Arrays and loops

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Arrays and loops

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when you are looking for a particular element (e.g. biggest or smallest) ... findDog({'mouse', 'lizard','snake'}) false. findDog({'beetle','hamster'}) false ... – PowerPoint PPT presentation

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Title: Arrays and loops

1
Arrays and loops

Often, you need a loop to visit all the elements
in the array
when you are looking for a particular element
(e.g. biggest or smallest)
you want to find a total of all the elements
you want to count the elements

2
Problem write a method will return true if one
of the pets is a dog

findDog("fish", "cat","rat","dog") ? true
findDog("mouse", "lizard","snake") ? false
findDog("beetle","hamster") ? false
findDog() ? false (empty array)

3
Problem write a method will return true if one
of the pets is a dog

boolean findDog(String pets)
??

4
Problem write a method will return true if one
of the pets is a dog

boolean findDog(String pets)
??
return true
??
return false

5
Problem write a method will return true if one
of the pets is a dog

boolean findDog(String pets)
if( ?? )
return true
??
return false

6
Problem write a method will return true if one
of the pets is a dog

boolean findDog(String pets)
if( pets ?? "dog" )
return true
??
return false

7
Problem write a method will return true if one
of the pets is a dog

boolean findDog(String pets)
if(pets??.equals("dog"))
return true
??
return false

8
Problem write a method will return true if one
of the pets is a dog

boolean findDog(String pets)
for( ?? ?? ?? )
if(pets??.equals("dog"))
return true
??
return false

9
Problem write a method will return true if one
of the pets is a dog

boolean findDog(String pets)
for( int nI 0 ?? ?? )
if(petsnI.equals("dog"))
return true
??
return false

10
Problem write a method will return true if one
of the pets is a dog

boolean findDog(String pets)
for( int nI 0 nI ltpets.lengthnI)
if(petsnI.equals("dog"))
return true
??
return false

11
else doesn't work here!!

boolean findDog(String pets)
for( int nI 0 nI ltpets.lengthnI)
if(petsnI.equals("dog"))
return true
else ??
return false

12
else doesn't work here!!

boolean findDog(String pets)
for( int nI 0 nI ltpets.lengthnI)
if(petsnI.equals("dog"))
return true
else ??
return false

13
now it works !!

boolean findDog(String pets)
for( int nI 0 nI ltpets.lengthnI)
if(petsnI.equals("dog"))
return true
return false

14
Problem write a method will count the pets that
are dogs OR cats

countDogsCats("fish", "cat","rat","dog") ? 2
countDogsCats("mouse", "lizard","snake") ? 0
countDogsCats("beetle","hamster") ? 0
countDogsCats() ? 0 (empty array)

15
Problem write a method will count the pets that
are dogs OR cats

int countDogsCats(String pets)
??

16
Problem write a method will count the pets that
are dogs OR cats

int countDogsCats(String pets)
int nCount 0
??
return nCount

17
Problem write a method will count the pets that
are dogs OR cats

int countDogsCats(String pets)
int nCount 0
for(int nI0nIltpets.lengthnI)
if(petsnI.equals("dog")
petsnI.equals("cat") )
nCount
return nCount

18
Problem write a method will return true if one
of the FIRST 3 pets is a dog

findDogIn3("fish", "cat","rat","dog") ? false
findDogIn3("mouse", "lizard","snake") ? false
findDogIn3("beetle","hamster") ? false
findDogIn3() ? false (empty array)

19
Problem write a method will return true if one
of the FIRST 3 pets is a dog

boolean findDogIn3(String pets)
??

20
Problem write a method will return true if one
of the FIRST 3 pets is a dog

boolean findDogIn3(String pets)
for( int nI 0 nI lt??nI)
if(petsnI.equals("dog"))
return true
return false

21
Problem write a method will return true if one
of the FIRST 3 pets is a dog

boolean findDogIn3(String pets)
int nEnd
??
for( int nI 0 nI ltnEndnI)
if(petsnI.equals("dog"))
return true
return false

22
Problem write a method will return true if one
of the FIRST 3 pets is a dog

boolean findDogIn3(String pets)
int nEnd
if(pets.length gt 3)
nEnd 3
else
nEnd pets.length
for( int nI 0 nI ltnEndnI)
if(petsnI.equals("dog"))
return true
return false

23
Problem write a method will return true if one
of the FIRST 3 pets is a dog

boolean findDogIn3(String pets)
for( int nI 0 nIlt3 nIltpets.lengthnI)
if(petsnI.equals("dog"))
return true
return false

