Classes
Chapter 4
Spring 2005
CS 101
Aaron Bloomfield
1
Preparation
 Scene so far has been background material and experience
 Computing systems and problem solving
 Variables
 Types
 Input and output
 Expressions
 Assignments
 Objects
 Standard classes and methods
 Decisions (if, switch)
 Loops (while, for, do-while)
 Now: Experience what Java is really about
 Design and implement objects representing information
2
and physical world objects
Object-oriented programming
 Basis
 Create and manipulate objects with attributes
behaviors that the programmer can specify
and
 Mechanism
 Classes
 Benefits
 An information type is design and implemented once
 Reused as needed
 No need reanalysis and re-justification of the
representation
3
Known Classes
 Classes we’ve seen
 BigInteger
 String
 StringBuffer
 Scanner
 System
 Classes we’ll be seeing soon
 BigDecimal
4
The Car class
5
More on classes vs. objects
6
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;
//...
}
7
Car’s instance variables
public class Car {
private Color color;
private int xpos;
private int ypos;
private int fuel;
//...
Car
}
- color
- fuel
- xpos
- ypos
+…
8
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:
Car
 Numeric instance
variables initialized to 0
- color = null
- xpos = 0
- fuel = 0
- ypos = 0
 Logical instance
variables initialized to
+…
false
 Object instance
variables initialized to
null
9
Car behaviors or methods
 What can a car do? And what can you do to a car?
 Move it
 Change it’s x and y positions
 Change it’s 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
10
Creating a new car
 To create a new Car, we call:
 Car car = 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;
}
11
Constructors
No return type!
For now, all
constructors
are public
EXACT same
name as class
public Car() {
fuel = 1000;
color = Color.BLUE;
}
12
Our Car class so far
public class Car {
private Color color;
private int xpos;
private int ypos;
private int fuel;
public class Car {
private Color color =
Color.BLUE;
private int xpos;
private int ypos;
private int fuel = 1000;
public Car() {
fuel = 1000;
color = Color.BLUE;
}
}
public Car() {
}
}
13
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;
Car
- color = Color.BLUE
- fuel = 1000
- xpos = 0
- ypos = 0
+ Car()
+…
public Car() {
}
}
 Called the default constructor
 The default constructor has no parameters
 If you don’t include one, Java will SOMETIMES put one
14
there automatically
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
15
Our Car class so far
Car
public class Car {
private Color color =
Color.BLUE;
private int xpos = 0;
private int ypos = 0;
private int fuel = 1000;
- color = Color.BLUE
- fuel = 1000
- xpos = 0
- ypos = 0
+ Car()
+ Car (Color, int, int, int)
+…
public Car() {
}
public Car (Color c, int x, int y, int f) {
color = c;
xpos = x;
ypos = y;
fuel = f;
}
}
16
DeCSS: The program
#include<stdlib.h> typedef unsigned int uint; char
ctb[512]="33733b2663236b763e7e362b6e2e667bd393db0643034b96de9ed60b4e0e4\
69b57175f82c787cf125a1a528fca8ac21fd999d10049094190d898d001480840913d7d35246\
d2d65743c7c34256c2c6475dd9dd5044d0d4594dc9cd4054c0c449559195180c989c11058185\
081c888c011d797df0247074f92da9ad20f4a0a429f53135b86c383cb165e1e568bce8ec61bb\
3f3bba6e3a3ebf6befeb6abeeaee6fb37773f2267276f723a7a322f6a2a627fb9f9b1a0e9a9e\
1f0b8f8b0a1e8a8e0f15d1d5584cd8dc5145c1c5485cc8cc415bdfdb5a4edade5f4bcfcb4a5e\
cace4f539793120692961703878302168286071b7f7bfa2e7a7eff2bafab2afeaaae2ff"; typedef
unsigned char uchar;uint tb0[11]={5,0,1,2,3,4,0,1,2,3,4};uchar* F=NULL; uint
lf0,lf1,out;void ReadKey(uchar* key){int i;char hst[3]; hst[2]=0;if(F==\
NULL){F=malloc(256);for(i=0;i<256;i++){hst[0]=ctb[2*i];hst[1]=ctb[2*i+1];F[i]=\
strtol(hst,NULL,16);}}out=0;lf0=(key[1]<<9)|key[0]|0x100;lf1=(key[4]<<16)|(key\
[3]<<8)|key[2];lf1=((lf1&0xfffff8)<<1)|(lf1&0x7)|0x8;}uchar Cipher(int sw1,\ int sw2){int
i,a,b,x=0,y=0;for(i=0;i<8;i++){a=((lf0>>2)^(lf0>>16))&1;b=((lf1\
>>12)^(lf1>>20)^(lf1>>21)^(lf1>>24))&1;lf0=(lf0<<1)|a;lf1=(lf1<<1)|b;x=(x>>1)\
|(a<<7);y=(y>>1)|(b<<7);}x^=sw1;y^=sw2;return out=(out>>8)+x+y;} void \
CSSdescramble(uchar *sec,uchar *key){uint i;uchar *end=sec+0x800;uchar KEY[5];
for(i=0;i<5;i++)KEY[i]=key[i]^sec[0x54+i];ReadKey(KEY);sec+=0x80;while(sec!=\
end)*sec++=F[*sec]^Cipher(255,0);}void CSStitlekey1(uchar *key,uchar *im) {uchar
k[5];int i; ReadKey(im);for(i=0;i<5;i++)k[i]=Cipher(0,0);for(i=9;i>=0;\ i-)key[tb0[i+1]]=k[tb0[i+1]]^F[key[tb0[i+1]]]^key[tb0[i]];}void CSStitlekey2\ (uchar
*key,uchar *im){uchar k[5];int i;ReadKey(im);for(i=0;i<5;i++)k[i]=\
Cipher(0,255);for(i=9;i>=0;i--)key[tb0[i+1]]=k[tb0[i+1]]^F[key[tb0[i+1]]]^key\
[tb0[i]];}void CSSdecrypttitlekey(uchar *tkey,uchar *dkey){int i;uchar im1[6]; uchar
im2[6]={0x51,0x67,0x67,0xc5,0xe0,0x00};for(i=0;i<6;i++)im1[i]=dkey[i];
CSStitlekey1(im1,im2);CSStitlekey2(tkey,im1);}
17
DeCSS: The shirt (and tie!)
18
DeCSS: The poem
How to decrypt a
DVD: in haiku form.
(Thanks, Prof. D. S. T.)
------------------------
Table Zero is:
Five, zero, one, two, three, four,
oh, one, two, three, four.
(I abandon my
exclusive rights to make or
perform copies of
Table One is long:
two to the eighth power bytes.
Ready? Here they are:
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
said "All is number"
long before he saw
computers and their effects,
or what they could do
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
19
DeCSS: The number

