Advanced Placement Computer Science Java

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Advanced Placement Computer Science Java

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Title: Advanced Placement Computer Science Java

1
Advanced PlacementComputer Science(Java)

Mr. Simon
Lowell High School
San Francisco, CA

2
Equivalent to 2 semesters of college science

Course Requirements
One semester of programming in C or Java with a
B or better
A web based email account (e.g. HotMail or Yahoo
mail) that you can access from the school lab
A web site (e.g. freewebs.com) where you will
publish your finished programs

3
Computer science vs. Computer programming

A computer programming course might focus on a
programming language, it's syntax and features
Computer science is more abstract and includes
studying
Algorithms
Data Structures

4
What youll learn

How to program in Java Applets and Applications
Object Oriented Programming including inheritance
How to write (some) HTML
How to maintain a website
Grid World Case Study
Searching and sorting
Recursion
Abstract classes (AB only)
Invariants (AB only)
Big O notation (AB only)
Linked Lists, Stacks, Queues, Trees, Heaps,
Hashes, Maps, Sets (AB only)
Good Programming Style
A good foundation for studying computer science
in college

5
Two exams A vs. AB

AB students will need to do more assignments
more difficult test questions
2008 results 85 pass rate

A exam Grade 5 8 Grade 4 5 Grade 3 1 Grade
2 3 Grade 1 - 1
AB exam Grade 5 6 Grade 4 1 Grade 3 1 Grade
2 0 Grade 1 - 0
6
Which exam should I take?A or AB?

Most students should take the A exam
Consider taking the AB exam if
You have already taken Computer Programming 2
You are consistently scoring in the top 25 on
class tests and quizzes
For most people, a 5 on the A exam looks a lot
better than a 3 on the AB

7
Grading Policy

Each six week grade
70 tests and quizzes
20 programming assignments
10 productive use of lab time
The Semester grade
Average of the 3 six week
grades and the final exam

8
Programming Assignments

Use free Windows software JCreator
Available online or on CD
Its critical to your success that you spend
adequate time programming and fully understand
the assignments
Programming can be very time consumingclass time
alone may not be enough
Send the code and url of your finished program to
room334_at_gmail.com

9
Style

Building a working program is not enough
Other programmers need to read, understand and
modify your program
Every class, book and job may use a slightly
different style
Good style will significantly improve your grade

10
10 point Program Grading Scale

1 pt Name, class, assignment in comments at top
2 pts URL/Sample Output
2 pts Correct Style
5 pts Program Correctness
Always put your name, class and assignment on the
subject line of the email message

11
Web and email

Send programs to room334_at_gmail.com
Send questions to mrsimon_at_earthling.net
Check mrsimon.net for assignments, grades,
software and other information

12
Logging On

User Name The last digit of graduation year
(Class of 20055) plus first initial and last
name, 9 characters maximum, in the user name
field
Password last 4 numbers of student id
Log on to Choose "Lowell" from pull down menu

13
Examples

Example 1
Student nameLinda Smith
Class of 2004
ID 123456789
User Name4lsmith
Password 6789
Example 2
Student nameLinda Shakespeare
Class of 2007
ID 111110012
User Name 7lshakesp
Password 12

14
Logging On

If Lisa Wong and Linda Wong are in the same
class, they would have the same user name (e.g.
5lwong)
To differentiate those students, a number was
added to the end (e.g. 5lwong2)
There is a list of those students online at
http//inside.lowell.edu

15
The first time you log on

You will be prompted to change your password
Choose a password that you can remember that is
less than 8 characters in length

16
Saving your work

In "My Computer" you should see two network
drives
Save your work to the network drive with your
username
If you check Tools Folder Options Offline
files Synchronize All offline files before
logging off your work will still be available
even if the school network "goes down"

17
Log Off!

Please remember to logoff each time you finish
with the computer, otherwise other people will be
able to access your home folder!