24
Javabat Warmup2 gt arrayFront9

http//www.javabat.com/prob?idWarmup2.arrayFront9
Given an array of ints, return true if one of the
first 4 elements in the array is a 9. The array
length may be less than 4.
arrayFront9(1, 2, 9, 3, 4) ? true
arrayFront9(1, 2, 3, 4, 9) ? false
arrayFront9(1, 2, 3, 4, 5) ? false

25
Javabat Warmup2 gt arrayCount9

http//www.javabat.com/prob?idWarmup2.arrayCount9
Given an array of ints, return the number of 9's
in the array.
arrayCount9(1, 2, 9) ? 1
arrayCount9(1, 9, 9) ? 2
arrayCount9(1, 9, 9, 3, 9) ? 3

26
Javabat Array2 gt has22

http//www.javabat.com/prob?idArray2.has22
Given an array of ints, return true if the array
contains a 2 next to a 2 somewhere.
has22(1, 2, 2) ? true
has22(1, 2, 1, 2) ? false
has22(2, 1, 2) ? false

27
Interfaces

public class Example extends Applet implements
ActionListener
When you use implements you are using
(implementing) an interface
So what's an interface?

28
Interfaces

An interface is like a class, but simpler
An interface is just a list of methods
When you implement an interface, you must write
the methods that are listed in that interface

29
Interfaces

public class Example extends Applet implements
ActionListener
To implement an ActionListener we must write the
following method
public void actionPerformed(ActionEvent e)
We're not limited to that one method, we can
write any others we want to

30
Interfaces

An interface says All classes that implement
this interface will be able to do these things
You can think of interfaces this way
I (Mr. Simon) implement the Teacher interface
You implement the Student interface

31
Interfaces

Inside of the Teacher interface there should be
teach and grade methodsso if you want to be a
teacher you have to be able to teach and grade
Inside of the Student interface there should be
study, listen and learn methodsso if you want to
be a student you have to be able to study, listen
and learn

32
Problem Create a SuperHero interface

interface SuperHero
public void fight()
public void specialPower()

33
Problem Create a SuperHero interface

In designing a SuperHero interface, we are
setting the standards for what all SuperHeroes
must do
They are not limited to the methods in the
interface--there can be additional methods as well

34
Problem Create a SuperHero interface

class Spiderman implements SuperHero
public void fight()
System.out.println
("I will spin a web to trap you!")
public void specialPower()
System.out.println
("I can swing through the city streets.")
public void spiderSense()
System.out.println
("Hmmm, something's not right")

35
Problem Create a SuperHero interface

class Superman implements SuperHero
public void fight()
System.out.println
("My great strength will defeat you!")
public void specialPower()
System.out.println("I have Xray vision.")
public void kryptonite()
System.out.println("Oh no!")

36
Problem Create a SuperHero interface

public class InterfaceDemo extends Applet
Superman ClarkKent new Superman()
Spiderman PeterParker new Spiderman()
public void paint(Graphics g)
ClarkKent.fight()
PeterParker.fight()
ClarkKent.specialPower()
PeterParker.specialPower()
ClarkKent.kryptonite()
PeterParker.spiderSense()
/ Sample Output
My great strength will defeat you!
I will spin a web to trap you!
I have Xray vision.
I can swing through the city streets.

37
All SuperHeros can be treated the same and
grouped together

public class InterfaceDemo extends Applet
Superhero theGoodGuys new Superhero2
public void paint(Graphics g)
theGoodGuys0 new Superman()
theGoodGuys1 new Spiderman()
for(int nI 0 nI lt 2 nI)
theGoodGuysnI.fight()
theGoodGuysnI.specialPower()
/ Sample Output
My great strength will defeat you!
I have Xray vision.
I will spin a web to trap you!
I can swing through the city streets.

38
The keyListener interface

Allows your applet to respond to key events
interface keyListener
public void keyTyped(KeyEvent ke)
public void keyPressed(KeyEvent ke)
public void keyReleased(KeyEvent ke)
Note You don't have to write this, it's included
in your program with import.java.awt.event.