The world’s first illegal prime number:
4856507896573978293098418946942861377074420873513579240196520736686985134010472
3744696879743992611751097377770102744752804905883138403754970998790965395522701
1712157025974666993240226834596619606034851742497735846851885567457025712547499
9648219418465571008411908625971694797079915200486670997592359606132072597379799
3618860631691447358830024533697278181391479795551339994939488289984691783610018
2597890103160196183503434489568705384520853804584241565482488933380474758711283
3959896852232544608408971119771276941207958624405471613210050064598201769617718
0947811362200272344827224932325954723468800292777649790614812984042834572014634
8968547169082354737835661972186224969431622716663939055430241564732924855248991
2257394665486271404821171381243882177176029841255244647445055834628144883356319
0272531959043928387376407391689125792405501562088978716337599910788708490815909
7548019285768451988596305323823490558092032999603234471140776019847163531161713
0785760848622363702835701049612595681846785965333100770179916146744725492728334
8691600064758591746278121269007351830924153010630289329566584366200080047677896
7984382090797619859493646309380586336721469695975027968771205724996666980561453
3820741203159337703099491527469183565937621022200681267982734457609380203044791
2277498091795593838712100058876668925844870047077255249706044465212713040432118
2610103591186476662963858495087448497373476861420880529443
20
DeCSS: The images
21
DeCSS: The recordings

