCSS The poem

How to decrypt a
DVD in haiku form.
(Thanks, Prof. D. S. T.)
------------------------
(I abandon my
exclusive rights to make or
perform copies of
this work, U. S. Code
Title Seventeen, section
One Hundred and Six.)
Muse! When we learned to
count, little did we know all
the things we could do
some day by shuffling
those numbers Pythagoras

Table Zero is Five, zero, one, two, three,
four, oh, one, two, three, four. Table One is
long two to the eighth power bytes. Ready? Here
they are Fifty one then one hundred fifteen
fifty nine thirty eight ninety nine thirty
five one hundred seven one hundred eighteen
sixty two one hundred twenty six fifty four
forty three one hundred ten then
101
DeCSS The number

The worlds first illegal prime number
4856507896573978293098418946942861377074420873513
57924019652073668698513401047237446968797439926117
51097377770102744752804905883138403754970998790965
39552270117121570259746669932402268345966196060348
51742497735846851885567457025712547499964821941846
55710084119086259716947970799152004866709975923596
06132072597379799361886063169144735883002453369727
81813914797955513399949394882899846917836100182597
89010316019618350343448956870538452085380458424156
54824889333804747587112833959896852232544608408971
11977127694120795862440547161321005006459820176961
77180947811362200272344827224932325954723468800292
77764979061481298404283457201463489685471690823547
37835661972186224969431622716663939055430241564732
92485524899122573946654862714048211713812438821771
76029841255244647445055834628144883356319027253195
90439283873764073916891257924055015620889787163375
99910788708490815909754801928576845198859630532382
34905580920329996032344711407760198471635311617130
78576084862236370283570104961259568184678596533310
07701799161467447254927283348691600064758591746278
12126900735183092415301063028932956658436620008004
76778967984382090797619859493646309380586336721469
69597502796877120572499666698056145338207412031593
37703099491527469183565937621022200681267982734457
60938020304479122774980917955938387121000588766689
25844870047077255249706044465212713040432118261010
35911864766629638584950874484973734768614208805294
43

102
DeCSS The images
103
DeCSS The recordings

All this info from http//www-2.cs.cmu.edu/dst/D
eCSS/Gallery/
Or do a Google search for decss gallery

104
DeCSS The movie
105
Overriding methods (and constructors)
Creates a Circle of radius 1.0

Consider the following code
Circle c1 new Circle ()
Circle c2 new Circle (2.0)
Java knows which constructor to call by the list
of parameters
This is called overloading
Meaning it means multiple things, depending on
the context
Weve seen overloading before
34 Performs integer addition
3.04.0 Performs floating-point addition
34 Performs string concatenation
The operator is overloaded

Creates a Circle of radius 2.0
106
Overriding methods (and constructors), take 2

The following Circle constructors would not be
allowed
We are assuming PI is not final for this example
public Circle()
radius 1.0
public Circle (double r)
radius r
public Circle (double p)
PI p

When Circle(1.0) is called, which one is meant?
107
Using mutators in the constructor

Our second constructor has a problem
public Circle (double r)
radius r
Consider the following code
Circle c new Circle (0.0)
System.out.println (c.oneOverRadius())

The method is dividing by zero (again)
108
Using mutators in the constructor

This is easily fixed!
Our revised constructors
public Circle()
setRadius (1.0)
public Circle (double r)
setRadius (r)
The mutator will properly set the radius (and
wont set it to zero)

109
Why we always use the mutators

Consider a modified version of our circle class
class Circle
double radius
double diameter
String size
// ...
Our mutator now looks like this
Thats a lot of code to copy if you decide not
to call the mutator!

void setRadius (double r) if ( radius ) radius 1.0 else radius r
diameter 2radius if ( radius size small else if ( radius size medium else if ( radius size large else size huge
110
End of lecture on 30 March 2007
111
Google April Fools Day Jokes (2007)

http//www.google.com/tisp/
http//mail.google.com/mail/help/paper/

112
End of lecture on 2 April 2007

We didnt cover any slides today. Instead, we
covered
HW 7
Review of exam 2
The course project

113
An aside scoping issues

Consider this class
class Foo
int x 0
public Foo (int x)
x x
Which x is being referred to in the constructor?
Remember that Java will use the most recent x
So this doesnt set the instance variable!
Instead, it sets the parameter equal to itself

114
An aside scoping issues

Lets modify that class
class Foo
int x 0
public Foo (int x)
this.x x
The this.x means the instance variable x
And the x means the parameter
Now this does the right thing

Another solution
class Foo
int x 0
public Foo (int y)
x y
By renaming the parameter, we achieve the same
effect
This very relevant to HW 7!