39
Keyboard Events

Every time you press (or release) a key it
generates an Event
We can have our Applet listen for key events by
making it a KeyListener
The KeyListener interface requires three methods
keyTyped, keyPressed and keyReleased

public class KeyDemo extends Applet
implements
KeyListener
char cChar
public void init()
addKeyListener(this)
public void paint(Graphics g)
g.drawString("You pressed " cChar, 20, 20)
public void keyTyped(KeyEvent ke)
cChar ke.getKeyChar()
repaint()
public void keyPressed (KeyEvent ke) //not
used
public void keyReleased (KeyEvent ke) //not
used

41
requestFocus()

Our applet is one of many programs running
simultaneously
When you type, how does Windows know where the
typing should go? Should it go in the URL field
of the browser, or name a folder or where?
One way to tell Windows is to click on the thing
before you start typing
The other is requestFocus()

42
A simple Applet that listens for key events

import java.awt.
import java.awt.event.
import java.applet.
public class KeyDemo
extends Applet implements KeyListener
char cKey
public void init()
addKeyListener(this)
public void paint(Graphics g)
g.drawString("The " cKey
" key was pressed", 50, 60 )
public void keyTyped(KeyEvent ke)
cKey ke.getKeyChar()
System.out.println(cKey)
repaint()

43
Javabat Array2 gt has77

http//www.javabat.com/prob?idArray2.has77
Given an array of ints, return true if the array
contains two 7's next to each other, or there are
two 7's separated by one element, such as with
7, 1, 7.
has77(1, 7, 7) ? true
has77(1, 7, 1, 7) ? true
has77(1, 7, 1, 1, 7) ? false

44
handles, aliases and Binky

Pointer Fun is a 3 minute video from the computer
science dept. at Stanford
In Java, we don't use the word pointer, we use
handle instead
An alias is where two handles refer to the same
mailbox

//The code from the Binky video
class IntObj
int value
IntObj x // Allocate the pointers x and y
IntObj y // (but not the IntObj pointees)
x new IntObj() // Allocate an IntObj pointee
// and set x to point to it
x.value 42 // Dereference x to store 42
in its
//pointee
y.value 13// CRASH -- y does not have a
pointee yet
y x // Pointer assignment sets y to point to
x's

IntObj x
IntObj y

x
y
47

x new IntObj()

x
y
48

x.value 42

42
x
y
49

y.value 13

42
x
y
50

y.value 13
//Null Pointer exception

42
x
y
CRASH!
51

42
x
y
52

y.value 13

13
x
y
53

//What would
System.out.println(x.value)
//Display?

13
x
y
54

//Answer 13

13
x
y
55

/Practice Quiz Question write some Java code
that will produce the following structure /

1
x
2
y
56
Why dot equals is generally preferred to
double equals for comparing objects

assigns two handles
to the same object
if( x y)asks
Are x and y referring
to the same object?
if( x.equals(y))asks
Do x and y have the same value?

57
X Y refer to the same object
2
x
y
58
X Y have the same value
2
x
2
y
59
Practice Quiz Question What is the output of
this applet?

import java.awt.
import java.applet.
class SomeType
String sMessage
SomeType(String sText)
sMessage sText
public class AliasDemo extends Applet
SomeType one new SomeType("one")
SomeType two new SomeType("two")
public void paint(Graphics g)
SomeType three two

SomeType one new SomeType("one")
SomeType two new SomeType("two")

one
one
two
two
61

SomeType three two
three.sMessage new String("three)
System.out.println(one.sMessage
", " two.sMessage)

one
one
three
two
three
62

three one
three.sMessage new String("four)
System.out.println(one.sMessage
", " two.sMessage)

four
one
three
two
three
63
final and static

In C, you made constants like this
const double dCM_IN_INCH 2.54
In Java, they look like this
final static double dCM_IN_INCH 2.54
final works like const, it "locks" the "mailbox"
static means there is only one "mailbox" for the
entire class

public class StaticDemo extends Applet
Student student1 new Student("Bob")
Student student2 new Student("Hannah")
public void paint(Graphics g)
System.out.println(student1.sName)
System.out.println(student2.sName)
student1.sSchool "Lincoln"
System.out.println(student1.sSchool)
System.out.println(student2.sSchool)

class Student String sName static String
sSchool "Lowell" Student(String
sWho) sName sWho
/ Sample Output Bob Hannah Lincoln Lincoln Exit
code 0 No Errors /
65
final and static
Bob
student1
Lowell
Hannah
student2
System.out.println(student1.sName) System.out.pri
ntln(student2.sName)
66
final and static
Bob
student1
Lowell
Hannah
student2
student1.sSchool "Lincoln"
67
final and static
Bob
student1
Lincoln
Hannah
student2
student1.sSchool "Lincoln"
68
final and static
Bob
student1
Lincoln
Hannah
student2
System.out.println(student1.sSchool) System.out.p
rintln(student2.sSchool)
69
Three references to the same "mailbox"
Bob
student1
Lincoln
Hannah
student2
student1.sSchool student2.sSchool Student.sSchool