All this info from
http://www-2.cs.cmu.edu/~dst/DeCSS/Gallery/

Or do a Google search for “decss gallery”
22
DeCSS: The movie
23
Using our Car class
 Now we can use both our constructors:
Car c1 = new Car();
Car c2 = new Car (Color.BLACK, 1, 2, 500);
c1
c2
Car
- color = Color.BLUE
- fuel = 1000
Car
- xpos = 0
- ypos = 0
+ Car()
+ Car (Color, int, int, int)
+…
- color = Color.BLACK - xpos = 1
- fuel = 500
- ypos = 2
+ Car()
+ Car (Color, int, int, int)
+…
24
So what does private mean?
 Consider the following code
Note that it’s a
different class!
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
25
 If fuel were public, the above code would work
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;
}
public int getYPos() {
return ypos;
}
public int getXPos() {
return xpos;
}
 As these methods are within the Car class, they can read the
private instance variables
 As the methods are public, anybody can call them
27
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;
}
public void setXPos (int x) {
xpos = x;
}
public void setYPos (int y) {
ypos = y;
}
 As these methods are within the Car class, they can read the
private instance variables
 As the methods are public, anybody can call them
28
Why use all this?
 These methods are called a
get/set pair
 Used with private variables
 We’ll see why one uses these
later in this slide set
 Our Car so far:
Car
- color = Color.BLUE
- fuel = 1000
- xpos = 0
- ypos = 0
+ Car()
+ Car (Color, int, int, int)
+ void setXPos (int x)
+ void setYPos (int y)
+ void setPos (int x, int y)
+ void setColor (Color c)
+ void setFuel (int f)
+ int getFuel()
+ int getXPos()
+ int getYPos()
+ Color getColor()
+…
29
A solution to commenting your
code

The commentator:
http://www.cenqua.com/commentator/
30
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 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 Car (Color c,
int x, int y, int f) {
setColor (c);
setXPos (x);
setYPos (y);
setFuel (f);
}
}
31
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
 We’ll see why later
32
So what’s 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 Car’s fuel
 A method to paint itself on the screen
 Let’s do the first:
public void setPos (int x, int y) {
setXPos (x);
setYPos (y);
}
 Notice that it calls the mutator methods
33
Using the Car’s 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
public void setXPos (int x) {
xpos = x;
}
public void setXPos (int x) {
fuel -= Math.abs
(getXPos()-x);
xpos = x;
}
public void setYPos (int y) {
ypos = y;
}
public void setYPos (int y) {
fuel -= Math.abs
(getYPos()-y);
ypos = y;
34
}
Using the Car’s fuel
public void setPos (int x,
int y) {
setXPos(x);
setYPos(y);
}
public void setPos (int x,
int y) {
setXPos(x);
setYPos(y);
}
 Notice that to access the instance variables, the accessor
methods are used
 Math.abs() gets the absolute value of the passed parameter
35
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 car’s height and width...
36
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!
37
Drawing the Car
private
private
private
private
private
final
final
final
final
final
int
int
int
int
int
CAR_WIDTH = 100;
CAR_HEIGHT = 200;
TIRE_WIDTH = 20;
TIRE_HEIGHT = 40;
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/2+TIRE_WIDTH),
getYPos()-(CAR_HEIGHT/2-TIRE_OFFSET), TIRE_WIDTH, TIRE_HEIGHT);
g.fillRect (getXPos()-(CAR_WIDTH/2+TIRE_WIDTH),
getYPos()+(CAR_HEIGHT/2-TIRE_OFFSET-TIRE_HEIGHT), Don’t worry about
TIRE_WIDTH, TIRE_HEIGHT);
this – just know that
g.fillRect (getXPos()+(CAR_WIDTH/2),
it draws four tires
getYPos()+(CAR_HEIGHT/2-TIRE_OFFSET-TIRE_HEIGHT),
TIRE_WIDTH, TIRE_HEIGHT);
g.fillRect (getXPos()+(CAR_WIDTH/2),
getYPos()-(CAR_HEIGHT/2-TIRE_OFFSET), TIRE_WIDTH, TIRE_HEIGHT);
}
38
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!”
 We’ll color the car red
 We’ll insert the following code at the beginning of the paint()
method:
if ( fuel < 0 ) {
color = Color.RED;
}
39
Drawing the Car
private
private
private
private
private
final
final
final
final
final
int
int
int
int
int
CAR_WIDTH = 100;
CAR_HEIGHT = 200;
TIRE_WIDTH = 20;
TIRE_HEIGHT = 40;
TIRE_OFFSET = 20;
public void paint (Graphics g) {
if ( fuel < 0 ) {
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/2+TIRE_WIDTH),
getYPos()-(CAR_HEIGHT/2-TIRE_OFFSET), TIRE_WIDTH, TIRE_HEIGHT);
g.fillRect (getXPos()-(CAR_WIDTH/2+TIRE_WIDTH),
getYPos()+(CAR_HEIGHT/2-TIRE_OFFSET-TIRE_HEIGHT),
TIRE_WIDTH, TIRE_HEIGHT);
g.fillRect (getXPos()+(CAR_WIDTH/2),
getYPos()+(CAR_HEIGHT/2-TIRE_OFFSET-TIRE_HEIGHT),
TIRE_WIDTH, TIRE_HEIGHT);
g.fillRect (getXPos()+(CAR_WIDTH/2),
getYPos()-(CAR_HEIGHT/2-TIRE_OFFSET), TIRE_WIDTH, TIRE_HEIGHT);
}
40
Our car in action