115
Back to the static discussion

Remember that there is one (and only one) static
PI field, regardless of how many objects are
declared
Consider the following method
double getPI()
return PI
It doesnt read or modify the state of any
object
In this example, it doesnt read/write the radius
In fact, that particular method doesnt care
anything about the objects declared
Its only accessing a static field

116
Make getPI() static

Consider the following
static double getPI()
return PI
As the method is static, it can ONLY access
static fields
A static method does not care about the state
of an object
Examples Math.sin(), Math.tan(), Math.cos()
They dont care about the state of any Math
object
They only perform the computation

117
Invoking static methods

As with static fields, they can be called using
either an object or the class name
Circle c new Circle()
System.out.println (c.getPI())
System.out.println (Circle.getPI())
Static methods are also called class methods

118
static methods and non-static fields

Consider the following (illegal) Circle method
static double getRadius()
return radius
And the code to invoke it
public static void main (String args)
Circle c1 new Circle()
Circle c2 new Circle()
Circle c3 new Circle()
Circle c4 new Circle()
System.out.println (Circle.getRadius())

119
What happening in memory

There are 4 Circle objects in memory
Which radius field does Circle.getRadius() want?

There are 1 million Circle objects in memory

There are no Circle objects in memory

c1000000
c4

3.1415926536
PI
120
The main static lesson

A static method cannot access or modify the state
of the object it is a part of
If you remember nothing else about static
methods, remember this!

121
static and non-static rules

Non-static fields and methods can ONLY be
accessed by the object name
Static fields and methods can be accessed by
EITHER the class name or the object name
Non-static methods can refer to BOTH static and
non-static fields
Static methods can ONLY access static fields of
the class they are part of

122
How well do you feel you understand static-ness?

Very well! This stuff is easy!
Fairly well with a little review, Ill be good
Okay. Its not great, but its not horrible,
either
Not well. Im kinda confused
Not at all. Im soooooo lost.

123
Very unofficial demotivators
124
Back to our main() method
Well learn about arrays in chapter 8

public static void main (String args)

The method does not return a value
Any code anywhere can call this method

Its a static method
Cant access non-static fields or methods
directly
Can be called only by the class name

125
Implications of main() being static

It can call other static methods within the same
class
class StaticMethods
static void method1()
System.out.println (hi!)
public static void main (String args)
method1()
Note that we didnt have to prefix method1() with
a object
Java assumes that it is in the same class

126
Another use of static methods

Lets say we want each Circle object to have a
unique identifier
The identifier will be a positive integer
So in the Circle class, we add a instance
variable
int id 0
Thus, each Circle object will have its own id
number
To keep track of the last assigned id number, we
will use a class variable
static int lastAssignedID 0
Thus, for all Circle objects created, we will
have a single lastAssignedID field

127
Circle ID numbers

We can then create a method to obtain a unique ID
number
public static int getID()
return lastAssignedID
This method is static, and only can access static
fields
In our Circle constructors, we put the following
line
id getID()

128
Todays demotivators
129
Debugging Java Code
130
Debugging Java code

In Main.java
public class Main
public static void main
(String args)
Error1 e1 new Error1()
e1.foo()
e1.bar()

In Error1.java
public class Error1
public Error1()
public void foo()
String s null
System.out.println
(s.substring(5))
public void bar()
foo()

This will cause a null pointer exception!
131
Program Demo

Errors/Main.java

132
What the error output means

The error output
Exception in thread "main" java.lang.NullPointerEx
ception
at Error1.foo(Error1.java6)
at Main.main(Main.java4)
This means that
A null point exception happened
In the Error1.foo() method on line 6 of
Error1.java
And that was called from Main.main() on line 4 of
Main.java
Note that the top two lines tell the most useful
information
So we comment out the e1.foo() call in main()

133
Debugging Java code

In Main.java
public class Main
public static void main
(String args)
Error1 e1 new Error1()
//e1.foo()
e1.bar()