70
Overriding update()

Normally, every time we repaint(), the screen is
completely erased first
"Behind the scenes", repaint() calls the method
update() which erases the screen and then calls
our paint() method
For the etch a sketch assignment, we don't want
the screen erased, so we will write a new version
of update()
This is called "overriding update()"

71
Overriding update()

public void update(Graphics g)
paint(g)

72
Symmetrical reflections (mirrors)
(50,30)
480
640
73
Symmetrical reflections (mirrors)
(50,30)
50
480
640
74
Symmetrical reflections (mirrors)
(50,30)
50
50
480
640
75
Symmetrical reflections (mirrors)
(50,30)
(640-50,30)
50
50
480
640
76
panels

Sometimes you want part of your applet to use one
layout and the rest of the applet to use a
different layout
For instance you might want a grid of buttons for
part of the applet, but with a few extra buttons
that aren't part of the grid

77
panels

You can think of a panel as an "Applet within an
Applet"
Panel buttonPanel new Panel()
add(buttonPanel)
buttonPanel.setLayout(new GridLayout
(nNUM_ROWS, nNUM_COLUMNS, 0, 0))
Once you have the panel, you can add buttons to
it pretty much like you would an applet
buttonPanel.add(buttonArraynNum)

78
panels

Panel aPanel
public void init()
aPanel new Panel()
add(aPanel)
aPanel.setLayout(new BorderLayout())
aPanel.add(new Button("North"),BorderLayout.NOR
TH)
aPanel.add(new Button("South"),BorderLayout.SOU
TH)
aPanel.add(new Button("East"),BorderLayout.EAST
)
aPanel.add(new Button("West"),BorderLayout.WEST
)
aPanel.add(new Button("Center"),BorderLayout.CE
NTER)

79
Message Boxes

You know those annoying Message Boxes that pop up
on some websites saying your computer is infected
with spyware?
Here's how to make them )

80
Message Boxes

import java.awt.
import java.applet.
import javax.swing.
public class MessageBoxDemo extends Applet
public void init()
JOptionPane.showMessageDialog(
null,"Some Annoying Message")

81
Input Dialog Box

import java.awt.
import java.applet.
import javax.swing.
public class MessageBoxDemo extends Applet
String sInput
public void init()
sInputJOptionPane.showInputDialog(
"Type
Something!")
public void paint(Graphics g)
g.drawString("You typed " sInput,20,20)

82
Converting to an int

import java.awt.
import java.applet.
import javax.swing.
public class MessageBoxDemo extends Applet
String sInput
int nNum
public void init()
sInputJOptionPane.showInputDialog("Type an
Integer")
nNum Integer.parseInt(sInput)
public void paint(Graphics g)
g.drawString("You typed " nNum,20,20)

83
boolean

A boolean is a primitive data type that can only
hold one of two values true or false
You can use a boolean anywhere that can use a
true or false
boolean bBool true
boolean bValue (nNum gt 0)
System.out.println(bValue)
if(bValue)
System.out.println("positive")

84
boolean methods

A boolean method returns true or false
boolean isOldEnough(int nAge)
if(nAge gt 18)
return true
else
return false

85
boolean methods

Here is a shorter version that does the same
thing
boolean isOldEnough(int nAge)
return (nAge gt 18)

public class MysteryBoolean extends Applet
FurBall Fred
FurBall Murray
public void init()
Fred new FurBall(Color.red,20)
Murray new FurBall(Color.green,25)
public void paint(Graphics g)
System.out.println(Fred.isBig())
System.out.println(Murray.isBig())
System.out.println(Fred.isBigAndGreen())
System.out.println(Murray.isBigAndGreen())
class FurBall

87
Exceptions

An exception is a problem that Java doesn't know
how to deal with
new Color(256,-1,100)
Exception occurred during event dispatching
java.lang.IllegalArgumentException Color
parameter outside of expected range Red Green

88
More examples of Exceptions

int naArray1,2,3
naArray30
//java.lang.ArrayIndexOutOfBoundsException
Button aButton
aButton.setBounds(3,4,5,6)
//java.lang.NullPointerException
System.out.println(5/0)
//java.lang.ArithmeticException / by zero

89
What happens if you press the wrong button?

Button press new Button("5")
Button dont new Button("DO NOT PRESS THIS
BUTTON!")
int nNum 0
public void init()
add(press)
add(dont)
press.addActionListener(this)
dont.addActionListener(this)
public void paint(Graphics g)
g.drawString("Number is " nNum, 20, 50)
public void actionPerformed(ActionEvent ae)
nNum Integer.parseInt(ae.getActionCommand())
repaint()