CarGUI.java
41
Miscellaneous Stuff
43
What I’m 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 it’s a good programming practice, and
follow it
 We’ll see why later
 Why instance variables are supposed to be private
 Just know that it’s a good programming practice, and
follow it
44
 Again, we’ll see why soon
Terminology
 An attribute of a class can be called:
 Instance or class variable (we’ll 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 and class variable when I am 45
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!
46
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 let’s try running it…
47
Program Demo

Car.java
48
Variable scoping

A variable is visible within the block it is declared in
 Called the “scope” of the variable
public class Scoping {
int z
This instance variable is visible
anywhere in the Scoping class
public static void foo (int x) {
// ...
}
This parameter is visible
only in the foo() method
public static void bar () {
// ...
}
public static void main (String[] args) {
int y;
This local variable is visible until
// ...
the end of the main() method
}
}
49
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
50
Motivational posters…
51
Rational class
52
What we’ve 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
expressed as a fraction
 Both the numerator and denominator must
integers!
 Discussed in section 4.8 of the textbook
53
be
be
What properties should our Rational class
have?
 The numerator (top part of the fraction)
 The denominator (bottom part of the fraction)
 Not much else…
54
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
55
Our first take at our Rational class
 Our first take
public class Rational {
private int numerator;
private int denominator;
//...
}
Rational
- numerator = 0 - denominator = 0
+…
 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!
56
Our next take at our Rational class
 Our next take
public class Rational {
private int numerator = 0;
private int denominator = 1;
//...
}
 We’ve defined the attributes
of our class
Rational
- numerator = 0 - denominator = 1
+…
 Next up: the behaviors
57
The default constructor
 Ready?
Rational
- numerator = 0 - denominator = 1
public Rational() {
}
+ Rational()
+…
 Yawn!
 Note that we could have initialized the instance variables here
instead
 The default constructor is called that because, if you don’t
specify ANY constructors, then Java includes one by default
58
 Default constructors do not take parameters
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);
Rational
setDenominator (denom);
- numerator = 0 - denominator = 1
}
 Note that the specific constructor
calls the mutator methods instead
of setting the instance variables
directly
 We’ll see why later
+ Rational()
+ Rational (int num, int denom)
+…
59
Accessor methods
 Our two accessor methods:
public int getNumerator () {
return numerator;
}
public int getDenominator () {
return denominator;
}
Rational
- numerator = 0 - denominator = 1
+ Rational()
+ Rational (int num, int denom)
+ int getNumerator()
+ int getDemonimator()
+…
60
Mutator methods
 Our two mutator methods:
public void setNumerator (int towhat) {
numerator = towhat;
}
public void setDenominator (int towhat) {
denominator = towhat;
}
61
Rational addition
 How to do Rational addition:
a c ad  bc
 