18
You can now access your work

on any classroom computer in any classroom
the library
the Mac lab
room 215
JROTC resource center

19
Assignment 0

Send an email to room334_at_gmail.com with the
following information
Your name, reg, class and computer
Your parents/guardians name, home phone and email
Your parents work/daytime phone
Secret Code (4 digits/characters, examples AOK1,
1776, KT85)
If time permits, sign up for a free website. I
recommend freewebs.com
Log Off when youre done

20
Getting your own website

There are several free providers of websites
I recommend freewebs.com, but there are many
others and you are free to choose

21
Getting your own website

Other free providers of websites
http//pages.google.com
http//www.brinkster.com
http//geocities.yahoo.com/
http//www.netfirms.com
http//www.50megs.com
http//www.angelfire.lycos.com/
http//www.100megs.com
Know of any others?

22
Just Enough HTML

HTML can be written in a text editor like
NotePad, WordPad or JCreator
HTML files must be named with an .htm or .html
extension
HTML commands are called tags
A tag is surrounded by e.g.,
HTML is NOT case-sensitive so, is the same
as

23
Just Enough HTML...

Some HTML tags are single and others are paired
The marks the end of what was begun by
the
HTML is whitespace insensitive

24
Just Enough HTML...

- begins/end HTML
- begins/ends head
- begins/ends title
- begins/ends body
- begins/ends centered text
- begins/ends a headline
- horizontal rule
- end a paragraph
- break the line
- begins/ends an anchor (link)

25
Just Enough HTML...

Heres a proper webpage
This is a tiny sample
View this page using a browser
Second lineSome more text
And, no P at the end of the previous line see
what happens!

26
Just Enough HTML...

Using Internet Explorer

27
An Introduction to Applets

There are two kinds of Java programs
applications and applets
All the programs we wrote last semester were
applications they have a main
Applets run within a browser environment (usually
on the Internet)

28
An Introduction to Applets

There are two kinds of Java programs
applications and applets
All the programs we wrote last semester were
applications they have a main
This semester we will write Applets
Applets run within a web page
For each assignment, we will create two files
the java program and the web page (.html)

29
Applets

When you visit a website and see this Java logo,
a Java Applet is being loaded into your web
browser
An applet can do almost anything that an
application can
But they look differentthere is no main

30
Writing a Hello World applet in JCreator

The first time you start JCreator, you will see a
screen like the one below
Click "Finish" to accept the file associations

31
Configuring JCreator at school to use your
network Z drive

Don't try this at home kids!
Choose Configure Options
Click on "Directories"
Under "Default Project Directory" enter Z\Java\

32
Writing a Hello World applet in JCreator

Open JCreator
Choose File Close Workspace to close any open
files
Choose File New Project
Click on Basic Java Applet and click Next
Give your class a NameIll call mine Assignment1
Click Finish
JCreator automatically generates both the Java
and html code

33
Writing a Hello World applet in JCreator

The Java code Assignment1.java looks like this

The html code Assignment1.htm looks like this

35
Java is very picky about names

Do NOT use spaces in class names.
It's difficult to change the name of a class.
Make sure you choose the name you want when you
make a new project. If you make a mistake with
the name, it's best just to start over.
You can change the name of the .html file
JCreator will give it the same name as the class
(Assignment1.htm in this case) but you can change
it to anything you want

36
Running the Applet

Choose Build Compile Project
You should see Process Completed
Choose Build Execute Project
You should see

37
Uploading the applet

When you build your applet, it creates a .class
file
To put your applet on the internet, you need to
upload
The .class file
The .html file
To find these files, go to your Z drive and
look in your Java folder. Find the Assignment1
folder, and inside of that the classes folder
You will then upload both Assignment1.class and
Assignment1.htm to your website
You will then put a link on your homepage to
Assignment1.htm

38
Uploading the applet

To upload to freewebs.com, log in, click on My
Site and then Single File Uploader
Then browse to your .class and .html files one at
a time

39
Building your homepage and linking to your
assignments

Click on Site Manager and then click on the Edit
icon next to home

40
Building your homepage and linking to your
assignments

Move your cursor to the content box where you
want the link to go, and click Link

41
Building your homepage and linking to your
assignments

Choose My Files, click on your html file, type
some text and click Insert

42
Building your homepage and linking to your
assignments

You should now see the link on your homepage

43
Applets

We you play a game at Yahoo games, or check your
balance in your bank account online, you are
probably running an applet
An applet can do almost anything that an
application can
But they look differentthere is no main