90
Exception handling try catch

Exception handling can be used to "bulletproof"
your code
Even if the user does the wrong thing, your
program won't crash
try
//code that may cause problem
catch (Exception e)
//"catch" the problem

91
Exception handling try catch

public void actionPerformed(ActionEvent ae)
try
nNum Integer.parseInt(ae.getActionCommand())
catch(Exception e)
nNum -9999999999999999
System.out.println("Follow directions!")
repaint()

92
We can set up multiple catch blocks to handle
different kinds of exceptions

Button b1 new Button('x')
Button b2
. . . .
public void actionPerformed(ActionEvent ae)
try
nNum Integer.parseInt(ae.getActionCommand())
catch(NumberFormatException e)
System.out.println("Not a number")
catch(NullPointerException e)
System.out.println("Button doesn't exist")
catch(Exception e)
System.out.println("Some Other problem")

93
Practice Quiz Question What is the output of
this code fragment?

int nTotal 0
String sMessages new String4
sMessages0 new String("13")
sMessages2 new String("abc")
sMessages3 new String("7")
for(int nI 0 nI lt 5 nI)
try
if(sMessagesnI.equals("abc"))
System.out.println("Equal")
int nNum Integer.parseInt(sMessagesnI)
nTotal nNum
catch(NullPointerException np)
System.out.println("Caught 1")
catch(NumberFormatException ne)
System.out.println("Caught 2")

94
Practice Quiz what will be displayed if buttons
one, two and three are pressed in that order?

public class TryCatchButton extends Applet
implements ActionListener
Button one, two, three
String sMessage
int nTotal 0
public void init()
one new Button("one")
two new Button("two")
three new Button("three")
add(one)
add(two)
add(three)
one.addActionListener(this)
two.addActionListener(this)
three.addActionListener(this)
public void actionPerformed(ActionEvent ae)
try
if(ae.getActionCommand().equals("one"))
nTotal nTotal/nTotal

95
What is the output?

String sMessages "1", "two", "3"
for(int nI 0 nI lt 4 nI)
try
int nNum Integer.parseInt(sMessagesnI)
System.out.println("The number is " nNum)
catch(NumberFormatException ne)
System.out.println("Caught 1")
catch(ArrayIndexOutOfBoundsException ae)
System.out.println("Caught 2")
catch(NullPointerException np)
System.out.println("Caught 3")
catch(Exception e)
System.out.println("Caught 4")

96
Object Oriented Programming

In OOP we think about our program as a collection
of interacting objects
In Assignment 10 (Battleship) we have a grid, and
hidden inside that grid is a ship
We could design the program with two classes one
for the grid (the applet) and one for the ship
We would start writing the ship class by asking
what a ship HAS, and also what it DOES.

97
Object Oriented Programming

We use methods to represent what the object does
and variables for what it has.
class Ship
//some variables
Ship() //constructor
//other methods

98
Object Oriented Programming

One thing a ship has is a position. If the ship
is four cells long, we could represent the
position as four ints (Button Numbers)
class Ship
int nShip1, nShip2, nShip3, nShip4
//or
int naShip
Ship() //constructor
//other methods

99
Object Oriented Programming

What a ship does is hide. It may also get hit
class Ship
int nShip1, nShip2, nShip3, nShip4
//or
int naShip
Ship() //constructor
void hide()
boolean isHit(int nButton)

100
Object Oriented Programming

Data Hiding variables are "hidden" so that they
can only be accessed using the methods
private means that the variables are only
available to the class member methods
public means that the methods can be used
anywhere in the program
Encapsulation Hiding the details of the code
within the class, so we as programmers can focus
on the "big picture"

101
public and private

We aren't suppose to know where the ship is
hidden, so the data should be private
class ship
private int nShip1, nShip2, nShip3, nShip4
//or
private int naShip
ship() //constructor
void hide()
boolean isHit(int nButton)

102
public and private

There is another benefit to making the data
private
Any other class that uses the ship doesn't need
to know whether the ship is represented with an
array or not, it just needs to use the member
methods
In fact, we could change from 4 separate ints to
an array without changing any code in the applet
class
This is an example of encapsulation
It makes code easier to use, modify and re-use

103
Using isHit()

public void actionPerformed(ActionEvent ae)
//lots of java
int nB Integer.parseInt(ae.getActionCommand()
)
if(theShip.isHit(nB))
buttonArraynB.setBackground(Color.red)
else
buttonArraynB.setBackground(Color.blue)
//lots more java
repaint()