b d
bd
 Our add() method:
public Rational add (Rational other) {
}
62
The this keyword
Rational
Returns:
- numerator = 5 - denominator = 6
+ Rational ()
+ Rational (int n, int d)
+ Rational add (Rational other)
+…
this
Rational
Rational
- numerator = 1 - denominator = 2
- numerator = 1 - denominator = 3
+ Rational ()
+ Rational (int n, int d)
+ Rational
Rational add
add(Rational
(other other) )
+…
+ Rational ()
+ Rational (int n, int d)
+ Rational add (Rational other)
+…
63
The this keyword
 this is a reference to whatever object we are currently in
 Will not work in static methods
 We’ll see why later
 Note that the main() method is a static method
 While we’re at it, when defining a class, note that NONE of
the methods so far were static
64
Rational addition
 How to do Rational addition:
a c ad  bc
 
b d
bd
 Our add() method:
public Rational add (Rational other) {
int a = this.getNumerator();
int b = this.getDenominator();
int c = other.getNumerator();
int d = other.getDenominator();
return new Rational (a*d+b*c, b*d);
}
65
Rational addition
 The following method is equivalent:
 Our add() method:
public Rational add (Rational other) {
int a = getNumerator();
int b = getDenominator();
int c = other.getNumerator();
int d = other.getDenominator();
return new Rational (a*d+b*c, b*d);
}
66
Rational addition
 The following method is equivalent, but not preferred:
 Our add() method:
public Rational add (Rational other) {
int a = numerator;
int b = denominator;
int c = other.numerator;
int d = other.nenominator;
return new Rational (a*d+b*c, b*d);
}
67
Rational addition
 The following method is equivalent, but not preferred:
 Our add() method:
public Rational add (Rational other) {
int a = this.numerator;
int b = this.denominator;
int c = other.numerator;
int d = other.nenominator;
return new Rational (a*d+b*c, b*d);
}
68
Today’s demotivators
69
Rational subtraction
 How to do Rational subtraction:
a c ad  bc
 
b d
bd
 Our subtract() method:
public Rational subtract (Rational other) {
int a = this.getNumerator();
int b = this.getDenominator();
int c = other.getNumerator();
int d = other.getDenominator();
return new Rational (a*d-b*c, b*d);
}
70
Rational multiplication
 How to do Rational multiplication:
a c ac
* 
b d bd
 Our multiply() method:
public Rational multiply (Rational other) {
int a = this.getNumerator();
int b = this.getDenominator();
int c = other.getNumerator();
int d = other.getDenominator();
return new Rational (a*c, b*d);
}
71
Rational division
 How to do Rational division:
a c ad
 
b d bc
 Our divide() method:
public Rational divide (Rational other) {
int a = this.getNumerator();
int b = this.getDenominator();
int c = other.getNumerator();
int d = other.getDenominator();
return new Rational (a*d, b*c);
}
72
Printing it to the screen
 If we try printing a Rational object to the screen:
Rational r = new Rational (1,2);
System.out.println (r);
 We get the following:
Rational@82ba41
 Ideally, we’d like something more informative printed to the
screen
 The question is: how does Java know how to print a custom
73
class to the screen?
The toString() method
 When an object is put into a print statement:
Rational r = new Rational (1,2);
System.out.println (r);
 Java will try to call the toString() method to covert the object
to a String
 If the toString() method is not found, a default one is
included
 Hence the Rational@82ba41 from the previous slide
 So let’s include our own toString() method
74
The toString() method
 Our toString() method is defined as follows:
public String toString () {
return getNumerator() + "/" + getDenominator();
}
 Note that the prototype must ALWAYS be defined as shown
 The prototype is the ‘public String toString()’
75
Printing it to the screen
 Now, when we try printing a Rational object to the screen:
Rational r = new Rational (1,2);
System.out.println (r);
 We get the following:
1/2
 Which is what we wanted!
 Note that the following two lines are (mostly) equivalent:
System.out.println (r);
System.out.println (r.toString());
76
Our full Rational class
Rational
- numerator = 0 - denominator = 1
+ Rational()
+ Rational (int num, int denom)
+ int getNumerator()
+ int getDemonimator()
+ void setNumerator (int num)
+ void setDenominator (int denom)
+ Rational add (Rational other)
+ Rational subtract (Rational other)
+ Rational multiply (Rational other)
+ Rational divide (Rational other)
+ String toString()
77
Our Rational class in use, part 1 of 4