44
Applets

Heres about the all-time simplest applet
import java.awt.
import java.applet.
public class SimpleApplet extends Applet
public void paint(Graphics g)
g.drawString("Hello world", 50, 50)
g.drawRect(30, 30, 100, 30)

45
Applets

It looks like this

46
Applets

An applet needs a web page to run
The html tags that place your applet in a web
page look like this
code "Assignment1.class"
width "500"
height "300"

47
Advantages of Java

Java is portable, the same Java program can run
on Windows, Macintosh and Linux computers
This is because a Java program runs inside
another program called the JVM (Java Virtual
Machine)
You must install Java first on a computer before
you can run a Java program

48
Applications vs. Applets

There are two kinds of programs you can create in
Java
Applications
Applets
Applications are stand-alone programs that are
installed on one computer
Applets are programs that run inside a browser
and downloaded over the internet

49
Guidelines for Applet Creation

Here are some guidelines to follow to help you
create applets
Well discuss the details of what they all mean
later on
Every applet must have a comment at the top
telling who wrote it, etc
/ Mr Simon, Java Mods 10-11
Assignment 1 /

50
Guidelines / Applet Creation

Every applet must have two import statements
import java.awt.
import java.applet.
Every applet is stored in a file whose name is
XYZ.java
Every applet has
public class XYZ extends Applet

51
Guidelines / Applet Creation

(Most) every applet contains a paint method
public void paint(Graphics g)
The body of the paint method does the drawing
that is required

52
Guidelines / Applet Creation

The size of the applet is set in the HTML code in
the applet tag
HEIGHT"300"
IMPORTANT the name of the class has to match
both the name of the .java file and the name used
in the applet tag.

53
Guidelines / Applet Creation

Good Style
The name of a class should always start with a
capital letter. (e.g. MyApplet)
Method (function) names should start with a
lowercase letter. If the name uses more that one
word, capitalize each additional word. (e.g.
drawArc)
Variable names should start with a lower case
letter numPeople, dDouble, cChar, sString

54
Arithmetic in Java

Java uses the following arithmetic operators
- / -- -
The operators can be combined with numbers to
make expressions.
g.drawString(5 3,50,60)
//displays 8
Parenthesis can be used to create more
complicated expressions
g.drawString(5 (2 3),50,60)

55
Literals vs. Expressions

If Java sees an expression it will evaluate it.
How could you display 5 3 to the screen, rather
than just 8?
With a literal . By placing the expression in
double quotes, Java will display it without
evaluating it.
g.drawString("5 3",50,60)

56
Integer arithmetic

Java does division differently with integers than
with decimals
g.drawString(5.0/4,50,60)
//displays 1.25
g.drawString(5/4,50,60)
//displays 1

57
Modulus and integer division

Remember how you did math in grade school?
g.drawString(8/5,50,60)
//displays 1
g.drawString(85,50,60)
//displays 3

58
Adding a number to a string

int nNum 5
g.drawString("Total " nNum, 20, 20)
//displays Total 5
The operator can be used to add a number to a
stringmuch easier than C!
The order makes a difference!
g.drawString("X" 2 3, 20, 20)
//displays X23
g.drawString("X" (2 3), 20, 20)
//displays X5

59
Primitive Data Types

Primitive Data Types can be thought of as a
"mailbox"
There are different size mailboxes (boolean,
char, int, double, byte, short, long, float,
etc.) for different types of data
Good Style Use only boolean, int, char and
double unless you have a good reason not to.

60
Wrapper classes

For each primitive data type (such as int or
double) there is a corresponding class (such as
Integer or Double)
These classes (commonly known as wrapper classes)
provide methods and constants for dealing with
primitive data type values.
Primitive data types do not have methods.
Therefore, methods related to a primitive data
type are located in the corresponding wrapper
class.
Some of the information provided by the wrapper
classes are constants to indicate the largest and
smallest values for a given data type. For
example, the largest integer is 2147483647.