In Error1.java
public class Error1
public Error1()
public void foo()
String s null
System.out.println
(s.substring(5))
public void bar()
foo()

134
Program Demo

Errors/Main.java

135
What the error output means

The error output
Exception in thread "main" java.lang.NullPointerEx
ception
at Error1.foo(Error1.java6)
at Error1.bar(Error1.java10)
at Main.main(Main.java5)
This means that
A null point exception happened
In the Error1.foo() method on line 6 of
Error1.java
And that was called from Error1.bar() on line 10
of Error.java
And that was called from Main.main() on line 5 of
Main.java
Again, note that the top two lines tell the most
useful information

136
Hondas best commercial

cog.mov

137
More on methods
138
Calling a method

Consider two Strings
String s foo
String t bar
Calling s.substring(2) is different than calling
t.substring(2)
Why?
Because of the object it is being called out of
The method works the same in both cases (returns
the substring)
But it returns different results
Whenever we are calling a method, we also need to
know which object we are calling it out of

139
Return values

Many methods return a value
Math.cos()
String.valueOf()
Consider double d Math.cos (90
Math.PI/180.0)
Lets consider the Math.cos() method
public double cos (double a)
double c 0.0
// compute cos somehow into a variable c
return c
The value c in the cos() method is copied into d

140
The return keyword

The return keyword does a few things
Immediately terminate the current method
Pass the value back to whoever called the method
You can have a return anywhere you want
Inside loops, ifs, etc.
You can have as may returns as you want as well
public String foo (int x)
if ( x 1 ) return one
else if ( x 2 ) return two
else return other

141
More on returns

Consider this class
public class Foo
// Default constructor omitted on this slide
public String bar (String s)
String t CS 101 s
return t
And the code to invoke it
Foo w new Foo()
String x rules
String y foo.bar (x)
System.out.println (y)
What happens in memory?

142
Foo w new Foo() String x rules String y
w.bar (x) System.out.println (y)
Foo w new Foo() String x rules String y
w.bar (x) System.out.println (y)
s
t
this
public String bar (String s) String t
CS 101 s return t
public String bar (String s) String t
CS 101 s return t
Foo() bar (String s) String
143
Returning an object from a method

We could rewrite our bar() method a number of
ways
public String bar (String s)
String t CS 101 s
return t
public String bar (String s)
return new String (CS 101 s)
public String bar (String s)
return CS 101 s

144
Returning a non-object from a method

In other words, returning a primitive type from a
method
public foo ()
// ...
return x y
This method evaluates xy, then returns that
value to the caller

145
Chapter 2 Computer bugs
146
Off to arrays.
147
Yale vs. Harvard
148
Rational class
149
What weve seen so far

An example of creating a class
Car
Up next another example
Rational
Represents rational numbers
A rational number is any number that can be
expressed as a fraction
Both the numerator and denominator must be
integers!
Discussed in section 4.8 of the textbook

150
What properties should our Rational class have?

The numerator (top part of the fraction)
The denominator (bottom part of the fraction)
Not much else

151
What do we want our Rational class to do?

Obviously, the ability to create new Rational
objects
Setting the numerator and denominator
Getting the values of the numerator and
denominator
Perform basic operations with rational numbers
- /
Ability to print to the screen

152
Our first take at our Rational class

Our first take
public class Rational
private int numerator
private int denominator
//...
This does not represent a valid Rational number!
Why not?
Java initializes instance variables to zero
Both the numerator and denominator are thus set
to zero
0/0 is not a valid number!

153
Our next take at our Rational class

Our next take
public class Rational
private int numerator 0
private int denominator 1
//...
Weve defined the attributes of our class
Next up the behaviors

154
The default constructor

Ready?
public Rational()
Yawn!
Note that we could have initialized the instance
variables here instead
The default constructor is called that because,
if you dont specify ANY constructors, then Java
includes one by default
Default constructors do not take parameters

155
The specific constructor

Called the specific constructor because it is one
that the user specifies
They take one or more parameters
public Rational (int num, int denom)
setNumerator (num)
setDenominator (denom)
Note that the specific constructor calls the
mutator methods instead of setting the instance
variables directly
Well see why later

156
Accessor methods