This code is in a main() method of a RationalDemo class
First, we extract the values for our first Rational object:
Scanner stdin = new Scanner(System.in);
System.out.println();
// extract values for rationals r and s
Rational r = new Rational();
System.out.print("Enter numerator of a rational number: ");
int a = stdin.nextInt();
System.out.print("Enter denominator of a rational number: ");
int b = stdin.nextInt();
r.setNumerator(a);
r.setDenominator(b);
78
Our Rational class in use, part 2 of 4
 Next, we extract the values for our second Rational object:
Rational s = new Rational();
System.out.print("Enter numerator of a rational number: ");
int c = stdin.nextInt();
System.out.print("Enter denominator of a rational number: “);
int d = stdin.nextInt();
s.setNumerator(c);
s.setDenominator(d);
 Notice that I didn’t create another Scanner object!
 Doing so would be bad
 I used the same one
79
Our Rational class in use, part 3 of 4
 Next, we do the arithmetic:
// operate on r and s
Rational sum = r.add(s);
Rational difference = r.subtract(s);
Rational product = r.multiply(s);
Rational quotient = r.divide(s);
80
Our Rational class in use, part 4 of 4
 Lastly, we print the results
// display operation results
System.out.println("For r = " + r.toString() + " and s = "
+ s.toString());
System.out.println("
r + s = " + sum.toString());
System.out.println("
r - s = " + difference.toString());
System.out.println("
r * s = " + product.toString());
System.out.println("
r / s = " + quotient.toString());
System.out.println();
81
A demo of our Rational class

RationalDemo.java
82
Other things we might want to add to our
Rational class
 The ability to reduce the fraction
 So that 2/4 becomes 1/2
 Not as easy as it sounds!
 More complicated arithmetic
 Such as exponents, etc.
 Invert
 Switches the numerator and denominator
 Negate
 Changes the rational number into its (additive) negation
 We won’t see any of that here
83
These images are not animated…
84
More on methods
86
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
87
Return values
 Many methods return a value
 Math.cos()
 String.valueOf()
 Consider: double d = Math.cos (90 * Math.PI/180.0);
 Let’s 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
88
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
if (
else
else
}
String
x == 1
if ( x
return
foo (int x) {
) return “one”;
== 2 ) return “two”;
“other”;
89
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?
90
Foo w = new Foo();
String x = “rules”;
String y = w.bar (x);
System.out.println (y);
x
s
t
w
this
“rules"
“CS 101 rules"
y
Foo
+ Foo()
+ bar (String s): String
+…
public String bar (String s) {
String t = “CS 101” + “ ” + s;
return t;
}
91
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;
}
92
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 x+y, then returns that value to the
caller
93
A bit of humor…
94
The Circle class
Introducing static-ness, visibilities,
etc.
95
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
96
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
97
Our Circle class
Circle c = new Circle();
c
Note the radius
field is not
initialized by us
public class Circle {
double radius;
double PI = 3.1415926536;
}
We’re ignoring the
public for now
Note the fields
are not static
Circle
- radius = 0.0
- PI = 3.14159…
-…
+…
98
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
c
 Example: System.out.println
(c.radius);
 Example: c.PI = 4;
This is bad – PI should
have been declared final
(this will be done later)
Circle
- radius = 0.0
- PI = 3.14159…
-…
+…
99
What’s 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);
Java will give a
}
“variable not
}
initialized” error

When a variable is declared as part of a method, Java does not
initialize it to a default value
100
What’s 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);
Java
}
}