61
Wrapper class constants

Byte.MAX_VALUE // the largest value of
type byte
Byte.MIN_VALUE // the smallest value of
type byte
Short.MAX_VALUE // the largest value of
type short
Short.MIN_VALUE // the smallest value of
type short
Character.MAX_VALUE // the largest value of
type char
Character.MIN_VALUE // the smallest value of
type char
Integer.MAX_VALUE // the largest value of
type int
Integer.MIN_VALUE // the smallest value of
type int
Long.MAX_VALUE // the largest value of
type long
Long.MIN_VALUE // the smallest value of
type long
Float.MAX_VALUE // the largest positive
value of type float
Float.MIN_VALUE // the smallest positive
value of type float
Double.MAX_VALUE // the largest positive
value of type double
Double.MIN_VALUE // the smallest positive
value of type double

62
Variable Declarations

To create a mailbox, you declare it.
int nNum
Once its declared you can store a number
nNum 5
This is called initializing
You can also do both at once
int nValue 17

63
Variable Declarations Initializations

int nStudents 2535
double dNaturalLogBase 2.718
char cLetter 'a'
Good Style Use a prefix to show the data type,
n for int, f for float, d for double, l for long
and c for char
This is called Hungarian Notation

64
Cast

Most of Java's syntax comes from C, but there
are some important differences
Java is stongly typed
This means it's very strict about data types
int nNum 5.6
In C this would store 5 in nNum, but in Java
it's an error. To fix it you need a cast
int nNum (int)5.6

65
ASCII code

Ultimately, everything in a computer is stored as
a (binary) number
The system of numbers that represent letters of
the alphabet is called ASCII American Standard
Code for Information Interchange
For example, the letter 'A' has the ASCII value
of 65, which is stored as the binary value
0000000001000001
There's a good website with all the ASCII codes
at http//www.asciichart.com/
If you cast a char as an int, it will display the
ASCII value
g.drawString((int)'A',50,60)
//displays 65

66
Graphics in Java

Unlike C, Java comes with built in graphics
Putting the following statement at the top of
your program allows you to use the graphics
methods
import java.awt.
There are various methods that allow us to draw
shapes including lines, rectangles, ovals and
polygons both unfilled and filled

67
An applet that draws a Red Square

import java.awt.
import java.applet.
public class RedSquare extends Applet
public void paint(Graphics g)
g.setColor(Color.red)
g.fillRect(20,20,100,100)

68
An applet that draws a Red Square
69
Some Graphics methods

void drawArc(int x, int y, int width,
int height,int startAngle, int arcAngle)
void drawLine(int x1, int y1, int x2, int y2)
void drawOval(int x, int y, int width,
int height)
void drawRect(int x, int y, int width,
int height)
void drawString(String s, int x, int y)
void fillArc(int x, int y, int width,
int height,int startAngle, int arcAngle)
void fillOval(int x, int y, int width,
int height)
void fillRect(int x, int y, int width,
int height)
Color getColor()
void setColor(Color color)

70
The Graphics Class

Drawing is done within the space specified by a
bounding rectangle

71
The Color Class

There are a collection of 13 predefined colors
Color.black, Color.blue, Color.cyan,
Color.darkGray, Color.gray, Color.green,
Color.lightGray, Color.magenta, Color.orange,
Color.pink, Color.red, Color.white, Color.yellow
And, you can create a new color using Color(red,
green, blue), where RGB are in the range 0-255
g.setColor(new Color(125,35,89))

72
The Color Class

Set the background color using
void setBackground(Color color)
Set the drawing color using
void setColor(Color color)
For example
setBackground(Color.yellow)
g.setColor(Color.white)
g.drawLine(50, 50, 100, 200)
g.setColor(Color.black)
g.drawRect(70, 80, 100, 25)

73
drawArc

For Assignment 2, you must use drawArc (you may
use other methods as well)
drawArc has the following prototype
void drawArc(int x, int y, int width,
int height, int startAngle,
int arcAngle)
x, y, width, and height set the bounding
rectangle

74
drawArc

startAngle and arcAngle set the starting point
and length of the arc
g.drawArc(20, 20,200,100,90,180)

90
180
0
270
75
drawArc

Of course, all you see on the screen is the arc
g.drawArc(20, 20,200,100,90,180)

76
System.out.print() System.out.println()

Useful for debugging
like cout
System.out.println("Hello") is the same as
cout
System.out.print("Hello") is the same as
cout

77
loops

C and Java both have the same three types of
loops
Here are three examples that all count from 1 to
10
for(int nI 1 nI
System.out.println(nI)
int nJ 1
while(nJ
System.out.println(nJ)
nJ
int nK 1
do
System.out.println(nK)
nK
while(nK

78
Math.random()

Inside of the Math class are many variables and
methods
One method random, returns a random decimal
between 0 and 1
0 n

79
Math.random()

Math.random()
0 n

80
Math.random()

Math.random()2
(0 n

81
Math.random()

Math.random()2
(0 n
A random decimal from 0 to 1.9999999

82
Math.random()

(int)(Math.random()2)
??