104
public and private

The methods should all probably be public, since
our applet will need to use them
class ship
private int nShip1, nShip2, nShip3, nShip4
//or
private int naShip
public ship() //constructor
public void hide()
public boolean isHit(int nButtonNumber)

105
public and private

Good Style Never declare object data
(variables) public
Good Style Never declare methods that other
classes will need private

106
Practice Quiz what will be displayed the first
time paint() is called?

public class ClassQuiz extends Applet
Mystery one, two
public void init()
one new Mystery(3)
two new Mystery(4)
public void paint(Graphics g)
one.count()
two.count()
g.drawString("1 " one.isSome() ", "
one.nNum ", "
one.dValue,20,20)
g.drawString("2 " two.isSome() ", "
two.nNum ", "
two.dValue,20,50)
class Mystery
int nNum
double dValue

107
Animation and threads

All animation is base on the same Erase Move
Draw Wait sequence. This is sometimes called
the Universal Animation Loop.

108
Animation and threads

All animation is base on the same Erase Move
Draw Wait sequence. This is sometimes called
the Universal Animation Loop.
Draw

109
Animation and threads

All animation is base on the same Erase Move
Draw Wait sequence. This is sometimes called
the Universal Animation Loop.
Erase

110
Animation and threads

All animation is base on the same Erase Move
Draw Wait sequence. This is sometimes called
the Universal Animation Loop.
Draw

111
Animation and threads

All animation is base on the same Erase Move
Draw Wait sequence. This is sometimes called
the Universal Animation Loop.
Erase

112
Animation and threads

All animation is base on the same Erase Move
Draw Wait sequence. This is sometimes called
the Universal Animation Loop.
Draw

113
Animation and threads

All animation is base on the same Erase Move
Draw Wait sequence. This is sometimes called
the Universal Animation Loop.
Erase

114
Animation and threads

All animation is base on the same Erase Move
Draw Wait sequence. This is sometimes called
the Universal Animation Loop.
Draw

115
Animation and threads

All animation is base on the same Erase Move
Draw Wait sequence. This is sometimes called
the Universal Animation Loop.
Erase

116
Animation and threads

All animation is base on the same Erase Move
Draw Wait sequence. This is sometimes called
the Universal Animation Loop.
Draw

117
Animation and threads

All animation is base on the same Erase Move
Draw Wait sequence. This is sometimes called
the Universal Animation Loop.
Erase

118
Animation and threads

All animation is base on the same Erase Move
Draw Wait sequence. This is sometimes called
the Universal Animation Loop.
Draw

119
Animation and threads

In C we used delay() to wait
In Java, your applet is just one of many
programs. A delay would slow them all
We'll avoid this problem by making our applet a
thread
We can do this by implementing the runnable
interface

120
Animation and threads

The runnable interface requires three methods
public void start()
public void run()
public void stop()
There is a "fill in the blank" outline for an
applet that implements runnable in the Pong
assignment
Use it a starting point for your program

121
Animation and threads

Let's say I want to make an animation of a
balloon that gets bigger, that is, it "inflates"
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 nX, nY
int nSize
Balloon()
nX nY nSize 0
void inflate()
nSize
void draw(Graphics g)
g.fillOval(nX,nY,nSize,nSize)

122
Animation and threads

Once I've written my balloon 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

123
Animation and threads

Once I've written my balloon 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.inflate()
repaint()
try
Thread.sleep(REFRESH_RATE)
catch(Exception exc)

124
Normal Animation Flicker

125
Normal Animation Flicker

126
Normal Animation Flicker

127
double buffering

The technique of double buffering eliminates
flickering in animation
It uses two "screens"
One is the normal screen (Graphics g)
One is an "invisible" screen (offscreen)
All drawing is done to offscreen first, and then
copied all at once to the visible screen

128
double buffering

g offscreen

129
double buffering

g offscreen

130
double buffering

g offscreen

131
double buffering

g offscreen

132
double buffering

g offscreen

133
double buffering

g offscreen

134
double buffering

g offscreen

g.drawImage(image,0,0,this)

135
double buffering

To implement double buffering you need to make 6
changes in your program
1. two added declarations
Graphics offscreen
Image image
2. two added initializations (in init())
image createImage(nSCREEN_WIDTH,
nSCREEN_HEIGHT)
offscreen image.getGraphics()