outputs 0.0!
When a variable is declared as part of a class, Java does initialize
it to a default value
101
What’s 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…
102
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
103
Another optical illusion
104
Calling the Circle constructor
 To create a Circle object:
c1
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 no
parameters (the ‘default’
constructor)
Circle
- radius = 0.0
- PI = 3.14159…
-…
+ Circle()
+ Circle (double r)
+…
 The constructor is always the first method called when
creating (or ‘constructing’) an object
105
Calling the Circle constructor
 To create a Circle object:
c1
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 1
double parameters (the ‘specific’
constructor)
Circle
- radius = 0.0
- PI = 3.14159…
-…
+ Circle()
+ Circle (double r)
+…
 The constructor is always the first method called when
creating (or ‘constructing’) an object
106
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!
107
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
Circle
c
- radius = 1.0
- PI = 3.1415926536
-…
+ Circle()
+ Circle (double r)
+…
Shorthand representation
c
Circle
- radius = 1.0
- PI = 3.14159
108
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();
}
}
109
What happens in memory
 There are 4 Circle objects in memory
 Taking up a total of 4*16 = 64 bytes of memory
c1
Circle
- radius = 1.0
- PI = 3.14159
c2
Circle
- radius = 1.0
- PI = 3.14159
c3
Circle
- radius = 1.0
- PI = 3.14159
c4
Circle
- radius = 1.0
- PI = 3.14159
110
Consider the following code
public class CircleTest {
public static void main (String[] args) {
Circle c1 = new Circle();
//...
Circle c1000000 = new Circle();
}
}
This program creates 1 million Circle objects!
111
What happens in memory
 There are 1 million Circle objects in memory
 Taking up a total of 1,000,000*16 ≈ 16 Mb of memory
c1
c2
Circle
- radius = 1.0
- PI = 3.14159
Circle
- radius = 1.0
- PI = 3.14159
c1000000
…
Circle
- radius = 1.0
- PI = 3.14159
Note that the final PI field is repeated 1 million times
112
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
16
(1+1=2 doubles)
Total memory
Total usage:
memory
8 Mb
usage:
+ 8 40
bytes
bytes
(1,000,000+1=1,000,001
(4+1=5
doubles)
c1
c2
c3
Circle
Circle
- radius = 1.0
- radius = 1.0
PI
…
c1000000
c4
Circle
Circle
- radius = 1.0
- radius = 1.0
3.1415926536
113
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);
Note you can refer
to static fields by
object.variable
 It prints 4.3
114
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
115
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);
116
Even even even more on static fields
 We’ve 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)
117
Back to our Circle class
public class Circle {
double radius;
final static double PI = 3.1415926536;
public Circle() {
radius = 1.0;
}
Note that PI is now final and static
public Circle (double r) {
radius = r;
}
}

But it doesn’t do much!
118
Adding a method
public class Circle {
double radius;
final static double PI = 3.1415926536;
// Constructors...
double computeArea () {
return PI*radius*radius;
}
}
Note that a (non-static) method can
use both instance and class variables
120
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
121
What happens when that method is called
public class Circle {
double radius;
final static double PI = 3.1415926536;
public Circle() {
radius = 1.0;
}
c
area 12.566
// other constructor
double computeArea () {
return PI*radius*radius;
}
}
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);
}
}
Circle
- radius = 2.0
0.0
1.0
- PI = 3.14159…
-…
+ Circle()
+ Circle (double r)
+ computeArea()
+…
122
Review of our Circle class
public class Circle {
double radius;
final static double PI = 3.1415926536;
public Circle() {
}
Slight change from before
public Circle (double r) {
radius = r;
}
double computeArea () {
return PI*radius*radius;
}
}
123
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++
124
Adding another method
double oneOverRadius() {
return 1.0/radius;
}

I couldn’t think of a good reason to divide something by the radius…
125
What happens now?
 Code in class CircleTest’s main() method
Circle c = new Circle(); // c.radius is now 0.0
System.out.println (c.oneOverRadius());

Java won’t crash, but many other programming languages (C and
C++, in particular) will
 So we’ll call this a ‘crash’ for the sake of this lecture

Java prints “Infinity”
 Not what we wanted, though!
126
One way to fix this…
public class Circle {
double radius = 1.0;
final static double PI = 3.1415926536;
Note that the radius
variable is now
initialized to 1.0
// Constructors...
double computeArea () {
return PI*radius*radius;
}
double oneOverRadius() {
return 1.0/radius;
}
}
127
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());
128
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());
}
}
Here is the badly written code
public class Circle {
double radius = 1.0;
final static double PI =
3.1415926536;
double computeArea () {
return PI*radius*radius;
}
double oneOverRadius() {
return 1.0/radius;
}
Here is where the “crash” occurs
129
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: Don’t allow other code to modify the radius field
 Give it private visibility

private means that only code within the class can modify the
field
130
One way to fix this…
public class Circle {
private double radius = 1.0;
final static double PI = 3.1415926536;
Note that the radius
variable is now private
// Constructors...
double computeArea () {
return PI*radius*radius;
}
double oneOverRadius() {
return 1.0/radius;
}
}
131
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
132
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
133
What we wish computers could do
134
The problem now…

But now you can’t have a Circle with a radius other than 1.0!