83
Math.random()

(int)(Math.random()2)
0, 1

84
Math.random()

(int)(Math.random()10)
??

85
Math.random()

(int)(Math.random()10)
0,1,2,3,4,5,6,7,8,9

86
Math.random()

(int)(Math.random()10) - 5
??

87
Math.random()

(int)(Math.random()10)- 5
0,1,2,3,4,5,6,7,8,9 - 5

88
Math.random()

(int)(Math.random()10)- 5
0,1,2,3,4,5,6,7,8,9 5
-5,-4,-3,-2,-1,0,1,2,3,4

89
Watch out for this mistake

(int)Math.random()101
is always zero!

90
Watch out for this mistake

(int)Math.random()101
If Math.random() is cast as an int before being
multiplied, the result is always zero
The fix
(int)(Math.random()101)

91
Practice quiz questions Write an expression

The following line of code was meant to display
the sum of 4 and 9, but it didn't work as
expected. Fix the problem.
g.drawString("4 9 " 4 9,50,60)
Write an expression that will make a random
decimal between 0 and 5 (including 0 but not 5)
Write an expression that will make a random
integer between 0 and 5 (including 0 but not 5)
Write an expression that will make a random
integer between -2 and 3 (including -2 but not 3)
What is the output of the following line
g.drawString(("One" 2) (3 4),20,20)

92
boolean

A primitive data type (Mailbox) that can hold
only two values true or false
boolean bBool true
boolean bGreater 5 3
boolean b3Multiple (5 3) 0
if(b3Multiple)
g.drawString("Is a multiple of 3",50,50)
else
g.drawString("Is NOT a multiple of 3",50,50)

93
Objects and class

You wouldn't call this thing "Two wheels, a
handlebar, a seat, some pedals and a frame"
Instead, you would use one name that describes
the entire object
A class is a collection of primitive data types
and methods that describe the data and behavior
of an object
class Bicycle
int nWheels
double dWeight
double dSpeed
void ride()
dSpeed 10
The different parts are accessed using the dot
operator

94
A simple class

A class represents an complex object
There are methods (functions) that represent what
the object does
There are variables (mailboxes) that represent
the object has
Methods that modify the variables are called
Modifiers or Mutators
A special method that initializes the variables
is called the constructor
Methods that retrieve the value of a variable are
called accessors
Note that in Java there are two types of things
Primitives and Objects

95
A coin flip class

Let's create a class that represents a coin flip
What it has is two sides heads and tails
What it is does is flip

96
A coin flip class

Let's create a class that represents a coin flip
What it has is two sides heads and tails
What it is does is flip
class CoinFlip
boolean bIsHeads
void flip()
//java code

97
A coin flip class

Let's create a class that represents a coin flip
What it has is two sides heads and tails
What it is does is flip
class CoinFlip
boolean bIsHeads
void flip()
if(Math.random()
bIsHeads true
else
bIsHeads false

98
Using the coin flip class

To use the coin flip class, we need to create a
new coin flip object
CoinFlip theCoin new CoinFlip()

99
Using the coin flip class

To use the coin flip class, we need to create a
new coin flip object
CoinFlip theCoin new CoinFlip()
This is called creating a new instance of
CoinFlip or instantiating a new object
Now we can flip the coin and see what it is
theCoin.flip()
if(theCoin.bIsHeads)
g.drawString(Heads!,50,50)
else
g.drawString(Tails!,50,50)

100
The Constructor

CoinFlip theCoin new CoinFlip()
What happens if we forget to flip the coin?
//theCoin.flip() this line removed
if(theCoin.bIsHeads)
g.drawString(Heads!,50,50)
else
g.drawString(Tails!,50,50)
What value is in bIsHeads?