136
double buffering

The next three changes are in paint()
3. erase offscreen
offscreen.setColor(Color.white)
offscreen.fillRect(0,0,nSCREEN_WIDTH,
nSCREEN_HEIGHT)
4. modify code so that all drawing is offscreen
theBall.draw(offscreen)
5. copy offscreen to visible screen
g.drawImage(image,0,0,this)

137
double buffering

The last change is to override update()
6. override update()
public void update(Graphics g)
paint(g)

138
The Technical Details

There are many technical details associated with
flicker and many other techniques including
"triple buffering" and "infinite buffering"
Here's one good website with a description
http//www.azillionmonkeys.com/qed/flicker.html

139
extends and inheritance

extends allows programmers to quickly and easily
build a new class from an existing class
This is called inheritance
The new class inherits all the member variables
and methods of the old class, except for
constructors

140
extends and inheritance

class Person
String sName
Person()
sName "John"
void speak()
System.out.println
("Hello, my name is " sName)

141
extends and inheritance

Now let's extend the Person class to make a Pro
Wrestler
What do Pro Wrestlers have that normal people
don't?

142
extends and inheritance

class Wrestler extends Person
String sGimmick
Wrestler()
sName "The incredible Hulk"
sGimmick
"Can you smell what's cooking?"
void speak()
System.out.println
("Hello, my name is " sName)
System.out.println(sGimmick)

143
extends and inheritance

public class InheritanceExample extends Applet
Person guy
Wrestler hulk
public void init()
guy new Person()
hulk new Wrestler()
public void paint(Graphics g)
guy.speak()
hulk.speak()
/ Sample Output
Hello, my name is John
Hello, my name is The incredible Hulk
Can you smell what's cooking?/

144
extends and inheritance

The Wrestler class (called the sub class)
inherits the sName variable and the speak()
method from the Person class (called the super
class)
The Wrestler class has an additional sGimmick
data member
The the speak() method is overridden ("replaced")
While constructors are not inherited, there is
an "invisible call" to the default (no argument)
constructor of the super class

145
What would happen if we didn't initialize the
wrestler's name?

extends and inheritance
class Wrestler extends Person
String sGimmick
Wrestler()
//sName "The incredible Hulk"
sGimmick "Can you smell what's cooking?"
void speak()
System.out.println
("Hello, my name is " sName)
System.out.println(sGimmick)

146
The wrestler is also named "John"!

public class InheritanceExample extends Applet
Person guy
Wrestler hulk
public void init()
guy new Person()
hulk new Wrestler()
public void paint(Graphics g)
guy.speak()
hulk.speak()
/ Sample Output
Hello, my name is John
Hello, my name is John
Can you smell what's cooking?/

147
extends and inheritance

When Java builds an instance of the Wrestler
class (called the sub class) it starts by
building an instance of the Person class (called
the super class)
Then, whatever is extra or different is added to
the wrestler.
In this case, wrestlers have an extra String
variable for the gimmick
And, the speak method is different ("overridden")

148
What is the output of this code?

class Wolf
int nLegs
int nSize
Wolf()
nLegs 4
nSize 150
int legs()
return nLegs
int size()
return nSize
String speak()

class Dog extends Wolf
String speak()
return "Bark!"
class Collie extends Dog
Collie()
nSize 70

149
What is the output of this code?

Wolf BigBad new Wolf()
Collie Lassie new Collie()
System.out.println("The Big Bad Wolf is "
BigBad.size()
" pounds has "
BigBad.legs()
" legs and says "
BigBad.speak())
System.out.println("Lassie is "
Lassie.size()
" pounds has "
Lassie.legs()
" legs and says "
Lassie.speak())

150
Practice Quiz Question Find the output

SuperClass one new SuperClass()
one.mystery()
System.out.println(one.getNum())
SubClass two new SubClass()
two.mystery()
System.out.println(two.getNum())

class SuperClass int nNum SuperClass()
nNum 1 System.out.println
("building a super" nNum) int
getNum() return nNum void
mystery() nNum
class SubClass extends SuperClass
SubClass() nNum 3 System.out.println
("building a sub" nNum) void mystery()
nNum 6
151
Interfacesa quick review

An interface is like a class, but simpler
An interface is just a list of methods
When you implement an interface, you must write
the methods that are listed in that interface

152
interfacesa quick review

interface Teacher
public void lecture()
class Simon implements Teacher
//lots of java not shown
public void lecture()
System.out.println("blah")

153
The difference between extends and implements

Both inheritance (extends) and interfaces
(implements) allow you to build new classes
quickly, using other classes as a starting point
Inheritance means the new class gets all of the
methods and data members of the parent class
(except constructors)
With an interface, the classes share only those
methods that are listedand no data