Solution: Use a get/set methods in Circle:

A mutator method:
void setRadius (double r) {
radius = r;
}

An accessor method:
double getRadius () {
return radius;
}
135
Our Circle class so far
public class Circle {
private double radius = 1.0;
final static double PI = 3.1415926536;
// Constructors...
double computeArea () {
return PI*radius*radius;
}
double oneOverRadius() {
return 1.0/radius;
}
void setRadius (double r) {
radius = r;
}
double getRadius () {
return radius;
}
}
136
Using the get/set methods
public class CircleTest {
public static void main
(String[] args) {
public class Circle {
private double radius = 1.0;
final static double PI =
3.1415926536;
double computeArea () {
return PI*radius*radius;
}
Circle c = new Circle();
c.setRadius (1.0);
double oneOverRadius() {
return 1.0/radius;
}
System.out.println
(c.computeArea());
void setRadius (double r) {
radius = r;
}
System.out.println
(c.getRadius());
}
double getRadius () {
return radius;
}
}
Here a method is invoked
}
137
Here the change to radius occurs
Wait! Another problem!
public class CircleTest {
public static void main (String[] args) {
Circle c = new Circle();
c.setRadius (0.0);
Here is the problem now…
System.out.println (c.oneOverRadius());
}
}
138
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
139
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 won’t get to that this semester

default: Almost the same as public
 This is the default (duh!)
 Note that it can’t be specified like the others
 Also called ‘package’
140
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 it’s
defined) and private outside the method
 You can also specify visibilities for methods and classes
 We won’t get to that in this course
141
Overriding methods (and constructors)

Consider the following code:
Circle c1 = new Circle ();
Circle c2 = new Circle (2.0);
Creates a Circle
of radius 1.0
Creates a Circle
of radius 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

We’ve seen overloading before:
 3+4
Performs integer addition
 3.0+4.0
Performs floating-point addition
 “3”+”4”
Performs string concatenation

The ‘+’ operator is overloaded
142
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;
}
When Circle(1.0)
is called, which
one is meant?
public Circle (double p) {
PI = p;
}
143
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)
144
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 won’t set it to
zero)
145
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:
 That’s a lot of code to
copy if you decide not
to call the mutator!
void setRadius (double r) {
if ( radius <= 0.0 )
radius = 1.0;
else
radius = r;
diameter = 2*radius;
if ( radius < 1.0 )
size = “small”;
else if ( radius < 5.0 )
size = “medium”;
else if ( radius < 10.0 )
size = “large”;
else
size = “huge”;
}
146
Today’s demotivators
147
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 doesn’t read or modify the “state” of any object
 In this example, it doesn’t read/write the radius
 In fact, that particular method doesn’t care anything about
the objects declared
 It’s only accessing a static field
148
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 don’t care about the state of any Math object
 They only perform the computation
149
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
150
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());
}
151
What happening in memory
 There are no
4 million
1
Circle
Circleobjects
Circle
objects
objects
ininmemory
memory
in memory
 Which radius field does Circle.getRadius() want?
c1
c2
c3
Circle
Circle
- radius = 1.0
- radius = 1.0
PI
…
c1000000
c4
Circle
Circle
- radius = 1.0
- radius = 1.0
3.1415926536
152
The main static lesson
 A static method cannot access or modify the state of the
object it is a part of
 If you remember
remember this!
nothing
else
about
static
methods,
153
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
154
Back to our main() method
public static void main (String[] args) We’ll learn about
arrays in chapter 8
The method does not return a value
Any code anywhere
can call this method
It’s a static method:
•Can’t access non-static fields or methods directly
•Can be called only by the class name
155
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 didn’t have to prefix method1() with a object
 Java assumes that it is in the same class
156
Today’s demotivators
157