101
The Constructor

CoinFlip theCoin new CoinFlip()
The constructor's job is to automatically set
up a new instance of CoinFlip
It has the same name as the class, and no return
type
class CoinFlip
boolean bIsHeads
CoinFlip()
flip()
void flip()
if(Math.random()
bIsHeads true
else
bIsHeads false

102
The Finished Program

public class SimpleClassDemo extends Applet
public void paint (Graphics g)
CoinFlip theCoin new CoinFlip()
theCoin.flip()
if(theCoin.bIsHeads)
g.drawString("Heads!",50,60)
else
g.drawString("Tails!",50,60)
class CoinFlip
boolean bIsHeads
CoinFlip()
flip()
void flip()
if(Math.random()

103
Instantiating new Objects

In Java, you don't get a new Object until you ask
for one using the Java keyword new
The fancy name for this is instantiating a new
Object.
CoinFlip theCoin new CoinFlip()

104
Getting the value inside an object using an
accessor method

One way to get the value out of an object is to
just use the dot operator
theCoin.bIsHeads
While this is the easiest way, unfortunately,
it's not the most common
Usually, Java programmers use an accessor method

105
an accessor method

class CoinFlip
boolean bIsHeads
CoinFlip()
flip()
void flip()
if(Math.random()
bIsHeads true
else
bIsHeads false
boolean getFlip()
return bIsHeads

106
an accessor method

if(theCoin.bIsHeads)
g.drawString("Heads!",50,50)
else
g.drawString("Tails!",50,50)

107
an accessor method

if(theCoin.bIsHeads)
g.drawString("Heads!",50,50)
else
g.drawString("Tails!",50,50)

108
an accessor method

if(theCoin.getFlip())
g.drawString("Heads!",50,50)
else
g.drawString("Tails!",50,50)

109
Practice Quiz Question

A. Find two lines of code that instantiate new
objects
B. Find two accessor methods
C. Find the output
public class SimpleClassQuiz extends Applet
public void paint(Graphics g)
Thingy Bob new Thingy()
g.drawString("Bob " Bob.getValue(),50,60
)
WhatsIt Evelyn new WhatsIt()
g.drawString("Evelyn "
Evelyn.getValue(),50,80)
class Thingy
int nNum
Thingy()
nNum 0
for(int nI0 nI
if(nI3 0)
nNum nI

class WhatsIt double dNum WhatsIt() dNum
1 for(int nI1 nI 0) dNum nI double
getValue() return dNum
110
The paint() and init() methods

The paint() method is called automatically when a
screen update is needed
The paint() method may be called many times. If
you have something that should only run once, put
it in init()
init() is called once, before paint(). It's
generally used to initialize values.

111
An applet with init() and paint() methods

import java.awt.
import java.applet.
public class TwoMethodDemo extends Applet
int nNum
public void init()
nNum 3
setBackground(Color.blue)
public void paint(Graphics g)
g.setColor(Color.red)
g.fillRect(10,10,200,200)
g.setColor(Color.white)
g.drawString("nNum is " nNum,105,105)

112
An applet with init() and paint() methods
113
Math.sqrt()

Inside of the Math class there is a square root
method
double sqrt(double a)
It takes a double as an argument and returns a
double
g.drawString("" Math.sqrt(100),50,50)
//displays 10.0

114
Boolean methods

Methods that return booleans are called boolean
methods or predicates
Good Style Boolean methods are usually given
names that start with is
isEmpty()
isEqual()
isOpen()
isUpperCase()

115
Practice Quiz Question

class Practice extends Applet
public void paint(Graphics g)
Mystery bob new Mystery()
g.drawString("Bob" bob.getValue(),50,50
)
class Mystery
double dValue
double getValue()
return dValue
Mystery()
dValue 0
for(int nI 1 nI
if(isSomething(nI))
dValue nI

116
A "Typical" class

A class may have any number of methods and any
number of variables
class Thingy
int nSomeInt
String sSomeString
Thingy()
int someMethod (double dNum)
//lots of java
void anotherMethod (double dNum)
//lots of java