154
The difference between extends and implements

Inheritance means the new class is fundamentally
the same as parent class
With an interface, the classes only share a few
methods

155
The difference between extends and implements

An analogy
I (Mr. Simon) extend the Simon class I inherited
all the data (hair, eye color, etc) and methods
(breathe, sleep, eat, etc) of my parents
I implement the Teacher interface I have teach
and grade methods
I also implement the Motorcyclist interface I
have a ride method

156
The difference between extends and implements

Inheritance is more powerful, but you can only
use it once a class cannot extend two different
classes
With an interface, you can implement as many
interfaces as you want however, you can not
make an instance of an inteface
new Simon() //OK
new Motorcyclist() //NO!
new Teacher()//NO!

157
Pong
158
Pong
159
Pong

Brought the first computers into people's homes
Written by Alan Alcorn (a Lowell grad) for Atari
First Pong game was installed in a Sunnyvale bar
in September 1972
Two weeks later the machine stopped
workingbecause all the quarters had jammed it
Original Paddles had eight segmentsdifferent
segments gave different angles
Was not the first computer game

160
Spacewar!
161
Spacewar!

Written by members of the MIT model railroad club
beginning in 1961
They also coined the word "hack"
Code was kept in an unlocked draweranyone was
free to modify it
Programmed on the Digital Equipment PDP-1, one of
the first computers with a monitor
Club members built the world's first joysticks
Later modified by Atari (stolen?) to become
Asteroids

162
Pong Spin-offs

Breakout 1976

163
Pong Spin-offs

Dr. Pong 1974

164
Pong Spin-offs

Pin Pong 1974

165
Pong Spin-offs

Pong Doubles 1973

166
Pong Spin-offs

Puppy Pong 1974

167
Pong Spin-offs

Quadra Pong 1974

168
Pong Spin-offs

Rebound 1974

169
Pong Spin-offs

Bomb Bee 1979

170
Pong Spin-offs

Super Pong 1974

171
Pong Spin-offs

3d pong

172
Pong Spin-offs

circular pong

173
Pong Spin-offs

Warlords

174
Pong Spin-offs

and many, many others. . .

175
Pong

Think in objects Applet, Paddle, Ball
Start with the Ball classwhat does it have, what
does it do?
class Ball
Ball()

176
Pong

class Ball
int nX, nY
Color myColor
Ball()

177
Pong

class Ball
int nX, nY
Color myColor
Ball()
nX 320
nY 240
myColor Color.red

178
Pong

class Ball
int nX, nY
Color myColor
Ball()
nX 320
nY 240
myColor Color.red
void move()
void bounce()
void draw (Graphics g)

179
Pong

class Ball
int nX, nY
Color myColor
Ball()
nX 320
nY 240
myColor Color.red
void move()
if( ?? )
nX nX 5
else
nX nX 5

180
Pong

class Ball
int nX, nY
Color myColor
boolean bRight, bUp
Ball()
nX 320
nY 240
bRight false
bUp true
myColor Color.red
void move()
if(bRight)
nX nX 5
else
nX nX 5

181
Pong

class Ball
int nX, nY
Color myColor
boolean bRight, bUp
Ball()
nX 320
nY 240
bRight false
bUp true
myColor Color.red
void move()
if(bRight)
nX nX 5
else
nX nX 5

182
Pong

class Ball
int nX, nY
Color myColor
boolean bRight, bUp
Ball()
nX 320
nY 240
bRight false
bUp true
myColor Color.red
void move()
if(bRight)
nX nX 5
else
nX nX 5

183
Practice Quiz Question Find the output

Couple bob new Couple()
Few jane new Few()
System.out.println(bob.mystery1())
System.out.println(jane.mystery1())
bob.mystery2()
jane.mystery2()
System.out.println("Bob's double is " bob.d1)
System.out.println("Jane's double is " jane.d1)

class Couple int n1 double d1
Couple() System.out.println("Couple")
n1 2 d1 1.5
int mystery1 () return (int)(n1d1)
void mystery2 () int nMys
mystery1() d1 nMys d1
class Few extends Couple String
sWord Few() System.out.println("Few") sWor
d "Few" int mystery1() d1
sWord.length() return (int)d1
184
How worms move

Starting from the back of the worm copy the x
and y coordinates of the next segment. Stop when
you get to the segment behind the head
The head coordinates change depending on the
direction of travel

head
head
(5,2)
(6,2)
(7,2)
(5,2)
(6,2)
(5,3)
(5,3)
(5,4)
185