117
Class member methods

A class may have any number of methods and any
number of variables
class Thingy
//lots of java
int someMethod (double dNum)
//lots of java
The return type specifies what type of data is
returned
The arguments specifies what types of data are
passed to the method

118
Class member methods

A class may have any number of methods and any
number of variables
class Thingy
//lots of java
int someMethod (double dNum)
//lots of java
Thingy()
//lots of java
Note that the constructor has no return type, not
even void

119
Class member methods

A constructor can use arguments just like any
other function
class Thingy
//lots of java
int someMethod (double dNum)
//lots of java
Thingy(int nArgument)
//lots of java
so now if we instatiate a Thingy with code like
Thingy bob new Thingy(5) 5 is copied into
nArgument

120
Modeling objects

Let's say I want to make an animation of a
balloon that gets bigger as it is inflated
I'll start by asking what a balloon has and what
it does
And then I'll write a class that models the
object
class Balloon
???

121
Modeling objects

Let's say I want to make an animation of a
balloon that gets bigger as it is inflated
I'll start by asking what a balloon has and what
it does
And then I'll write a class that models the
object
class Balloon
int nSize
Color myColor

122
The constructor's job is to initialize the member
variables

It puts stuff in the mailboxes when a new object
is created
class Balloon
int nSize
Color myColor
Balloon()
nSize 0
myColor Color.red

123
What do balloon's do?

class Balloon
int nSize
Color myColor
Balloon()
nSize 0
myColor Color.red
???

124
What do balloon's do?

class Balloon
int nSize
Color myColor
Balloon()
nSize 0
myColor Color.red
void inflate()
nSize

125
What do balloon's do?

class Balloon
int nSize
Color myColor
Balloon()
nSize 0
myColor Color.red
void inflate()
nSize
void draw(Graphics g)
g.setColor(myColor)
g.fillOval(0,0,nSize,nSize)

126
Animation and threads

Once I've written my ball class, I can "plug it
into" the outline for a program with a thread
//declare other class data members
Balloon theBalloon
public void init()
//initialize class data members
theBalloon new Balloon()
public void paint(Graphics g)
theBalloon.draw(g)
//painting

127
Animation and threads

Once I've written my ball class, I can "plug it
into" the outline for a program with a thread
public void run()
while(true)
//put code that runs in loop here
theBalloon.grow()
repaint()
try
Thread.sleep(REFRESH_RATE)
catch(Exception exc)

128
Arrays

An array is a collection of items of the same
data type that is accessed by index
String saAnswers "yes","no","maybe"
g.drawString("" saAnswers1, 50,50)
int naNums 17,3,-5,42
g.drawString("" naNums2,50,70)
g.drawString("" saAnswers.length,50,90)
//note that length for arrays is NOT a function
g.drawString("" naNums.length,50,110)
/Sample Output
no
-5
3
4 /

129
Arrays

Arrays and loops are commonly used together
String saAnswers "yes","no","maybe"
for(int nI 0 nI
System.out.println(saAnswersnI)

130
Arrays

When an array is an argument of a function, the
size of the array is not specified.
int someMethod(int naNums)
//lots of java
for(int nI 0 nI
//do something with naNumsnI
//return some int

131
Arrays

Arrays in Java work almost identically to C,
but there are some differences in style
String saAnswers "yes","no","maybe"
//Java Style
String saAnswers "yes","no","maybe"
//C Style
Practice Quiz Question www.javabat.com
Array1Basic Array ProblemscountEvens()

132
Practice Quiz Question

http//javabat.com/prob?idArray2.sum13
Return the sum of the numbers in the array,
returning 0 for an empty array. Except the number
"13" is very unlucky, so it does not count and
numbers that come immediately after a 13 also do
not count.
Hint Don't worry about 13 at first, just find
the sum of of the array
Then, just exclude 13
Finally, figure out how to exclude the number
after 13

133
Practice Quiz Question

http//javabat.com/prob?idArray2.sum13
public int sum13(int nums)

134
Arrays of Objects

In Java, Arrays are objects. So if you make an
array of objects, you need to use new twice once
to make a new array, and then again to put new
objects in the array.
Particle aBunch new Particle10
for(int nI 0 nI
aBunchnI new Particle()

135
Arrays of Objects